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  • Chemistry  (9,239)
  • General Chemistry  (2,207)
  • Polymer and Materials Science  (2,009)
  • Fluid Mechanics and Thermodynamics
  • Industrial Chemistry
  • Inorganic Chemistry
  • Seismology
  • 2010-2014  (786)
  • 1950-1954  (9,302)
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  • 101
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    Automatic picking of direct P, S seismic phases and fault zone head waves (2014)
    Oxford University Press
    Details
    Publication Date: 2014-08-13
    Description: We develop a set of algorithms for automatic detection and picking of direct P and S waves, as well as fault zone head waves (FZHW), generated by earthquakes on faults that separate different lithologies and recorded by local seismic networks. The S -wave picks are performed using polarization analysis and related filters to remove P -wave energy from the seismograms, and utilize STA/LTA and kurtosis detectors in tandem to lock on the phase arrival. The early portions of P waveforms are processed with STA/LTA, kurtosis and skewness detectors for possible first-arriving FZHW. Identification and picking of direct P and FZHW is performed by a multistage algorithm that accounts for basic characteristics (motion polarities, time difference, sharpness and amplitudes) of the two phases. The algorithm is shown to perform well on synthetic seismograms produced by a model with a velocity contrast across the fault, and observed data generated by earthquakes along the Parkfield section of the San Andreas fault and the Hayward fault. The developed techniques can be used for systematic processing of large seismic waveform data sets recorded near major faults.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 102
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    A level-set adjoint-state method for crosswell transmission-reflection traveltime tomography (2014)
    Oxford University Press
    Details
    Publication Date: 2014-08-13
    Description: We propose a level-set adjoint-state method for crosswell traveltime tomography using both first-arrival transmission and reflection traveltime data. Since our entire formulation is based on solving eikonal and advection equations on finite-difference meshes, our traveltime tomography strategy is carried out without computing rays explicitly. We incorporate reflection traveltime data into the formulation so that possible reflectors (slowness interfaces) in the targeted subsurface model can be recovered as well as the slowness distribution itself. Since a reflector may assume a variety of irregular geometries, we propose to use a level-set function to implicitly parametrize the shape of a reflector. Therefore, a mismatch functional is established to minimize the traveltime data misfit with respect to both the slowness distribution and the level-set function, and the minimization is achieved by using a gradient descent method with gradients computed by solving adjoint state equations. To assess uncertainty or reliability of reconstructed slowness models, we introduce a labelling function to characterize first-arrival ray coverage of the computational domain, and this labelling function satisfies an advection equation. We apply fast-sweeping type methods to solve eikonal, adjoint-state and advection equations arising in our formulation. Numerical examples demonstrate that the proposed algorithm is robust to noise in the measurements, and can recover complicated structure even with little information on the reflector.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
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  • 103
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    Application of teleseismic long-period surface waves from ambient noise in regional surface wave tomography: a case study in western USA (2014)
    Oxford University Press
    Details
    Publication Date: 2014-07-19
    Description: Since the emerging of ambient noise tomography (ANT) in 2005, it has become a routine method to image the structures of crust and uppermost mantle because of its exclusive capability to extract short-period surface waves. Most of previous ANT studies focus on surface waves at periods shorter than 40/50 s. There are only a few studies of long-period surface wave tomography from ambient noise (longer than 50 s) in global scale. No tomography studies have been performed using teleseismic long-period surface waves from ambient noise in a regional scale, probably due to the two reasons that (1) energy of long-period ambient noise is weaker and it is harder to retrieve good signal-to-noise ratio long-period surface waves from portable stations with several years of ambient noise data and (2) long-period dispersion measurements from ambient noise may have larger uncertainties than those at shorter periods (〈40/50 s). In this study, I investigate the feasibility of using teleseismic long-period surface waves from ambient noise in regional surface wave tomography and also evaluate the accuracy of long-period dispersion measurements at periods up to 150 s. About 300 USArray/Transportable Array (TA) stations located in the Colorado Plateau and surrounding areas and 400 teleseismic stations relative to the TA stations are selected. Clear, strong, and coherent long-period teleseismic surface waves at periods much longer than 50 s are observed in the teleseismic cross-correlations between the TA stations and the teleseismic stations. Using long-period dispersion curves from ambient noise, I generate phase velocity maps at 50–150 s periods and then compare them with phase velocity maps from teleseismic earthquake data. The results show that phase velocity maps from ambient noise data and earthquake data are similar at the 50–150 s period range, verifying the validity of using long-period surface wave from ambient noise in regional surface wave tomography.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
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  • 104
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    Retrieval of reflections from ambient noise using illumination diagnosis (2014)
    Oxford University Press
    Details
    Publication Date: 2014-07-19
    Description: Seismic interferometry (SI) enables the retrieval of virtual sources at the location of receivers. In the case of passive SI, no active sources are used for the retrieval of the reflection response of the subsurface, but ambient-noise recordings only. The resulting retrieved response is determined by the illumination characteristics of the recorded ambient noise. Characteristics like geometrical distribution and signature of the noise sources, together with the complexity of the medium and the length of the noise records, determine the quality of the retrieved virtual-shot events. To retrieve body wave reflections, one needs to correlate body-wave noise. A source of such noise might be regional seismicity. In regions with notable human presence, the dominant noise sources are generally located at or close to the surface. In the latter case, the noise will be dominated by surface waves and consequently also the retrieved virtual common-source panels will contain dominant retrieved surface waves, drowning out possible retrieved reflections. In order to retrieve reflection events, suppression of the surface waves becomes the most important pre-processing goal. Because of the reasons mentioned above, we propose a fast method to evaluate the illumination characteristics of ambient noise using the correlation results from ambient-noise records. The method is based on the analysis of the so-called source function of the retrieved virtual-shot panel, and evaluates the apparent slowness of arrivals in the correlation results that pass through the position of the virtual source and at zero time. The results of the diagnosis are used to suppress the retrieval of surface waves and therefore to improve the quality of the retrieved reflection response. We explain the approach using modelled data from transient and continuous noise sources and an example from a passive field data set recorded at Annerveen, Northern Netherlands.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 105
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    Three-dimensional inverse modelling of damped elastic wave propagation in the Fourier domain (2014)
    Oxford University Press
    Details
    Publication Date: 2014-07-19
    Description: 3-D full waveform inversion (FWI) of seismic wavefields is routinely implemented with explicit time-stepping simulators. A clear advantage of explicit time stepping is the avoidance of solving large-scale implicit linear systems that arise with frequency domain formulations. However, FWI using explicit time stepping may require a very fine time step and (as a consequence) significant computational resources and run times. If the computational challenges of wavefield simulation can be effectively handled, an FWI scheme implemented within the frequency domain utilizing only a few frequencies, offers a cost effective alternative to FWI in the time domain. We have therefore implemented a 3-D FWI scheme for elastic wave propagation in the Fourier domain. To overcome the computational bottleneck in wavefield simulation, we have exploited an efficient Krylov iterative solver for the elastic wave equations approximated with second and fourth order finite differences. The solver does not exploit multilevel preconditioning for wavefield simulation, but is coupled efficiently to the inversion iteration workflow to reduce computational cost. The workflow is best described as a series of sequential inversion experiments, where in the case of seismic reflection acquisition geometries, the data has been laddered such that we first image highly damped data, followed by data where damping is systemically reduced. The key to our modelling approach is its ability to take advantage of solver efficiency when the elastic wavefields are damped. As the inversion experiment progresses, damping is significantly reduced, effectively simulating non-damped wavefields in the Fourier domain. While the cost of the forward simulation increases as damping is reduced, this is counterbalanced by the cost of the outer inversion iteration, which is reduced because of a better starting model obtained from the larger damped wavefield used in the previous inversion experiment. For cross-well data, it is also possible to launch a successful inversion experiment without laddering the damping constants. With this type of acquisition geometry, the solver is still quite effective using a small fixed damping constant. To avoid cycle skipping, we also employ a multiscale imaging approach, in which frequency content of the data is also laddered (with the data now including both reflection and cross-well data acquisition geometries). Thus the inversion process is launched using low frequency data to first recover the long spatial wavelength of the image. With this image as a new starting model, adding higher frequency data refines and enhances the resolution of the image. FWI using laddered frequencies with an efficient damping schemed enables reconstructing elastic attributes of the subsurface at a resolution that approaches half the smallest wavelength utilized to image the subsurface. We show the possibility of effectively carrying out such reconstructions using two to six frequencies, depending upon the application. Using the proposed FWI scheme, massively parallel computing resources are essential for reasonable execution times.
    Keywords: Seismology
    Print ISSN: 0956-540X
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    Topics: Geosciences
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  • 106
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    Up-dip directivity in near-source during the 2009 L'Aquila main shock (2014)
    Oxford University Press
    Details
    Publication Date: 2014-07-19
    Description: In this study we have investigated the directivity associated with the initial up-dip rupture propagation during the 2009 April 6 ( M w 6.1) L'Aquila normal-faulting earthquake. The objective is the understanding of how the peculiar initial behaviour of rupture history during the main shock has affected the near-source recorded ground motions in the L'Aquila town and surrounding areas. We have modelled the observed ground velocities at the closest near-source recording sites by computing synthetic seismograms using a discrete wavenumbers and finite difference approach in the low frequency bandwidth (0.02–0.4 Hz) to avoid site effects contaminations. We use both the rupture model retrieved by inverting ground motion waveforms and continuous high sampling-rate GPS time-series as well as uniform-slip constant-rupture speed models. Our results demonstrate that the initial up-dip rupture propagation, characterizing the first 3 s of the rupture history during the L'Aquila main shock and releasing only ~25 per cent of total seismic moment, controls the observed ground motions in the near-source. This initial stage of the rupture is characterized by the generation of ground velocity pulses, which we interpret as a forward directivity effect. Our modelling results confirm a heterogeneous distribution of rupture velocity during the initial up-dip rupture propagation, since uniform rupture speed models overestimate up-dip directivity effects in the footwall of the causative fault. The up-dip directivity observed in the near field during the 2009 L'Aquila main shock is that expected for a normal faulting earthquake, but it differs from that inferred from far-field observations that conversely provide evidence of along-strike directivity. This calls for a careful analysis as well as for the realistic inclusion of rupture directivity to predict ground motions in the near source.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
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  • 107
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    Effective orthorhombic anisotropic models for wavefield extrapolation (2014)
    Oxford University Press
    Details
    Publication Date: 2014-07-20
    Description: Wavefield extrapolation in orthorhombic anisotropic media incorporates complicated but realistic models to reproduce wave propagation phenomena in the Earth's subsurface. Compared with the representations used for simpler symmetries, such as transversely isotropic or isotropic, orthorhombic models require an extended and more elaborated formulation that also involves more expensive computational processes. The acoustic assumption yields more efficient description of the orthorhombic wave equation that also provides a simplified representation for the orthorhombic dispersion relation. However, such representation is hampered by the sixth-order nature of the acoustic wave equation, as it also encompasses the contribution of shear waves. To reduce the computational cost of wavefield extrapolation in such media, we generate effective isotropic inhomogeneous models that are capable of reproducing the first-arrival kinematic aspects of the orthorhombic wavefield. First, in order to compute traveltimes in vertical orthorhombic media, we develop a stable, efficient and accurate algorithm based on the fast marching method. The derived orthorhombic acoustic dispersion relation, unlike the isotropic or transversely isotropic ones, is represented by a sixth order polynomial equation with the fastest solution corresponding to outgoing P waves in acoustic media. The effective velocity models are then computed by evaluating the traveltime gradients of the orthorhombic traveltime solution, and using them to explicitly evaluate the corresponding inhomogeneous isotropic velocity field. The inverted effective velocity fields are source dependent and produce equivalent first-arrival kinematic descriptions of wave propagation in orthorhombic media. We extrapolate wavefields in these isotropic effective velocity models using the more efficient isotropic operator, and the results compare well, especially kinematically, with those obtained from the more expensive anisotropic extrapolator.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 108
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    High-resolution phase-velocity maps using data from an earthquake recorded at regional distances by a dense broad-band seismic network (2014)
    Oxford University Press
    Details
    Publication Date: 2014-07-12
    Description: The large regional earthquake (2008 February 21, M = 6.0) with epicentre near Wells, Nevada, occurred within a few hundred kilometres of the High Lava Plains (HLP) seismic experiment stations when the network was near its full deployment (〉100 stations with 10–30 km station spacing). The Wells earthquake provides an excellent opportunity to address two questions: What is the effect of small epicentral distances on surface-wave analyses at periods that are used in the analysis of teleseisms? Can one take advantage of a high-density seismic network to obtain improved phase-velocity maps? As small epicentral distances may introduce systematic errors in the surface-wave analysis for longer periods, we test for such effects by generating synthetic waveforms at locations for all regional-distance stations recording the Wells earthquake. Analysis of the synthetics suggests that our surface-waves analyses can be applied for the Wells earthquake up to periods of ~50 s. Applying the same method to data, we estimate two-station Rayleigh-wave fundamental-mode phase-velocities at selected periods and, for each acceptable path, assign the calculated phase velocity to the geographic location of the centre of the path. We contour the phase velocities for all path centres using a local gridding algorithm. The resulting maps for the Wells earthquake have well-constrained phase velocities up to 40–50 s period and allow us to see phase-velocity gradients not observed in earlier studies that used data from teleseisms or ambient noise tomography.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 109
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    A Bayesian approach to determine the average shape of the slip-rate function and the rupture velocity using near-field phases: the 2003 Mw6.6 Bam earthquake example (2014)
    Oxford University Press
    Details
    Publication Date: 2014-08-24
    Description: Slip-rate function and the rupture velocity are two important parameters that are critical in understanding the physics of earthquakes. When conventional objective functions are used, the slip-rate function is not well resolved from seismic data. Here, we propose a new method to obtain the slip-rate function by utilizing the near-field phases recorded near the fault rupture. First we illustrate the sensitivity of near-field phases to the moment accumulation and modify the objective function in order to take advantage of this sensitivity. By utilizing near-field P waves along with S pulses on the near-source records and using a Bayesian approach, we show that we can constrain the average slip-rate function as well as the average rupture velocity for a strike-slip earthquake. As a case example, we apply this technique to the record of the 2003, M w 6.6 Bam Earthquake. Our results indicate an asymmetric slip-rate function, with acceleration duration of about 0.4 s, and deceleration duration of 1.4 s. The slip-rate function obtained from kinematic modelling of the 2003 Bam earthquake is consistent with those predicted by dynamic rupture simulations. The rupture velocity is about 82–90 per cent of the shear wave velocity, implying a sub-Rayleigh rupture velocity close to the Rayleigh wave speed. In future cases where abundant near-source strong-motion data exist and slip is well constrained, the method described in this study can be applied to obtain the variation of the slip-rate function along the fault which would improve our understanding of earthquake rupture physics.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
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  • 110
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    Changes in shear wave splitting before the 2010 Eyjafjallajokull eruption in Iceland (2014)
    Oxford University Press
    Details
    Publication Date: 2014-08-02
    Description: We use shear wave splitting (SWS) above microearthquakes to monitor stress variations before the 2010 March and April flank and summit eruptions of Eyjafjallajökull volcano in Iceland. SWS time delays before Eyjafjallajökull show characteristic variations similar to those seen before earthquakes. The time delays display a nearly linear increase before the eruption, an abrupt change of slope and a rapid nearly linear decrease until the flank eruption begins. Similar variations before earthquakes are interpreted as stress-accumulation increases, and stress-relaxation decreases as microcracks coalesce onto the eventual fault plane. The changes in SWS before Eyjafjallajökull are interpreted as a similar stress-accumulation increase, as magma penetrates the crust, and stress-relaxation decrease as microcracks coalesce onto the magma conduit prior to magma release. We suggest that the remarkable similarity between stress changes before eruption and earthquake is strong evidence for the New Geophysics of a critically microcracked crust.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
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  • 111
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    A comprehensive dispersion model of surface wave phase and group velocity for the globe (2014)
    Oxford University Press
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    Publication Date: 2014-08-02
    Description: A new method is developed to measure Rayleigh- and Love-wave phase velocities globally using a cluster analysis technique. This method clusters similar waveforms recorded at different stations from a single event and allows users to make measurements on hundreds of waveforms, which are filtered at a series of frequency ranges, at the same time. It also requires minimal amount of user interaction and allows easy assessment of the data quality. This method produces a large amount of phase delay measurements in a manageable time frame. Because there is a strong trade-off between the isotropic part of the Rayleigh-wave phase velocity and azimuthal anisotropy, we include the effect of azimuthal anisotropy in our inversions in order to obtain reliable isotropic phase velocity. We use b-splines to combine these isotropic phase velocity maps with our previous group velocity maps to produce an internally consistent global surface wave dispersion model.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
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  • 112
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    Ps-splitting analysis for multilayered anisotropic media by azimuthal stacking and layer stripping (2014)
    Oxford University Press
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    Publication Date: 2014-08-03
    Description: We present a robust technique for the analysis of shear wave splitting in layered anisotropic media by using converted shear phases. In (weakly) anisotropic media, the P -to- S converted phases ( Ps ) exhibit a distinct variation in arrival time (cosine moveout) as function of backazimuth. This variation can be exploited by time-shifting and stacking radial receiver functions to constrain a range of possible splitting parameters (i.e. the fast-polarization direction and the delay time) for an anisotropic layer. Then, the minimization of the transverse-component energy is used to select the pair of splitting parameters that best describes the anisotropic properties of the layer. This two-step approach stabilizes the inversion process and significantly reduces the time for computing the best splitting parameters. In multilayered anisotropic media, the distinct cosine moveout of the radial component is also observed for Ps phases from deeper discontinuities. The direct application of the method leads to apparent (or effective) splitting parameters for all anisotropic layers above the converting interface. Explicit expressions for the apparent splitting parameters are derived. The splitting parameters for the individual layers can be inferred by a layer-stripping approach, where the splitting effect due to shallower layers on converted phases from deeper discontinuities is successively corrected. In comparison to the approach by Liu & Niu ( 2012 ), the method represents a computationally efficient extension to multiple anisotropic layers. We further investigate the influences of noise, gaps in the azimuthal distribution of events, and interface dip on the splitting analysis. As an example, we apply the method to data from two stations of the Iranian National Seismic Network. At station GHIR the observed azimuthal variations of the Ps phase yield splitting parameters t  = 0.26 s and f  = 48° for a 48-km-thick crust. At station SHGR anisotropic effects on the Ps phases can be accounted for by two anisotropic crustal layers with a total thickness of 44 km. For the upper 22-km-thick layer we obtain a delay time t 1  = 0.25 s and a fast axis 1  = 35°. Values for the lower layer are t 2  = 0.19 s and 2  = –88°. The delay times correspond to anisotropies of about 4 and 3 per cent in the upper and lower layer, respectively. The method also provides a robust tool to separate the anisotropic properties of the crust and mantle and, thus, to isolate the anisotropic signature of mantle-deformation processes.
    Keywords: Seismology
    Print ISSN: 0956-540X
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  • 113
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    Synthetic seismic anisotropy models within a slab impinging on the core-mantle boundary (2014)
    Oxford University Press
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    Publication Date: 2014-08-03
    Description: The lowermost few hundreds of kilometres of the Earth's mantle are elastically anisotropic; seismic velocities vary with direction of propagation and polarization. Observations of strong seismic anisotropy correlate with regions where subducted slab material is expected. In this study, we evaluate the hypothesis that crystal preferred orientation (CPO) in a slab, as it impinges on the core–mantle boundary, is the cause of the observed anisotropy. Next, we determine if fast polarization directions seen by shear waves can be mapped to directions of geodynamic flow. This approach is similar to our previous study performed for a 2-D geodynamic model. In this study, we employ a 3-D geodynamic model with temperature-dependent viscosity and kinematic velocity boundary conditions defined at the surface of the Earth to create a broad downwelling slab. Tracers track the deformation that we assume to be accommodated by dislocation creep. We evaluate the models for the presence of perovskite or post-perovskite and for different main slip systems along which dislocation creep may occur in post-perovskite [(100),(010) and (001)]—resulting in four different mineralogical models of CPO. Combining the crystal pole orientations with single crystal elastic constants results in seismically distinguishable models of seismic anisotropy. The models are evaluated against published seismic observations by analysing different anisotropic components: the radial anisotropy, the splitting for (sub-)vertical phases (i.e. azimuthal anisotropy), and the splitting for subhorizontal phases. The patterns in radial anisotropy confirm our earlier results in 2-D. Observations of radial anisotropy and splitting in subhorizontal phases are mostly consistent with our models of post-perovskite with (010)-slip and (001)-slip. Our model of (001)-slip predicts stronger splitting than for (010)-slip for horizontally propagating phases in all directions. The strongest seismic anisotropy in this model occurs where the slab impinges on the core–mantle boundary. The azimuthal anisotropy pattern for (001)-slip shows fast axis directions at the edges of the slab (sub-)parallel to flow directions, suggesting horizontal flows may be mapped out in the lowermost mantle using seismic observations.
    Keywords: Seismology
    Print ISSN: 0956-540X
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    Topics: Geosciences
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  • 114
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    Changchun (2014)
    Nature Publishing Group (NPG)
    Details
    Publication Date: 2014-12-18
    Description: 〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉England -- Nature. 2014 Dec 18;516(7531):S72. doi: 10.1038/516S72a.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/25517243" target="_blank"〉PubMed〈/a〉
    Keywords: Chemistry ; China ; Cities ; Periodicals as Topic/statistics & numerical data ; Research/standards/*statistics & numerical data/trends ; Universities/statistics & numerical data
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Published by Nature Publishing Group (NPG)
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  • 115
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    Hangzhou (2014)
    Nature Publishing Group (NPG)
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    Publication Date: 2014-12-18
    Description: 〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉England -- Nature. 2014 Dec 18;516(7531):S69. doi: 10.1038/516S69a.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/25517242" target="_blank"〉PubMed〈/a〉
    Keywords: Chemistry ; China ; Cities ; Periodicals as Topic/statistics & numerical data ; Physics ; Research/standards/*statistics & numerical data/trends ; Universities/statistics & numerical data
    Print ISSN: 0028-0836
    Electronic ISSN: 1476-4687
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
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  • 116
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    Retrospective. Frederick Sanger (1918-2013) (2014)
    American Association for the Advancement of Science (AAAS)
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    Publication Date: 2014-01-18
    Description: 〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Brenner, Sydney -- New York, N.Y. -- Science. 2014 Jan 17;343(6168):262. doi: 10.1126/science.1249912.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Institute of Molecular and Cell Biology, 61 Biopolis Drive, Singapore, 138673.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/24436413" target="_blank"〉PubMed〈/a〉
    Keywords: Chemistry ; England ; History, 20th Century ; History, 21st Century ; Molecular Biology/*history ; *Nobel Prize ; Sequence Analysis, DNA/*history/methods
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 117
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    3-D crustal structure of the western United States: application of Rayleigh-wave ellipticity extracted from noise cross-correlations (2014)
    Oxford University Press
    Details
    Publication Date: 2014-06-22
    Description: We present a new 3-D seismic model of the western United States crust derived from a joint inversion of Rayleigh-wave phase velocity and ellipticity measurements using periods from 8 to 100 s. Improved constraints on upper-crustal structure result from use of short-period Rayleigh-wave ellipticity, or Rayleigh-wave H/V (horizontal to vertical) amplitude ratios, measurements determined using multicomponent ambient noise cross-correlations. To retain the amplitude ratio information between vertical and horizontal components, for each station, we perform daily noise pre-processing (temporal normalization and spectrum whitening) simultaneously for all three components. For each station pair, amplitude measurements between cross-correlations of different components (radial–radial, radial–vertical, vertical–radial and vertical–vertical) are then used to determine the Rayleigh-wave H/V ratios at the two station locations. We use all EarthScope/USArray Tranportable Array data available between 2007 January and 2011 June to determine the Rayleigh-wave H/V ratios and their uncertainties at all station locations and construct new Rayleigh-wave H/V ratio maps in the western United States between periods of 8 and 24 s. Combined with previous longer period earthquake Rayleigh-wave H/V ratio measurements and Rayleigh-wave phase velocity measurements from both ambient noise and earthquakes, we invert for a new 3-D crustal and upper-mantle model in the western United States. Correlation between the inverted model and known geological features at all depths suggests good resolution in five crustal layers. Use of short-period Rayleigh-wave H/V ratio measurements based on noise cross-correlation enables resolution of distinct near surface features such as the Columbia River Basalt flows, which overlie a thick sedimentary basin.
    Keywords: Seismology
    Print ISSN: 0956-540X
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    Topics: Geosciences
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    Rayleigh wave tomography of the British Isles from ambient seismic noise (2014)
    Oxford University Press
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    Publication Date: 2014-06-22
    Description: We present the first Rayleigh wave group speed maps of the British Isles constructed from ambient seismic noise. The maps also constitute the first surface wave tomography study of the crust under the British Isles at a relatively high resolution. We computed interferometric, interstation Rayleigh waves from vertical component records of ambient seismic noise recorded on 63 broad-band and short-period stations across the UK and Ireland. Group velocity measurements were made from the resulting surface wave dispersion curves between 5 and 25 s using a multiple phase-matched filter method. Uncertainties in the group velocities were computed by calculating the standard deviation of four dispersion curves constructed by stacking a random selection of daily cross-correlations. Where an uncertainty could not be obtained for a ray path using this method, we estimated it as a function of the interreceiver distance. Group velocity maps were computed for 5–25-s period using the Fast Marching forward solution of the eikonal equation and iterative, linearized inversion. At short and intermediate periods, the maps show remarkable agreement with the major geological features of the British Isles including: terrane boundaries in Scotland; regions of late Palaeozoic basement uplift; areas of exposed late Proterozoic/early Palaeozoic rocks in southwest Scotland, northern England and northwest Wales and, sedimentary basins formed during the Mesozoic such as the Irish Sea Basin, the Chester Basin, the Worcester Graben and the Wessex Basin. The maps also show a consistent low-velocity anomaly in the region of the Midlands Platform, a Proterozoic crustal block in the English Midlands. At longer periods, which are sensitive velocities in the lower crustal/upper mantle, the maps suggest that the depth of Moho beneath the British Isles decreases towards the north and west. Areas of fast velocity in the lower crust also coincide with areas thought to be associated with underplating of the lower crust such as Northern Ireland, the eastern Irish Sea and northwest Wales.
    Keywords: Seismology
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    Receiver-pair seismic interferometry applied to body-wave USArray data (2014)
    Oxford University Press
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    Publication Date: 2014-06-28
    Description: With seismic interferometry, reflections can be retrieved between stations positioned on the Earth's surface. In the classical form, the reflections are retrieved by a crosscorrelation of observations and an integration over subsurface sources. For a specific data set, however, the actual source distribution might not be sufficient to approximate the source integral. Yet, there might be a dense distribution of receivers allowing an integration over the receiver domain. We rewrite the source integral to an integration over receiver pairs and call it receiver-pair seismic interferometry (RPSI). With this formulation, reflections can be retrieved even in the limiting case of only a single source. We illustrate the new relation both with synthetic data and data from the USArray, which is a large grid of stations covering the USA. The field observations are from an earthquake in Mexico. We show that RPSI can be applied both for a line and grid of receivers. When using isolated phases recorded over a line of stations inline with the earthquake, reflections are retrieved from the core-mantle boundary, which reflections can be ascribed to specific virtual source and receiver locations within the USArray. When using full responses as input to RPSI, the retrieved phases are an average over multiple virtual sources and receivers. Location is then only possible when the integrand is spatially windowed or when a clear leading term is identified. When using a grid of receivers, the location of the source does not need to be known, but spatially averaged instead of localized responses are obtained, also when isolated arrivals are used as input to RPSI.
    Keywords: Seismology
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    Improved implementation of the fk and Capon methods for array analysis of seismic noise (2014)
    Oxford University Press
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    Publication Date: 2014-06-28
    Description: The frequency–wavenumber (fk) and Capon methods are widely used in seismic array studies of background or ambient noise to infer the backazimuth and slowness of microseismic sources. We present an implementation of these techniques for the analysis of microseisms (0.05–2 Hz) which draws on array signal processing literature from a range of disciplines. The presented techniques avoid frequency mixing in the cross-power spectral density and therefore yield an accurate slowness vector estimation of the incoming seismic waves. Using synthetic data, we show explicitly how the frequency averaged broad-band approach can result in a slowness-shifted spectrum. The presented implementation performs the slowness estimations individually for each frequency bin and sums the resulting slowness spectra over a specific frequency range. This may be termed an incoherently averaged signal, or IAS, approach. We further modify the method through diagonal loading to ensure a robust solution. The synthetic data show good agreement between the analytically derived and inferred error in slowness. Results for real (observed) data are compared between the approximate and IAS methods for two different seismic arrays. The IAS method results in the improved resolution of features, particularly for the Capon spectrum, and enables, for instance, Rg and Lg arrivals from similar backazimuths to be separated in the case of real data.
    Keywords: Seismology
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    Tracking earthquake source evolution in 3-D (2014)
    Oxford University Press
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    Publication Date: 2014-06-28
    Description: Starting from the hypocentre, the point of initiation of seismic energy, we seek to estimate the subsequent trajectory of the points of emission of high-frequency energy in 3-D, which we term the ‘evocentres’. We track these evocentres as a function of time by energy stacking for putative points on a 3-D grid around the hypocentre that is expanded as time progresses, selecting the location of maximum energy release as a function of time. The spatial resolution in the neighbourhood of a target point can be simply estimated by spatial mapping using the properties of isochrons from the stations. The mapping of a seismogram segment to space is by inverse slowness, and thus more distant stations have a broader spatial contribution. As in hypocentral estimation, the inclusion of a wide azimuthal distribution of stations significantly enhances 3-D capability. We illustrate this approach to tracking source evolution in 3-D by considering two major earthquakes, the 2007 M w 8.1 Solomons islands event that ruptured across a plate boundary and the 2013 M w 8.3 event 610 km beneath the Sea of Okhotsk. In each case we are able to provide estimates of the evolution of high-frequency energy that tally well with alternative schemes, but also to provide information on the 3-D characteristics that is not available from backprojection from distant networks. We are able to demonstrate that the major characteristics of event rupture can be captured using just a few azimuthally distributed stations, which opens the opportunity for the approach to be used in a rapid mode immediately after a major event to provide guidance for, for example tsunami warning for megathrust events.
    Keywords: Seismology
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    Shear-wave velocity profile and seismic input derived from ambient vibration array measurements: the case study of downtown L'Aquila (2014)
    Oxford University Press
    Details
    Publication Date: 2014-06-28
    Description: Downtown L'Aquila suffered severe damage (VIII-IX EMS98 intensity) during the 2009 April 6 M w 6.3 earthquake. The city is settled on a top flat hill, with a shear-wave velocity profile characterized by a reversal of velocity at a depth of the order of 50–100 m, corresponding to the contact between calcareous breccia and lacustrine deposits. In the southern sector of downtown, a thin unit of superficial red soils causes a further shallow impedance contrast that may have influenced the damage distribution during the 2009 earthquake. In this paper, the main features of ambient seismic vibrations have been studied in the entire city centre by using array measurements. We deployed six 2-D arrays of seismic stations and 1-D array of vertical geophones. The 2-D arrays recorded ambient noise, whereas the 1-D array recorded signals produced by active sources. Surface-wave dispersion curves have been measured by array methods and have been inverted through a neighbourhood algorithm, jointly with the H/V ambient noise spectral ratios related to Rayleigh waves ellipticity. We obtained shear-wave velocity ( Vs ) profiles representative of the southern and northern sectors of downtown L'Aquila. The theoretical 1-D transfer functions for the estimated Vs profiles have been compared to the available empirical transfer functions computed from aftershock data analysis, revealing a general good agreement. Then, the Vs profiles have been used as input for a deconvolution analysis aimed at deriving the ground motion at bedrock level. The deconvolution has been performed by means of EERA and STRATA codes, two tools commonly employed in the geotechnical engineering community to perform equivalent-linear site response studies. The waveform at the bedrock level has been obtained deconvolving the 2009 main shock recorded at a strong motion station installed in downtown. Finally, this deconvolved waveform has been used as seismic input for evaluating synthetic time-histories in a strong-motion target site located in the middle Aterno river valley. As a target site, we selected the strong-motion station of AQV 5 km away from downtown L'Aquila. For this site, the record of the 2009 L'Aquila main shock is available and its surface stratigraphy is adequately known making possible to propagate the deconvolved bedrock motion back to the surface, and to compare recorded and synthetic waveforms.
    Keywords: Seismology
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    The 2013 September-October seismic sequence offshore Spain: a case of seismicity triggered by gas injection? (2014)
    Oxford University Press
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    Publication Date: 2014-06-28
    Description: A spatially localized seismic sequence originated few tens of kilometres offshore the Mediterranean coast of Spain, close to the Ebro river delta, starting on 2013 September 5, and lasting at least until 2013 October. The sequence culminated in a maximal moment magnitude M w 4.3 earthquake, on 2013 October 1. The most relevant seismogenic feature in the area is the Fosa de Amposta fault system, which includes different strands mapped at different distances to the coast, with a general NE–SW orientation, roughly parallel to the coastline. However, no significant known historical seismicity has involved this fault system in the past. The epicentral region is also located near the offshore platform of the Castor project, where gas is conducted through a pipeline from mainland and where it was recently injected in a depleted oil reservoir, at about 2 km depth. We analyse the temporal evolution of the seismic sequence and use full waveform techniques to derive absolute and relative locations, estimate depths and focal mechanisms for the largest events in the sequence (with magnitude mbLg larger than 3), and compare them to a previous event (2012 April 8, mbLg 3.3) taking place in the same region prior to the gas injection. Moment tensor inversion results show that the overall seismicity in this sequence is characterized by oblique mechanisms with a normal fault component, with a 30° low-dip angle plane oriented NNE–SSW and a subvertical plane oriented NW–SE. The combined analysis of hypocentral location and focal mechanisms could indicate that the seismic sequence corresponds to rupture processes along shallow low-dip surfaces, which could have been triggered by the gas injection in the reservoir, and excludes the activation of the Amposta fault, as its known orientation is inconsistent with focal mechanism results. An alternative scenario includes the iterated triggering of a system of steep faults oriented NW–SE, which were identified by prior marine seismics investigations.
    Keywords: Seismology
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    Bayesian inversion for finite fault earthquake source models - II: the 2011 great Tohoku-oki, Japan earthquake (2014)
    Oxford University Press
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    Publication Date: 2014-06-28
    Description: We present a fully Bayesian inversion of kinematic rupture parameters for the 2011 M w 9 Tohoku-oki, Japan earthquake. Albeit computationally expensive, this approach to kinematic source modelling has the advantage of producing an ensemble of slip models that are consistent with physical a priori constraints, realistic data uncertainties, and realistic but simplistic uncertainties in the physics of the kinematic forward model, all without being biased by non-physical regularization constraints. Combining 1 Hz kinematic GPS, static GPS offsets, seafloor geodesy and near-field and far-field tsunami data into a massively parallel Monte Carlo simulation, we construct an ensemble of samples of the posterior probability density function describing the evolution of fault rupture. We find that most of the slip is concentrated in a depth range of 10–20 km from the trench, and that slip decreases towards the trench with significant displacements at the toe of wedge occurring in just a small region. Estimates of static stress drop and rupture velocity are ambiguous. Due to the spatial compactness of the fault rupture, the duration of the entire rupture was less than approximately 150 s.
    Keywords: Seismology
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    The Millikan shaking experiments and high-frequency seismic wave propagation in Southern California (2014)
    Oxford University Press
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    Publication Date: 2014-06-28
    Description: In order to study high-frequency seismic wave propagation, seismic wavefields generated by resonant-shaking experiments of the Millikan Library, on the campus of California Institute Technology (Pasadena, California, USA), were analysed. Because the resonant shaking frequencies are 1.12 Hz (the east–west direction) and 1.64 Hz (the north–south direction), this active-source experiment can provide opportunities for studying high-frequency seismic wave propagation in Southern California. Because they are very narrow frequency band data, the analyses must be quite different from ordinary time-domain analyses. We show, theoretically, that the signals must be dominated by surface waves. Adopting this surface wave assumption, we proceed to make two separate analyses, one on spectral amplitude and the other on phase. We present a new method to derive group velocity from phase based on the cross correlations between the station in the Millikan Library (MIK) and stations in the regional network. Our results support that an active-source experiment by resonant shaking of a building is a feasible approach for high-frequency seismic wave studies.
    Keywords: Seismology
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    Seismic velocity structure in the Hot Springs and Trifurcation areas of the San Jacinto fault zone, California, from double-difference tomography (2014)
    Oxford University Press
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    Publication Date: 2014-06-28
    Description: We present tomographic images of crustal velocity structures in the complex Hot Springs and Trifurcation areas of the San Jacinto Fault Zone (SJFZ) based on double-difference inversions of earthquake arrival times. We invert for V P , V S and hypocentre location within 50 x 50 x 20 km 3 volumes, using 266 969 P and 148 249 S arrival times. We obtain high-fidelity images of seismic velocities with resolution on the order of a few kilometres from 2 to 12 km depth and validate the results using checkerboard tests. Due to the relatively large proportion of S -wave arrival times, we also obtain stable maps of V P / V S ratios in both regions. The velocity of the Trifurcation Area as a whole is lower than adjacent unfaulted material. We interpret a 4-km-wide low velocity zone with high V P / V S ratio in the trifurcation itself as related to fault zone damage. We also observe clear velocity contrasts across the Buck Ridge, Clark and Coyote Creek segments of the SJFZ. The Anza segment of the SJFZ, to the NW of the trifurcation area, displays a strong (up to 27 per cent) contrast of V S from 2 to 9 km depth. In the Hot Springs area, a low velocity zone between the Claremont and Casa Loma Strands narrows with depth, with clear velocity contrasts observed across both segments. A roughly 10-km-wide zone of low velocity and low V P / V S ratio at the NW tip of the Hot Springs fault is indicative of either unconsolidated sediments associated with the San Jacinto basin, or fluid-filled cracks within a broad deformation zone. High V P / V S ratios along the Anza segment could indicate a preferred nucleation location for future large earthquakes, while the across-fault velocity contrast suggests a preferred northwest rupture propagation direction for such events.
    Keywords: Seismology
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    Seismic array analysis and redetermination of depths of earthquakes in Tien-Shan: implications for strength of the crust and lithosphere (2014)
    Oxford University Press
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    Publication Date: 2014-06-30
    Description: We have redetermined focal depths of moderate and major earthquakes with reported lower-crust and upper-mantle depths that have occurred in Tien-Shan, since the availability of broad-band array data. Records of earthquakes at global arrays have been used for identification and modelling of depth phases in order to make accurate estimation of focal depths. Our results show that half of the purportedly deep earthquakes are indeed originating from depths attributable to middle-crust and lower-crust regions. Also one exceptional event in the northern foreland of Tien-Shan in Junggar Basin is located in the upper mantle at the depth of 64 km. Such unusually deep earthquakes for intraplate continental tectonic domain are all located at the margin of Tien-Shan with its adjacent stable blocks and at least some of them have occurred where the brittle behaviour of continental rocks is not highly expected. The reverse mechanisms of all these earthquakes and their proximity to formerly subducting and later colliding and underplating stable blocks and their interactions with overlying Tien-Shan are clues to explain this extremity.
    Keywords: Seismology
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    Improved forward wave propagation and adjoint-based sensitivity kernel calculations using a numerically stable finite-element PML (2014)
    Oxford University Press
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    Publication Date: 2014-07-23
    Description: In recent years, the application of time-domain adjoint methods to improve large, complex underground tomographic models at the regional scale has led to new challenges for the numerical simulation of forward or adjoint elastic wave propagation problems. An important challenge is to design an efficient infinite-domain truncation method suitable for accurately truncating an infinite domain governed by the second-order elastic wave equation written in displacement and computed based on a finite-element (FE) method. In this paper, we make several steps towards this goal. First, we make the 2-D convolution formulation of the complex-frequency-shifted unsplit-field perfectly matched layer (CFS-UPML) derived in previous work more flexible by providing a new treatment to analytically remove singular parameters in the formulation. We also extend this new formulation to 3-D. Furthermore, we derive the auxiliary differential equation (ADE) form of CFS-UPML, which allows for extension to higher order time schemes and is easier to implement. Secondly, we rigorously derive the CFS-UPML formulation for time-domain adjoint elastic wave problems, which to our knowledge has never been done before. Thirdly, in the case of classical low-order FE methods, we show numerically that we achieve long-time stability for both forward and adjoint problems both for the convolution and the ADE formulations. In the case of higher order Legendre spectral-element methods, we show that weak numerical instabilities can appear in both formulations, in particular if very small mesh elements are present inside the absorbing layer, but we explain how these instabilities can be delayed as much as needed by using a stretching factor to reach numerical stability in practice for applications. Fourthly, in the case of adjoint problems with perfectly matched absorbing layers we introduce a computationally efficient boundary storage strategy by saving information along the interface between the CFS-UPML and the main domain only, thus avoiding the need to solve a backward wave propagation problem inside the CFS-UPML, which is known to be highly ill-posed. Finally, by providing several examples we show numerically that our formulation is efficient at absorbing acoustic waves for normal to near-grazing incident body waves as well as surface waves.
    Keywords: Seismology
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    Models and Frechet kernels for frequency-(in)dependent Q (2014)
    Oxford University Press
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    Publication Date: 2014-07-29
    Description: We present a new method for the modelling of frequency-dependent and frequency-independent Q in time-domain seismic wave propagation. Unlike previous approaches, attenuation models are constructed such that Q as a function of position in the Earth appears explicitly as a parameter in the equations of motion. This feature facilitates the derivation of Fréchet kernels for Q using adjoint techniques. Being simple products of the forward strain field and the adjoint memory variables, these kernels can be computed with no additional cost, compared to Fréchet kernels for elastic properties. The same holds for Fréchet kernels for the power-law exponent of frequency-dependent Q , that we derive as well. To illustrate our developments, we present examples from regional- and global-scale time-domain wave propagation.
    Keywords: Seismology
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    Influence of a sedimentary basin infilling description on the 2-D P-SV wave propagation using linear and non-linear constitutive models (2014)
    Oxford University Press
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    Publication Date: 2014-07-23
    Description: Numerical modelling is a useful tool to understand the role of different parameters (site geometry, impedance contrast, material properties, constitutive model, input motion) governing site effects. We focus here on the 2-D P – SV seismic wave propagation in a simple-shaped asymmetric model. We consider two ways of describing the sedimentary infilling properties: the basin is either composed of distinct homogeneous geological layers or is described through properties progressively changing as a function of depth (smooth model). In order to study the wave propagation in the two basins with similar soil properties, P and S velocities of the layered basin are deduced from the smooth basin ones, making both models compatible in a kinematic point of view. P and S velocities globally increase with depth. Following EPRI, non-linear properties are considered as constant in layers for the two basins. We assess the influence of the model choice on the wave propagation while taking into account linear and non-linear constitutive models. We test the influence of the input motion by propagating two different input motions, the first being a simple impulsive one and the second a real complex accelerogram. We find that transfer functions computed in each basin are quite similar irrespective of the soil constitutive behaviour and the input motion. Moreover, we show that the maximum shear strain spatial distribution depends on the input motion frequency content and maximum values depend on the input motion strength and complexity. Maximum shear strains are generally higher for superficial layers than for deeper ones. We show that the highest shear strain values are located above discontinuities, where velocities are lower and the soil is more non-linear than in the underlying layer, regardless of the input motion. Indeed, in a layered basin, the shear strain is higher above interfaces for all the soil constitutive models that were used. In the smooth visco-elastoplastic model, a kind of layering appears, absent in a viscoelastic model, due to the different non-linear properties in each layer. Moreover, no matter what the model structure and the input motion used, maximum shear strain levels are higher in the visco-elastoplastic soil than in the viscoelastic one. Such a numerical study helps to understand the non-linearity location and therefore the consequences of a model choice when performing a site-specific study.
    Keywords: Seismology
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    Fingerprinting ordered diffractions in multiply diffracted waves (2014)
    Oxford University Press
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    Publication Date: 2014-07-23
    Description: We show how to ‘fingerprint’ individual diffractors inside an acoustic medium using interrogative wave energy from arrays of sources and receivers. For any recorded multiply diffracted wave observed between any source and any receiver, the set of such fingerprints is sufficient information to identify all diffractors involved in the corresponding diffraction path, and the sequential order in which diffractors are encountered. The method herein thus decomposes complex, multiply diffracted wavefields into constituent, single-diffraction interactions.
    Keywords: Seismology
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    A full waveform tomography algorithm for teleseismic body and surface waves in 2.5 dimensions (2014)
    Oxford University Press
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    Publication Date: 2014-07-23
    Description: We describe a 2.5-D, frequency domain, viscoelastic waveform tomography algorithm for imaging with seismograms of teleseismic body and surface waves recorded by quasi-linear arrays. The equations of motion are discretized with p -adaptive finite elements that allow for geometric flexibility and accurate solutions as a function of wavelength. Artificial forces are introduced into the media by specifying a known wavefield along the model edges and solving for the corresponding scattered field. Because of the relatively low frequency content of teleseismic data, regional scale tectonic settings can be parametrized with a modest number of variables and perturbations can be determined directly from a regularized Gauss–Newton system of equations. Waveforms generated by the forward problem compare well with analytic solutions for simple 1-D and 2-D media. Tests of different approaches to the inverse problem show that the use of an approximate Hessian serves to properly focus the scattered field. We also find that while full waveform inversion can provide significantly better resolution than standard techniques for both body and surface wave tomography modelled individually, joint inversion both enhances resolution and mitigates potential artefacts.
    Keywords: Seismology
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    The spatial cross-correlation method for dispersive surface waves (2014)
    Oxford University Press
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    Publication Date: 2014-07-31
    Description: Dispersive surface waves are routinely used to estimate the subsurface shear-wave velocity distribution, at all length scales. In the well-known Spatial Autocorrelation method, dispersion information is gained from the correlation of seismic noise signals recorded on the vertical (or radial) components. We demonstrate practical advantages of including the cross-correlation between radial and vertical components of the wavefield in a spatial cross-correlation method. The addition of cross-correlation information increases the resolution and robustness of the phase velocity dispersion information, as demonstrated in numerical simulations and a near-surface field study with active seismic sources, where our method confirms the presence of a fault-zone conduit in a geothermal field.
    Keywords: Seismology
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    Radiative transfer theory for a random distribution of low velocity spheres as resonant isotropic scatterers (2014)
    Oxford University Press
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    Publication Date: 2014-07-31
    Description: Short-period seismograms of earthquakes are complex especially beneath volcanoes, where the S wave mean free path is short and low velocity bodies composed of melt or fluid are expected in addition to random velocity inhomogeneities as scattering sources. Resonant scattering inherent in a low velocity body shows trap and release of waves with a delay time. Focusing of the delay time phenomenon, we have to consider seriously multiple resonant scattering processes. Since wave phases are complex in such a scattering medium, the radiative transfer theory has been often used to synthesize the variation of mean square (MS) amplitude of waves; however, resonant scattering has not been well adopted in the conventional radiative transfer theory. Here, as a simple mathematical model, we study the sequence of isotropic resonant scattering of a scalar wavelet by low velocity spheres at low frequencies, where the inside velocity is supposed to be low enough. We first derive the total scattering cross-section per time for each order of scattering as the convolution kernel representing the decaying scattering response. Then, for a random and uniform distribution of such identical resonant isotropic scatterers, we build the propagator of the MS amplitude by using causality, a geometrical spreading factor and the scattering loss. Using those propagators and convolution kernels, we formulate the radiative transfer equation for a spherically impulsive radiation from a point source. The synthesized MS amplitude time trace shows a dip just after the direct arrival and a delayed swelling, and then a decaying tail at large lapse times. The delayed swelling is a prominent effect of resonant scattering. The space distribution of synthesized MS amplitude shows a swelling near the source region in space, and it becomes a bell shape like a diffusion solution at large lapse times.
    Keywords: Seismology
    Print ISSN: 0956-540X
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    Coseismic and potential early afterslip distribution of the 2009 Mw 6.3 L'Aquila, Italy earthquake (2014)
    Oxford University Press
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    Publication Date: 2014-07-31
    Description: In view of significant discrepancy between the slip models of the 2009 April 6 L'Aquila, (Italy) earthquake based on seismic waveforms or geodetic data, a new inversion strategy is proposed to constrain the coseismic rupture model and early post-seismic afterslip model of this earthquake. Both models were jointly inverted simultaneously from the near-source strong ground motion recordings, teleseismic P waves, long-period Rayleigh waves, GPS displacement vectors and InSAR line-of-sight displacement image. The deformations are mapped onto the fault with strike 140° and dip 50° to the southwest. The coseismic period here represents the first 10 s after the rupture initiation, when over 99 per cent of our total seismic moment (3.07 10 18 Nm) is already released in our preferred coseismic model. Most coseismic slip occurs in a small rupture area of approximately 16 km along strike and in a depth range of 3–10 km. The average slip is 0.45 m; peak slip reaches about 0.90 m. Our result reveals that L'Aquila rupture has a lag of 0.6 s before it first propagates updip to break an asperity equivalent to an M w 5.7. And then the rupture propagates along strike to break a large patch located about 8.0 km along strike and 4.0 km updip from the hypocentre. The weighted average of the rise time and slip velocity are 0.71 s and 0.77 m s –1 , respectively. Our estimate of radiated seismic energy is 1.67 10 13 J, yielding an energy to seismic moment ratio of 0.55 10 –5 , considerably smaller than the global average for shallow earthquakes. The early post-seismic period here represents the time window from 10 s to about first day. During this period, our preferred post-seismic model has an accumulated seismic moment of 6.0 10 17 Nm, equivalent to an M w of 5.8. The afterslip distribution features two separated high slip fault patches, locating either near the hypocentre or the region with peak coseismic slip. Their locations only partially overlap with the fault patches with significant afterslip after the first day. In particular, the slip distribution of the shallower patch is quantitatively consistent with the Amoruso and Crescentini’ diffusive afterslip model. Therefore, the coseismic rupture of the 2009 L'Aquila earthquake was following by a slow-slip event with significant moment release in first hour.
    Keywords: Seismology
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    Iterative joint inversion for stress and fault orientations from focal mechanisms (2014)
    Oxford University Press
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    Publication Date: 2014-07-31
    Description: Stress inversions from focal mechanisms require knowledge of which nodal plane is the fault. If such information is missing, and faults and auxiliary nodal planes are interchanged, the stress inversions can produce inaccurate results. It is shown that the linear inversion method developed by Michael is reasonably accurate when retrieving the principal stress directions even when the selection of fault planes in focal mechanisms is incorrect. However, the shape ratio is more sensitive to the proper choice of the fault and substituting the faults by auxiliary nodal planes introduces significant errors. This difficulty is removed by modifying Michael's method and inverting jointly for stress and for fault orientations. The fault orientations are determined by applying the fault instability constraint and the stress is calculated in iterations. As a by-product, overall friction on faults is determined. Numerical tests show that the new iterative stress inversion is fast and accurate and performs much better than the standard linear inversion. The method is exemplified on real data from central Crete and from the West-Bohemia swarm area of the Czech Republic. The joint iterative inversion identified correctly 36 of 38 faults in the central Crete data. In the West Bohemia data, the faults identified by the inversion were close to the principal fault planes delineated by foci clustering. The overall friction on faults was estimated to be 0.75 and 0.85 for the central Crete and West Bohemia data, respectively.
    Keywords: Seismology
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    Estimation of azimuthal anisotropy in the NW Pacific from seismic ambient noise in seafloor records (2014)
    Oxford University Press
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    Publication Date: 2014-07-31
    Description: We analysed background surface waves in seismic ambient noise by cross-correlating continuous records of eight ocean bottom seismometers and nine differential pressure gauges deployed in the northwestern Pacific Ocean by the PLATE project. After estimating the clock delay and instrumental phase responses of differential pressure gauges by using cross-correlation functions, we measured average phase velocities in the area of the array for the fundamental-, first higher- and second higher-mode Rayleigh waves, and the fundamental-mode Love waves at a period range of 3–40 s by waveform fitting. We then measured phase-velocity anomalies of fundamental-mode and first higher-mode Rayleigh waves for each pair of stations at a period range of 5–25 s, and corrected the effect of variation in water-depths. The seismic anomalies imply the presence of strong azimuthal anisotropy beneath the eastern part of array. The direction of maximum velocity is approximately N35°E in the fossil seafloor spreading direction perpendicular to magnetic lineations from the ancient triple junction at this location. The peak-to-peak intensity of shear-wave velocity anisotropy in the mantle is ~7 per cent.
    Keywords: Seismology
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    A parimutuel gambling perspective to compare probabilistic seismicity forecasts (2014)
    Oxford University Press
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    Publication Date: 2014-07-31
    Description: Using analogies to gaming, we consider the problem of comparing multiple probabilistic seismicity forecasts. To measure relative model performance, we suggest a parimutuel gambling perspective which addresses shortcomings of other methods such as likelihood ratio, information gain and Molchan diagrams. We describe two variants of the parimutuel approach for a set of forecasts: head-to-head, in which forecasts are compared in pairs, and round table, in which all forecasts are compared simultaneously. For illustration, we compare the 5-yr forecasts of the Regional Earthquake Likelihood Models experiment for M 4.95+ seismicity in California.
    Keywords: Seismology
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    Fundamental and higher two-dimensional resonance modes of an Alpine valley (2014)
    Oxford University Press
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    Publication Date: 2014-06-28
    Description: We investigated the sequence of 2-D resonance modes of the sediment fill of Rhône Valley, Southern Swiss Alps, a strongly overdeepened, glacially carved basin with a sediment fill reaching a thickness of up to 900 m. From synchronous array recordings of ambient vibrations at six locations between Martigny and Sion we were able to identify several resonance modes, in particular, previously unmeasured higher modes. Data processing was performed with frequency domain decomposition of the cross-spectral density matrices of the recordings and with time-frequency dependent polarization analysis. 2-D finite element modal analysis was performed to support the interpretation of processing results and to investigate mode shapes at depth. In addition, several models of realistic bedrock geometries and velocity structures could be used to qualitatively assess the sensitivity of mode shape and particle motion dip angle to subsurface properties. The variability of modal characteristics due to subsurface properties makes an interpretation of the modes purely from surface observations challenging. We conclude that while a wealth of information on subsurface structure is contained in the modal characteristics, a careful strategy for their interpretation is needed to retrieve this information.
    Keywords: Seismology
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    Seismogenic magma intrusion before the 2010 eruption of Eyjafjallajokull volcano, Iceland (2014)
    Oxford University Press
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    Publication Date: 2014-06-28
    Description: We present relatively relocated earthquake hypocentres for 〉1000 microearthquakes ( M L 〈 3) that occurred during the 2 weeks immediately prior to the 2010 March 20 fissure eruption at Fimmvörðuháls on the flank of Eyjafjallajökull volcano in Iceland. Our hypocentre locations lie predominantly in horizontally separated clusters spread over an area of 10 km 2 and approximately 4 km below sea level (5 km below the surface). Seismic activity in the final 4 d preceding the eruption extended to shallower levels 〈2 km below sea level and propagated to the surface at the Fimmvörðuháls eruption site on the day the eruption started. We demonstrate using synthetic data that the observed apparent ~1 km vertical elongation of seismic clusters is predominantly an artefact caused by only small errors (0.01–0.02 s) in arrival time data. Where the signal-to-noise ratio was sufficiently good to make subsample arrival time picks by cross-correlation of both P - and S -wave arrivals, the mean depth of 103 events in an individual cluster were constrained to 3.84 ± 0.06 km. Epicentral locations are significantly less vulnerable to arrival time errors than are depths for the seismic monitoring network we used. Within clusters of typically 100 recorded earthquakes, most of the arrivals exhibit similar waveforms and identical patterns of P -wave first-motion polarities across the entire monitoring network. The clusters of similar events comprise repetitive sources in the same location with the same orientations of failure, probably on the same rupture plane. The epicentral clustering and similarity of source mechanisms suggest that much of the seismicity was generated at approximately static constrictions to magma flow in an inflating sill complex. These constrictions may act as a form of valve in the country rock, which ruptures when the melt pressure exceeds a critical level, then reseals after a pulse of melt has passed through. This would generate recurring similar source mechanisms on the same weak fault plane as the connection between segments of the sill system is repeatedly refractured in the same location. We infer that the magmatic intrusion causing most of the seismicity was likely to be a laterally inflating complex of sills at about 4 km depth, with seismogenic pinch-points occurring between aseismic compartments of the sills, or between adjacent magma lobes as they inflated. During the final 4 d preceding the eruption onset between 22:30 and 23:30 UTC on 2010 March 20, the seismicity suggests that melt progressed upwards to a depth of ~2 km. This seismicity was probably caused by fracturing of the country rock at the margins of the propagating dyke. Subsequently, on the morning of the eruption a dyke propagated eastward from the region of precursory seismic activity to the Fimmvörðuháls eruption site.
    Keywords: Seismology
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    Beyond conventional migration: non-linear elastic subsalt imaging with transmissions and two-sided illumination (2014)
    Oxford University Press
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    Publication Date: 2014-06-30
    Description: Source–receiver interferometric imaging can be used to synthesize a subsurface acoustic or elastic image, consisting of a zero-time, zero-offset response (or Green's function) between a colocated pseudo-source and pseudo-receiver placed at each point in the subsurface image. However, if the imaging process does not properly account for multiple reflections, and enclosing boundaries of sources and receivers are not available, the image shows artefacts, poorly illuminated areas and distorted image amplitudes. Here we demonstrate with numerical examples that two-sided non-linear imaging provides the best elastic pure-mode ( PP and SS ) and converted-mode ( PS ) images, having higher resolution and more uniform illumination than those obtained from both one-sided linear imaging and from other intermediate steps of imaging (e.g. non-linear one-sided, linear two-sided). We also propose practical approaches to construct the additional fields required by two-sided non-linear imaging without the need for a detailed velocity model and receivers (and/or sources) in the subsurface. Moreover, when conversions are used for imaging, ‘true-amplitude’ images (here true-amplitude means properly retrieving amplitudes that represent the zero-time, zero-offset elastic response) should theoretically vanish because neither P -to- S or S -to- P conversions arise at zero-time and zero-offset. Applying a correction procedure that accounts for the polarity reversal in PS (or SP ) single-shot images helps with their structural interpretation but results in an unphysical estimate of the subsurface response and uninterpretable amplitudes. This suggests that there are advantages in exploiting pure-mode SS reflections/transmissions, in addition to converted waves only, because they require no polarity correction and the resulting image contains meaningful amplitudes that are proportional to the local shear-wave properties of the medium.
    Keywords: Seismology
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    A smoothed stochastic earthquake rate model considering seismicity and fault moment release for Europe (2014)
    Oxford University Press
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    Publication Date: 2014-06-30
    Description: We present a time-independent gridded earthquake rate forecast for the European region including Turkey. The spatial component of our model is based on kernel density estimation techniques, which we applied to both past earthquake locations and fault moment release on mapped crustal faults and subduction zone interfaces with assigned slip rates. Our forecast relies on the assumption that the locations of past seismicity is a good guide to future seismicity, and that future large-magnitude events occur more likely in the vicinity of known faults. We show that the optimal weighted sum of the corresponding two spatial densities depends on the magnitude range considered. The kernel bandwidths and density weighting function are optimized using retrospective likelihood-based forecast experiments. We computed earthquake activity rates ( a - and b -value) of the truncated Gutenberg–Richter distribution separately for crustal and subduction seismicity based on a maximum likelihood approach that considers the spatial and temporal completeness history of the catalogue. The final annual rate of our forecast is purely driven by the maximum likelihood fit of activity rates to the catalogue data, whereas its spatial component incorporates contributions from both earthquake and fault moment-rate densities. Our model constitutes one branch of the earthquake source model logic tree of the 2013 European seismic hazard model released by the EU-FP7 project ‘Seismic HAzard haRmonization in Europe’ (SHARE) and contributes to the assessment of epistemic uncertainties in earthquake activity rates. We performed retrospective and pseudo-prospective likelihood consistency tests to underline the reliability of our model and SHARE's area source model (ASM) using the testing algorithms applied in the collaboratory for the study of earthquake predictability (CSEP). We comparatively tested our model's forecasting skill against the ASM and find a statistically significant better performance for testing periods of 10–20 yr. The testing results suggest that our model is a viable candidate model to serve for long-term forecasting on timescales of years to decades for the European region.
    Keywords: Seismology
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    Microseismic source imaging in a compressed domain (2014)
    Oxford University Press
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    Publication Date: 2014-06-30
    Description: Microseismic monitoring is an essential tool for the characterization of hydraulic fractures. Fast estimation of the parameters that define a microseismic event is relevant to understand and control fracture development. The amount of data contained in the microseismic records however, poses a challenge for fast continuous detection and evaluation of the microseismic source parameters. Work inspired by the emerging field of Compressive Sensing has showed that it is possible to evaluate source parameters in a compressed domain, thereby reducing processing time. This technique performs well in scenarios where the amplitudes of the signal are above the noise level, as is often the case in microseismic monitoring using downhole tools. This paper extends the idea of the compressed domain processing to scenarios of microseismic monitoring using surface arrays, where the signal amplitudes are commonly at the same level as, or below, the noise amplitudes. To achieve this, we resort to the use of an imaging operator, which has previously been found to produce better results in detection and location of microseismic events from surface arrays. The operator in our method is formed by full-waveform elastodynamic Green's functions that are band-limited by a source time function and represented in the frequency domain. Where full-waveform Green's functions are not available, ray tracing can also be used to compute the required Green's functions. Additionally, we introduce the concept of the compressed inverse, which derives directly from the compression of the migration operator using a random matrix. The described methodology reduces processing time at a cost of introducing distortions into the results. However, the amount of distortion can be managed by controlling the level of compression applied to the operator. Numerical experiments using synthetic and real data demonstrate the reductions in processing time that can be achieved and exemplify the process of selecting the compression rate that produces a tolerable amount of distortion into the results.
    Keywords: Seismology
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    Optimal observables for multiparameter seismic tomography (2014)
    Oxford University Press
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    Publication Date: 2014-06-30
    Description: We propose a method for the design of seismic observables with maximum sensitivity to a target model parameter class, and minimum sensitivity to all remaining parameter classes. The resulting optimal observables thereby minimize interparameter trade-offs in multiparameter inverse problems. Our method is based on the linear combination of fundamental observables that can be any scalar measurement extracted from seismic waveforms. Optimal weights of the fundamental observables are determined with an efficient global search algorithm. While most optimal design methods assume variable source and/or receiver positions, our method has the flexibility to operate with a fixed source–receiver geometry, making it particularly attractive in studies where the mobility of sources and receivers is limited. In a series of examples we illustrate the construction of optimal observables, and assess the potentials and limitations of the method. The combination of Rayleigh-wave traveltimes in four frequency bands yields an observable with strongly enhanced sensitivity to 3-D density structure. Simultaneously, sensitivity to S velocity is reduced, and sensitivity to P velocity is eliminated. The original three-parameter problem thereby collapses into a simpler two-parameter problem with one dominant parameter. By defining parameter classes to equal earth model properties within specific regions, our approach mimics the Backus–Gilbert method where data are combined to focus sensitivity in a target region. This concept is illustrated using rotational ground motion measurements as fundamental observables. Forcing dominant sensitivity in the near-receiver region produces an observable that is insensitive to the Earth structure at more than a few wavelengths’ distance from the receiver. This observable may be used for local tomography with teleseismic data. While our test examples use a small number of well-understood fundamental observables, few parameter classes and a radially symmetric earth model, the method itself does not impose such restrictions. It can easily be applied to large numbers of fundamental observables and parameters classes, as well as to 3-D heterogeneous earth models.
    Keywords: Seismology
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    Slowness estimation from noisy time delays observed on non-planar arrays (2014)
    Oxford University Press
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    Publication Date: 2014-06-30
    Description: To estimate the slowness vector of infrasound waves propagating across an infrasound array, it is often considered that the sensors are located in a perfectly horizontal plane (2-D approximation). However, the arrays are not planar and differences between sensor altitudes cannot be neglected without introducing biases when estimating the parameters of interest (backazimuth and incidence). A closed-formula of this error depending on the geometry of the array and the wave incidence is presented. Since the unbiased 3-D estimation of the slowness parameters results in a significant alteration of the variance, a metric based on the mean square error is proposed. We found it useful to compare the 3-D to the 2-D processed results. We show the 3-D estimator does not always give the best estimates. We propose also a formulation of the boundary that allow us to choose between these two estimators depending on the situation (arrays, incidence and signal-to-noise ratios). Finally, we compare these two approaches (2-D/3-D) to all International Monitoring System arrays with synthetic data, and perform a comparative estimation of backazimuth for real data.
    Keywords: Seismology
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    Dynamic earthquake rupture modelled with an unstructured 3-D spectral element method applied to the 2011 M9 Tohoku earthquake (2014)
    Oxford University Press
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    Publication Date: 2014-06-30
    Description: An important goal of computational seismology is to simulate dynamic earthquake rupture and strong ground motion in realistic models that include crustal heterogeneities and complex fault geometries. To accomplish this, we incorporate dynamic rupture modelling capabilities in a spectral element solver on unstructured meshes, the 3-D open source code SPECFEM3D, and employ state-of-the-art software for the generation of unstructured meshes of hexahedral elements. These tools provide high flexibility in representing fault systems with complex geometries, including faults with branches and non-planar faults. The domain size is extended with progressive mesh coarsening to maintain an accurate resolution of the static field. Our implementation of dynamic rupture does not affect the parallel scalability of the code. We verify our implementation by comparing our results to those of two finite element codes on benchmark problems including branched faults. Finally, we present a preliminary dynamic rupture model of the 2011 M w 9.0 Tohoku earthquake including a non-planar plate interface with heterogeneous frictional properties and initial stresses. Our simulation reproduces qualitatively the depth-dependent frequency content of the source and the large slip close to the trench observed for this earthquake.
    Keywords: Seismology
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    Line-source simulation for shallow-seismic data. Part 2: full-waveform inversion--a synthetic 2-D case study (2014)
    Oxford University Press
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    Publication Date: 2014-07-10
    Description: Full-waveform inversion (FWI) of shallow-seismic surface waves is able to reconstruct lateral variations of subsurface elastic properties. Line-source simulation for point-source data is required when applying algorithms of 2-D adjoint FWI to recorded shallow-seismic field data. The equivalent line-source response for point-source data can be obtained by convolving the waveforms with $\sqrt{t^{-1}}$ ( t : traveltime), which produces a phase shift of /4. Subsequently an amplitude correction must be applied. In this work we recommend to scale the seismograms with $\sqrt{2 r v_{\rm ph}}$ at small receiver offsets r , where v ph is the phase velocity, and gradually shift to applying a $\sqrt{t^{-1}}$ time-domain taper and scaling the waveforms with $r\sqrt{2}$ for larger receiver offsets r . We call this the hybrid transformation which is adapted for direct body and Rayleigh waves and demonstrate its outstanding performance on a 2-D heterogeneous structure. The fit of the phases as well as the amplitudes for all shot locations and components (vertical and radial) is excellent with respect to the reference line-source data. An approach for 1-D media based on Fourier–Bessel integral transformation generates strong artefacts for waves produced by 2-D structures. The theoretical background for both approaches is presented in a companion contribution. In the current contribution we study their performance when applied to waves propagating in a significantly 2-D-heterogeneous structure. We calculate synthetic seismograms for 2-D structure for line sources as well as point sources. Line-source simulations obtained from the point-source seismograms through different approaches are then compared to the corresponding line-source reference waveforms. Although being derived by approximation the hybrid transformation performs excellently except for explicitly back-scattered waves. In reconstruction tests we further invert point-source synthetic seismograms by a 2-D FWI to subsurface structure and evaluate its ability to reproduce the original structural model in comparison to the inversion of line-source synthetic data. Even when applying no explicit correction to the point-source waveforms prior to inversion only moderate artefacts appear in the results. However, the overall performance is best in terms of model reproduction and ability to reproduce the original data in a 3-D simulation if inverted waveforms are obtained by the hybrid transformation.
    Keywords: Seismology
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    Bayesian historical earthquake relocation: an example from the 1909 Taipei earthquake (2014)
    Oxford University Press
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    Publication Date: 2014-07-10
    Description: Locating earthquakes from the beginning of the modern instrumental period is complicated by the fact that there are few good-quality seismograms and what traveltimes do exist may be corrupted by both large phase-pick errors and clock errors. Here, we outline a Bayesian approach to simultaneous inference of not only the hypocentre location but also the clock errors at each station and the origin time of the earthquake. This methodology improves the solution for the source location and also provides an uncertainty analysis on all of the parameters included in the inversion. As an example, we applied this Bayesian approach to the well-studied 1909 M w 7 Taipei earthquake. While our epicentre location and origin time for the 1909 Taipei earthquake are consistent with earlier studies, our focal depth is significantly shallower suggesting a higher seismic hazard to the populous Taipei metropolitan area than previously supposed.
    Keywords: Seismology
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    Multiple-frequency tomography of the upper mantle beneath the African/Iberian collision zone (2014)
    Oxford University Press
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    Publication Date: 2014-07-10
    Description: During the Cenozoic, the geodynamics of the western Mediterranean domain has been characterized by a complex history of subduction of Mesozoic oceanic lithosphere. The final stage of these processes is proposed to have led to the development of the Calabria and Gibraltar arcs, whose formation is still under debate. In this study, we take advantage of the dense broad-band station networks now available in the Alborán Sea region, to develop a high-resolution 3-D tomographic P velocity model of the upper mantle beneath the African/Iberian collision zone that will better constraint the past dynamics of this zone. The model is based on 13200 teleseismic arrival times recorded between 2008 and 2012 at 279 stations for which cross-correlation delays are measured with a new technique in different frequency bands centred between 0.03 and 1.0 Hz, and for the first time interpreted using multiple frequency tomography. Our model shows, beneath the Alborán Sea, a strong (4 per cent) fast vertically dipping anomaly observed to at least 650 km depth. The arched shape of this anomaly, and its extent at depth, are coherent with a lithospheric slab, thus favouring the hypothesis of a westward consumption of the Ligurian ocean slab by roll-back during Cenozoic. In addition to this fast anomaly in the deep upper mantle, high intensity slow anomalies are widespread in the lithosphere and asthenosphere beneath Morocco and southern Spain. These anomalies are correlated at the surface with the position of the Rif and Atlas orogens and with Cenozoic volcanic fields. We thus confirm the presence, beneath Morocco, of an anomalous (hot?) upper mantle, but without clear indication for a lateral spreading of the Canary plume to the east.
    Keywords: Seismology
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    Crustal and upper-mantle structure beneath the western Atlas Mountains in SW Morocco derived from receiver functions (2014)
    Oxford University Press
    Details
    Publication Date: 2014-07-10
    Description: The High Atlas and the Anti Atlas are fold-belts linked to former and still ongoing continent–continent collisions. Despite their high elevation, studies indicate a lack of a deep crustal root (〈40 km) while the lithosphere underneath is thinned (〈100 km). Previous explanations for this thinning include asthenospheric upwelling due to small-scale convection or a small plume. We use data recorded at stations in SW Morocco to analyse teleseismic P - and S -wave receiver functions. Our study yields a crustal thickness ranging from 24 km near the Atlantic coast to 44 km beneath the High Atlas with an average crustal V p / V s ratio of 1.77 in the entire region. A crustal thickness of 40 km cannot entirely support the topography in this region. Furthermore, we find the lithosphere–asthenosphere boundary at ~80 km depth. The lithosphere beneath SW Morocco is thinner than beneath northern Morocco (〉150 km). This lithospheric thinning supports the theory of thermal compensation of the mountain ranges. The mantle transition zone thickness amounts to 240 ± 10 km. The transition zone seems to be slightly thinned which might indicate a higher mantle temperature in this region.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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    Estimation of shear velocity contrast from transmitted Ps amplitude variation with ray-parameter (2014)
    Oxford University Press
    Details
    Publication Date: 2014-07-10
    Description: Amplitude versus offset of P to P reflection is commonly used by the exploration seismology community for hydrocarbon exploration. In this paper, we investigate the feasibility to estimate crustal velocity structure from transmitted P- to S -wave amplitude variation with ray-parameter. First the transmission coefficient for the plane P wave converting to S wave ( P - to - S ) is approximated and expressed as a function of slowness. The resulting linear relation involves two coefficients (intercept, X and gradient, Y), which are functions of velocities and densities. Due to the stable nature of X and the fact that P - to - S amplitudes are weakly dependent on the density contrast, we use this parameter next to estimate the shear wave velocity contrast across an interface using the forward scattered P - to - S amplitude versus slowness data. We report on the effectiveness of the approach using various synthetics data sets. The present methodology is also tested on real data sets from two broad-band seismic stations from HYB and COR.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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    CyberShake-derived ground-motion prediction models for the Los Angeles region with application to earthquake early warning (2014)
    Oxford University Press
    Details
    Publication Date: 2014-07-10
    Description: Real-time applications such as earthquake early warning (EEW) typically use empirical ground-motion prediction equations (GMPEs) along with event magnitude and source-to-site distances to estimate expected shaking levels. In this simplified approach, effects due to finite-fault geometry, directivity and site and basin response are often generalized, which may lead to a significant under- or overestimation of shaking from large earthquakes ( M  〉 6.5) in some locations. For enhanced site-specific ground-motion predictions considering 3-D wave-propagation effects, we develop support vector regression (SVR) models from the SCEC CyberShake low-frequency (〈0.5 Hz) and broad-band (0–10 Hz) data sets. CyberShake encompasses 3-D wave-propagation simulations of 〉415 000 finite-fault rupture scenarios (6.5 ≤ M ≤ 8.5) for southern California defined in UCERF 2.0. We use CyberShake to demonstrate the application of synthetic waveform data to EEW as a ‘proof of concept’, being aware that these simulations are not yet fully validated and might not appropriately sample the range of rupture uncertainty. Our regression models predict the maximum and the temporal evolution of instrumental intensity (MMI) at 71 selected test sites using only the hypocentre, magnitude and rupture ratio, which characterizes uni- and bilateral rupture propagation. Our regression approach is completely data-driven (where here the CyberShake simulations are considered data) and does not enforce pre-defined functional forms or dependencies among input parameters. The models were established from a subset (~20 per cent) of CyberShake simulations, but can explain MMI values of all 〉400 k rupture scenarios with a standard deviation of about 0.4 intensity units. We apply our models to determine threshold magnitudes (and warning times) for various active faults in southern California that earthquakes need to exceed to cause at least ‘moderate’, ‘strong’ or ‘very strong’ shaking in the Los Angeles (LA) basin. These thresholds are used to construct a simple and robust EEW algorithm: to declare a warning, the algorithm only needs to locate the earthquake and to verify that the corresponding magnitude threshold is exceeded. The models predict that a relatively moderate M 6.5–7 earthquake along the Palos Verdes, Newport-Inglewood/Rose Canyon, Elsinore or San Jacinto faults with a rupture propagating towards LA could cause ‘very strong’ to ‘severe’ shaking in the LA basin; however, warning times for these events could exceed 30 s.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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    Line-source simulation for shallow-seismic data. Part 1: theoretical background (2014)
    Oxford University Press
    Details
    Publication Date: 2014-07-10
    Description: Equivalent line-source seismograms can be obtained from shallow seismic field recordings by (1) convolving the waveforms with $\sqrt{t^{-1}}$ , (2) applying a $\sqrt{t^{-1}}$ time-domain taper, where t is traveltime and (3) scaling the waveform with $r_{{\rm offset}}\sqrt{2}$ , where r offset is source-to-receiver offset. We require such a procedure when applying algorithms of 2-D adjoint full-waveform inversion (FWI) to shallow-seismic data. Although derived from solutions for acoustic waves in homogeneous full space this simple procedure performs surprisingly well when applied to vertical and radial components of shallow-seismic recordings from hammer blows or explosions. This is the case even in the near field of the force, although the procedure is derived from a far-field approximation. Similar approximative procedures recommended in literature are optimized for reflected waves and do not convert the amplitudes of all shallow seismic wavefield constituents equally well. We demonstrate the suitability of the proposed method for the viscoelastic case by numerical examples as well as analytical considerations. In contrast to the proposed single-trace procedure, integral-transform approaches are exact for all viscoelastic wavefield constituents of the near- and far-field in unknown 1-D-heterogeneous structure. Unfortunately, integral-transform approaches suffer from artefacts in applications to data sampled on 2-D structures. Here, we use the Fourier–Bessel integral transformation as a reference in 1-D heterogeneous structures. We unroll the wave-theoretical background of both approaches in order to demonstrate, why the simplistic single-trace simulation approach derived from the asymptotic acoustic case can perform so well when applied to the shallow elastic wavefield. Further we give recommendations for practical implementation and application to field data of the proposed simulation method and compare to the results of alternative conversion rules. The performance of the conversion procedure to data recorded on 2-D heterogeneous structures is presented in a companion study by FWI reconstruction tests.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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    On the initiation of sustained slip-weakening ruptures by localized stresses (2014)
    Oxford University Press
    Details
    Publication Date: 2014-12-19
    Description: Numerical simulations of dynamic earthquake rupture require an artificial initiation procedure, if they are not integrated in long-term earthquake cycle simulations. A widely applied procedure involves an ‘overstressed asperity’, a localized region stressed beyond the static frictional strength. The physical properties of the asperity (size, shape and overstress) may significantly impact rupture propagation. In particular, to induce a sustained rupture the asperity size needs to exceed a critical value. Although criteria for estimating the critical nucleation size under linear slip-weakening friction have been proposed for 2-D and 3-D problems based on simplifying assumptions, they do not provide general rules for designing 3-D numerical simulations. We conduct a parametric study to estimate parameters of the asperity that minimize numerical artefacts (e.g. changes of rupture shape and speed, artificial supershear transition, higher slip-rate amplitudes). We examine the critical size of square, circular and elliptical asperities as a function of asperity overstress and background (off-asperity) stress. For a given overstress, we find that asperity area controls rupture initiation while asperity shape is of lesser importance. The critical area obtained from our numerical results contrasts with published theoretical estimates when background stress is low. Therefore, we derive two new theoretical estimates of the critical size under low background stress while also accounting for overstress. Our numerical results suggest that setting the asperity overstress and area close to their critical values eliminates strong numerical artefacts even when the overstress is large. We also find that properly chosen asperity size or overstress may significantly shorten the duration of the initiation. Overall, our results provide guidelines for determining the size of the asperity and overstress to minimize the effects of the forced initiation on the subsequent spontaneous rupture propagation.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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    Seismological versus geodetic reference frames for seismic dislocation: consistency under momentum conservations (2014)
    Oxford University Press
    Details
    Publication Date: 2014-12-23
    Description: We raise attention to the issue of consistency between the reference frame with respect to which the seismological model calculations of displacement are made on one hand, and that to which the geodetic measurements of crustal deformation refer (e.g. the ITRF) on the other. This issue is critical in principle if the seismologically calculated displacement (or gravity change) is to be compared or used in joint inversion with geodetic measurements. A necessary set of conditions to be satisfied by inertial frames is the conservations of linear and angular momentums: no net change in them can be induced by a seismic source indigenous to the Earth. We show that the momentums are embodied in the degree-1 terms of the vector spherical-harmonic expansion of the displacement field. Using three largest recent earthquakes as case examples we find that the algorithms of seismological dislocation modelling in the literature do not conserve the momentums. However, quantitatively this inconsistency amounts to two orders of magnitude smaller than the current precision in the definition of the ITRF, hence insignificant in practice. Some caveats are raised.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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    Capillary Flows Along Open Channel Conduits: The Open-Star Section (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-19
    Description: Capillary rise in tubes, channels, and grooves has received significant attention in the literature for over 100 years. In yet another incremental extension of such work, a transient capillary rise problem is solved for spontaneous flow along an interconnected array of open channels forming what is referred to as an 'open-star' section. This geometry possesses several attractive characteristics including passive phase separations and high diffusive gas transport. Despite the complex geometry, novel and convenient approximations for capillary pressure and viscous resistance enable closed form predictions of the flow. As part of the solution, a combined scaling approach is applied that identifies unsteady-inertial-capillary, convective-inertial-capillary, and visco-capillary transient regimes in a single parameter. Drop tower experiments are performed employing 3-D printed conduits to corroborate all findings.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: JSC-CN-31794 , Annual Meeting of the American Physical Society Division of Fluid Dynamics; Nov 23, 2014 - Nov 25, 2014; San Francisco, CA; United States
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    Long Hole Film Cooling Dataset for CFD Development - Flow and Film Effectiveness (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: An experiment investigating flow and heat transfer of long (length to diameter ratio of 18) cylindrical film cooling holes has been completed. In this paper, the thermal field in the flow and on the surface of the film cooled flat plate is presented for nominal freestream turbulence intensities of 1.5 and 8 percent. The holes are inclined at 30 deg above the downstream direction, injecting chilled air of density ratio 1.0 onto the surface of a flat plate. The diameter of the hole is 0.75 in. (approx. 0.02 m) with center to center spacing (pitch) of 3 hole diameters. Coolant was injected into the mainstream flow at nominal blowing ratios of 0.5, 1.0, 1.5, and 2.0. The Reynolds number of the freestream was approximately 11,000 based on hole diameter. Thermocouple surveys were used to characterize the thermal field. Infrared thermography was used to determine the adiabatic film effectiveness on the plate. Hotwire anemometry was used to provide flowfield physics and turbulence measurements. The results are compared to existing data in the literature. The aim of this work is to produce a benchmark dataset for Computational Fluid Dynamics (CFD) development to eliminate the effects of hole length to diameter ratio and to improve resolution in the near-hole region. In this report, a Time Filtered Navier Stokes (TFNS), also known as Partially Resolved Navier Stokes (PRNS), method that was implemented in the Glenn-HT code is used to model coolant-mainstream interaction. This method is a high fidelity unsteady method that aims to represent large scale flow features and mixing more accurately.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: E-662718 , AIAA/ASME/SAE/ASEE Joint Propulsion Conference; Jul 28, 2014 - Jul 30, 2014; Cleveland, OH; United States
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    Simulations of NOx Emissions from Low Emissions Discrete Jet Injector Combustor Tests (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: An experimental and computational study was conducted to evaluate the performance and emissions characteristics of a candidate Lean Direct Injection (LDI) combustor configuration with a mix of simplex and airblast injectors. The National Combustion Code (NCC) was used to predict the experimentally measured EINOx emissions for test conditions representing low power, medium power, and high-power engine cycle conditions. Of the six cases modeled with the NCC using a reduced-kinetics finite-rate mechanism and lagrangian spray modeling, reasonable predictions of combustor exit temperature and EINOx were obtained at two high-power cycle conditions.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: AIAA Propulsion and Energy Forum 2014; Jul 28, 2014 - Jul 30, 2014; Cleveland, OH; United States
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    Long Hole Film Cooling Dataset for CFD Development - Flow and Film Effectiveness (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: No abstract available
    Keywords: Fluid Mechanics and Thermodynamics
    Type: E-662711 , AIAA/ASME/SAE/ASEE Joint Propulsion Conference; Jul 28, 2014 - Jul 30, 2014; Cleveland, OH; United States
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    Applying Flammability Limit Probabilities and the Normoxic Upward Limiting Pressure Concept to NASA STD-6001 Test 1 (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Repeated Test 1 extinction tests near the upward flammability limit are expected to follow a Poisson process trend. This Poisson process trend suggests that rather than define a ULOI and MOC (which requires two limits to be determined), it might be better to define a single upward limit as being where 1/e (where e (approx. equal to 2.7183) is the characteristic time of the normalized Poisson process) of the materials burn, or, rounding, where approximately 1/3 of the samples fail the test (and burn). Recognizing that spacecraft atmospheres will not bound the entire oxygen-pressure parameter space, but actually lie along the normoxic atmosphere control band, we can focus the materials flammability testing along this normoxic band. A Normoxic Upward Limiting Pressure (NULP) is defined that determines the minimum safe total pressure for a material within the constant partial pressure control band. Then, increasing this pressure limit by a factor of safety, we can define the material as being safe to use at the NULP + SF (where SF is on the order of 10 kilopascal, based on existing flammability data). It is recommended that the thickest material to be tested with the current Test 1 igniter should be 3 mm thick (1/8 inches) to avoid the problem of differentiating between an ignition limit and a true flammability limit.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: ICES-2014-179 , GRC-E-DAA-TN13759 , International Conference on Environmental Systems (ICES 2014); Jul 13, 2014 - Jul 17, 2014; Tuscon, AZ; United States
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    Potential Follow on Experiments for the Zero Boil Off Tank Experiment (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Cryogenic Storage &Transfer are enabling propulsion technologies in the direct path of nearly all future human or robotic missions; It is identified by NASA as an area with greatest potential for cost saving; This proposal aims at resolving fundamental scientific issues behind the engineering development of the storage tanks; We propose to use the ISS lab to generate & collect archival scientific data:, raise our current state-of-the-art understanding of transport and phase change issues affecting the storage tank cryogenic fluid management (CFM), develop and validate state-of-the-art CFD models to innovate, optimize, and advance the future engineering designs
    Keywords: Fluid Mechanics and Thermodynamics
    Type: GRC-E-DAA-TN18539 , American Society for Gravitational and Space Research (ASGSR) Annual Meeting; Oct 22, 2014 - Oct 26, 2014; Pasadena, CA; United States
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    On Characterizing Particle Shape (2014)
    In:  CASI
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    Publication Date: 2019-07-13
    Description: It is well known that particle shape affects flow characteristics of granular materials, as well as a variety of other solids processing issues such as compaction, rheology, filtration and other two-phase flow problems. The impact of shape crosses many diverse and commercially important applications, including pharmaceuticals, civil engineering, metallurgy, health, and food processing. Two applications studied here include the dry solids flow of lunar simulants (e.g. JSC-1, NU-LHT-2M, OB-1), and the flow properties of wet concrete, including final compressive strength. A multi-dimensional generalized, engineering method to quantitatively characterize particle shapes has been developed, applicable to both single particle orientation and multi-particle assemblies. The two-dimension, three dimension inversion problem is also treated, and the application of these methods to DEM model particles will be discussed. In the case of lunar simulants, flow properties of six lunar simulants have been measured, and the impact of particle shape on flowability - as characterized by the shape method developed here -- is discussed, especially in the context of three simulants of similar size range. In the context of concrete processing, concrete construction is a major contributor to greenhouse gas production, of which the major contributor is cement binding loading. Any optimization in concrete rheology and packing that can reduce cement loading and improve strength loading can also reduce currently required construction safety factors. The characterization approach here is also demonstrated for the impact of rock aggregate shape on concrete slump rheology and dry compressive strength.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: M14-3774 , AIChE (American Institute of Chemical Engineers) Annual Meeting; Nov 16, 2014 - Nov 21, 2014; Atlanta, GA; United States
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    Temperature Control with Two Parallel Small Loop Heat Pipes for GLM Program (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The concept of temperature control of an electronic component using a single Loop Heat Pipe (LHP) is well established for Aerospace applications. Using two LHPs is often desirable for redundancy/reliability reasons or for increasing the overall heat source-sink thermal conductance. This effort elaborates on temperature controlling operation of a thermal system that includes two small ammonia LHPs thermally coupled together at the evaporator end as well as at the condenser end and operating "in parallel". A transient model of the LHP system was developed on the Thermal Desktop (TradeMark) platform to understand some fundamental details of such parallel operation of the two LHPs. Extensive thermal-vacuum testing was conducted with two thermally coupled LHPs operating simultaneously as well as with only one LHP operating at a time. This paper outlines the temperature control procedures for two LHPs operating simultaneously with widely varying sink temperatures. The test data obtained during the thermal-vacuum testing, with both LHPs running simultaneously in comparison with only one LHP operating at a time, are presented with detailed explanations.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: GSFC-E-DAA-TN19620 , Frontiers in Heat Pipes (FHP) ; 5; 9; 013009
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    CVB: the Constrained Vapor Bubble Capillary Experiment on the International Space Station MARANGONI FLOW REGION (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The Constrained Vapor Bubble (CVB) is a wickless, grooved heat pipe and we report on a full- scale fluids experiment flown on the International Space Station (ISS). The CVB system consists of a relatively simple setup a quartz cuvette with sharp corners partially filled with either pentane or an ideal mixture of pentane and isohexane as the working fluids. Along with temperature and pressure measurements, the two-dimensional thickness profile of the menisci formed at the corners of the quartz cuvette was determined using the Light Microscopy Module (LMM). Even with the large, millimeter dimensions of the CVB, interfacial forces dominate in these exceedingly small Bond Number systems. The experiments were carried out at various power inputs. Although conceptually simple, the transport processes were found to be very complex with many different regions. At the heated end of the CVB, due to a high temperature gradient, we observed Marangoni flow at some power inputs. This region from the heated end to the central drop region is defined as a Marangoni dominated region. We present a simple analysis based on interfacial phenomena using only measurements from the ISS experiments that lead to a predictive equation for the thickness of the film near the heated end of the CVB. The average pressure gradient for flow in the film is assumed due to the measured capillary pressure at the two ends of the liquid film and that the pressure stress gradient due to cohesion self adjusts to a constant value over a distance L. The boundary conditions are the no slip condition at the wall interface and an interfacial shear stress at the liquid- vapor interface due to the Marangoni stress, which is due to the high temperature gradient. Although the heated end is extremely complex, since it includes three- dimensional variations in radiation, conduction, evaporation, condensation, fluid flow and interfacial forces, we find that using the above simplifying assumptions, a simple successful model can be developed.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: GRC-E-DAA-TN19372 , American Society for Gravitational and Space Research Annual Meeting 2014; Oct 22, 2014 - Oct 26, 2014; Pasadena, CA; United States
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    Simulation of Fluid Flow and Collection Efficiency for an SEA Multi-element Probe (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Numerical simulations of fluid flow and collection efficiency for a Science Engineering Associates (SEA) multi-element probe are presented. Simulation of the flow field was produced using the Glenn-HT Navier-Stokes solver. Three-dimensional unsteady results were produced and then time averaged for the heat transfer and collection efficiency results. Three grid densities were investigated to enable an assessment of grid dependence. Simulations were completed for free stream velocities ranging from 85-135 meters per second, and free stream total pressure of 44.8 and 93.1 kilopascals (6.5 and 13.5 pounds per square inch absolute). In addition, the effect of angle of attack and yaw were investigated by including 5 degree deviations from straight for one of the flow conditions. All but one of the cases simulated a probe in isolation (i.e. in a very large domain without any support strut). One case is included which represents a probe mounted on a support strut within a finite sized wind tunnel. Collection efficiencies were generated, using the LEWICE3D code, for four spherical particle sizes, 100, 50, 20, and 5 micron in diameter. It was observed that a reduction in velocity of about 20% occurred, for all cases, as the flow entered the shroud of the probe. The reduction in velocity within the shroud is not indicative of any error in the probe measurement accuracy. Heat transfer results are presented which agree quite well with a correlation for the circular cross section heated elements. Collection efficiency results indicate a reduction in collection efficiency as particle size is reduced. The reduction with particle size is expected, however, the results tended to be lower than the previous results generated for isolated two-dimensional elements. The deviation from the two-dimensional results is more pronounced for the smaller particles and is likely due to the reduced flow within the protective shroud. As particle size increases differences between the two-dimensional and three dimensional results become negligible. Taken as a group, the total collection efficiency of the elements including the effects of the shroud has been shown to be in the range of 0.93 to 0.99 for particles above 20 microns. The 3D model has improved the estimated collection efficiency for smaller particles where errors in previous estimates were more significant.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: GRC-E-DAA-TN15066 , AIAA Aviation and Aeronautics Forum and Exposition (Aviation 2014); Jun 16, 2014 - Jun 20, 2014; Atlanta, GA; United States
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    A Thermal Analysis of a Hot-Wire Probe for Icing Applications (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: This paper presents a steady-state thermal model of a hot-wire instrument applicable to atmospheric measurement of water content in clouds. In this application, the power required to maintain the wire at a given temperature is used to deduce the water content of the cloud. The model considers electrical resistive heating, axial conduction, convection to the flow, radiation to the surroundings, as well as energy loss due to the heating, melting, and evaporation of impinging liquid and or ice. All of these parameters can be varied axially along the wire. The model further introduces a parameter called the evaporation potential which locally gauges the maximum fraction of incoming water that evaporates. The primary outputs of the model are the steady-state power required to maintain a spatially-average constant temperature as well as the variation of that temperature and other parameters along the wire. The model is used to understand the sensitivity of the hot-wire performance to various flow and boundary conditions including a detailed comparison of dry air and wet (i.e. cloud-on) conditions. The steady-state power values are compared to experimental results from a Science Engineering Associates (SEA) Multi-Element probe, a commonly used water-content measurement instrument. The model results show good agreement with experiment for both dry and cloud-on conditions with liquid water content. For ice, the experimental measurements under read the actual water content due to incomplete evaporation and splashing. Model results, which account for incomplete evaporation, are still higher than experimental results where the discrepancy is attributed to splashing mass-loss which is not accounted in the model.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: GRC-E-DAA-TN15563 , (AVIATION 2014) AIAA Aviation and Aeronautics Forum and Exposition; Jun 16, 2014 - Jun 20, 2014; Atlanta, GA; United States
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    Propulsion System Simulation Using the Toolbox for the Modeling and Analysis of Thermodynamic System (T-MATS) (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A simulation toolbox has been developed for the creation of both steady-state and dynamic thermodynamic software models. This presentation describes the Toolbox for the Modeling and Analysis of Thermodynamic Systems (T-MATS), which combines generic thermodynamic and controls modeling libraries with a numerical iterative solver to create a framework for the development of thermodynamic system simulations, such as gas turbine engines. The objective of this presentation is to present an overview of T-MATS, the theory used in the creation of the module sets, and a possible propulsion simulation architecture.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: GRC-E-DAA-TN16854 , AIAA/ASME/SAE/ASEE Joint Propulsion Conference; Jul 28, 2014 - Jul 30, 2014; Cleveland, OH; United States
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    Subsonic Round and Rectangular Twin Jet Flow Effects (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Subsonic and supersonic aircraft concepts proposed by NASAs Fundamental Aeronautics Program have integrated propulsion systems with asymmetric nozzles. The asymmetry in the exhaust of these propulsion systems creates asymmetric flow and acoustic fields. The flow asymmetries investigated in the current study are from two parallel round, 2:1, and 8:1 aspect ratio rectangular jets at the same nozzle conditions. The flow field was measured with streamwise and cross-stream particle image velocimetry (PIV). A large dataset of single and twin jet flow field measurements was acquired at subsonic jet conditions. The effects of twin jet spacing and forward flight were investigated. For round, 2:1, and 8:1 rectangular twin jets at their closest spacings, turbulence levels between the two jets decreased due to enhanced jet mixing at near static conditions. When the flight Mach number was increased to 0.25, the flow around the twin jet model created a velocity deficit between the two nozzles. This velocity deficit diminished the effect of forward flight causing an increase in turbulent kinetic energy relative to a single jet. Both of these twin jet flow field effects decreased with increasing twin jet spacing relative to a single jet. These variations in turbulent kinetic energy correlate with changes in far-field sound pressure level.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: GRC-E-DAA-TN16376 , AIAA/ASME/SAE/ASEE Joint Propulsion Conference; Jul 28, 2014 - Jul 30, 2014; Cleveland, OH; United States
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    Method of Minimizing Size of Heat Rejection Systems for Thermoelectric Coolers to Cool Detectors in Space (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A thermal design concept of attaching the thermoelectric cooler (TEC) hot side directly to the radiator and maximizing the number of TECs to cool multiple detectors in space is presented. It minimizes the temperature drop between the TECs and radiator. An ethane constant conductance heat pipe transfers heat from the detectors to a TEC cold plate which the cold side of the TECs is attached to. This thermal design concept minimizes the size of TEC heat rejection systems. Hence it reduces the problem of accommodating the radiator within a required envelope. It also reduces the mass of the TEC heat rejection system. Thermal testing of a demonstration unit in vacuum verified the thermal performance of the thermal design concept.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: GSFC-E-DAA-TN15640 , AIAA/ASME/SAE/ASEE Joint Propulsion Conference; Jul 28, 2014 - Jul 30, 2014; Cleveland, OH; United States|International Energy Conversion Engineering Conference; Jul 28, 2014 - Jul 30, 2014; Cleveland, OH; United States
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  • 170
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    Combustion-Powered Actuation for Dynamic Stall Suppression - Simulations and Low-Mach Experiments (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: An investigation on dynamic-stall suppression capabilities of combustion-powered actuation (COMPACT) applied to a tabbed VR-12 airfoil is presented. In the first section, results from computational fluid dynamics (CFD) simulations carried out at Mach numbers from 0.3 to 0.5 are presented. Several geometric parameters are varied including the slot chordwise location and angle. Actuation pulse amplitude, frequency, and timing are also varied. The simulations suggest that cycle-averaged lift increases of approximately 4% and 8% with respect to the baseline airfoil are possible at Mach numbers of 0.4 and 0.3 for deep and near-deep dynamic-stall conditions. In the second section, static-stall results from low-speed wind-tunnel experiments are presented. Low-speed experiments and high-speed CFD suggest that slots oriented tangential to the airfoil surface produce stronger benefits than slots oriented normal to the chordline. Low-speed experiments confirm that chordwise slot locations suitable for Mach 0.3-0.4 stall suppression (based on CFD) will also be effective at lower Mach numbers.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: NF1676L-18571 , American Helicopter Society (AHS) Annual Forum; May 20, 2014 - May 22, 2014; Montreal; Canada
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    An Experimental Study of Roughness-Induced Instabilities in a Supersonic Boundary Layer (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Progress on an experimental study of laminar-to-turbulent transition induced by an isolated roughness element in a supersonic laminar boundary layer is reported in this paper. Here, the primary focus is on the effects of roughness planform shape on the instability and transition characteristics. Four different roughness planform shapes were considered (a diamond, a circle, a right triangle, and a 45 degree fence) and the height and width of each one was held fixed so that a consistent frontal area was presented to the oncoming boundary layer. The nominal roughness Reynolds number was 462 and the ratio of the roughness height to the boundary layer thickness was 0.48. Detailed flow- field surveys in the wake of each geometry were performed via hot-wire anemometry. High- and low-speed streaks were observed in the wake of each roughness geometry, and the modified mean flow associated with these streak structures was found to support a single dominant convective instability mode. For the symmetric planform shapes - the diamond and circular planforms - the instability characteristics (mode shapes, growth rates, and frequencies) were found to be similar. For the asymmetric planform shapes - the right-triangle and 45 degree fence planforms - the mode shapes were asymmetrically distributed about the roughness-wake centerline. The instability growth rates for the asymmetric planforms were lower than those for the symmetric planforms and therefore, transition onset was delayed relative to the symmetric planforms.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: AIAA Paper 2014-2501 , NF1676L-17666 , AIAA Fluid Dynamics Conference; Jun 16, 2014 - Jun 20, 2014; Atlanta, GA; United States
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    Effects of Forward- and Backward-Facing Steps on the Crossflow Receptivity and Stability in Supersonic Boundary Layers (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The effects of forward- and backward-facing steps on the receptivity and stability of three-dimensional supersonic boundary layers over a swept wing with a blunt leading edge are numerically investigated for a freestream Mach number of 3 and a sweep angle of 30 degrees. The flow fields are obtained by solving the full Navier-Stokes equations. The evolution of instability waves generated by surface roughness is simulated with and without the forward- and backward-facing steps. The separation bubble lengths are about 5-10 step heights for the forward-facing step and are about 10 for the backward-facing step. The linear stability calculations show very strong instability in the separated region with a large frequency domain. The simulation results show that the presence of backward-facing steps decreases the amplitude of the stationary crossflow vortices with longer spanwise wavelengths by about fifty percent and the presence of forward-facing steps does not modify the amplitudes noticeably across the steps. The waves with the shorter wavelengths grow substantially downstream of the step in agreement with the linear stability prediction.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: AIAA Paper 2014-2639 , NF1676L-17654 , AIAA Fluid Dynamics Conference; Jun 16, 2014 - Jun 20, 2014; Atlanta, GA; United States
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    On Formulations of Discontinuous Galerkin and Related Methods for Conservation Laws (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A formulation for the discontinuous Galerkin (DG) method that leads to solutions using the differential form of the equation (as opposed to the standard integral form) is presented. The formulation includes (a) a derivative calculation that involves only data within each cell with no data interaction among cells, and (b) for each cell, corrections to this derivative that deal with the jumps in fluxes at the cell boundaries and allow data across cells to interact. The derivative with no interaction is obtained by a projection, but for nodal-type methods, evaluating this derivative by interpolation at the nodal points is more economical. The corrections are derived using the approximate (Dirac) delta functions. The formulation results in a family of schemes: different approximate delta functions give rise to different methods. It is shown that the current formulation is essentially equivalent to the flux reconstruction (FR) formulation. Due to the use of approximate delta functions, an energy stability proof simpler than that of Vincent, Castonguay, and Jameson (2011) for a family of schemes is derived. Accuracy and stability of resulting schemes are discussed via Fourier analyses. Similar to FR, the current formulation provides a unifying framework for high-order methods by recovering the DG, spectral difference (SD), and spectral volume (SV) schemes. It also yields stable, accurate, and economical methods.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: NASA/TM-2014-218135 , E-18938 , International Conference on Spectral and High-Order Methods (ICOSAHOM) 2014; Jun 23, 2014 - Jun 27, 2014; Salt Lake City, UT; United States
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    Gaseous Non-Premixed Flame Research Planned for the International Space Station (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Thus far, studies of gaseous diffusion flames on the International Space Station (ISS) have been limited to research conducted in the Microgravity Science Glovebox (MSG) in mid-2009 and early 2012. The research was performed with limited instrumentation, but novel techniques allowed for the determination of the soot temperature and volume fraction. Development is now underway for the next experiments of this type. The Advanced Combustion via Microgravity Experiments (ACME) project consists of five independent experiments that will be conducted with expanded instrumentation within the stations Combustion Integrated Rack (CIR). ACMEs goals are to improve our understanding of flame stability and extinction limits, soot control and reduction, oxygen-enriched combustion which could enable practical carbon sequestration, combustion at fuel lean conditions where both optimum performance and low emissions can be achieved, the use of electric fields for combustion control, and materials flammability. The microgravity environment provides longer residence times and larger length scales, yielding a broad range of flame conditions which are beneficial for simplified analysis, e.g., of limit behaviour where chemical kinetics are important. The detailed design of the modular ACME hardware, e.g., with exchangeable burners, is nearing completion, and it is expected that on-orbit testing will begin in 2016.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: GRC-E-DAA-TN13358 , 2014 Spring Technical Meeting of the Central States Section of The Combustion Institute; Mar 16, 2014 - Mar 18, 2014; Tulsa, OK; United States
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    Development of Naphthalene PLIF for Visualizing Ablation Products From a Space Capsule Heat Shield (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The Orion Multi-Purpose Crew Vehicle (MPCV) will use an ablative heat shield. To better design this heat shield and others that will undergo planetary entry, an improved understanding of the ablation process would be beneficial. Here, a technique developed at The University of Texas at Austin that uses planar laser-induced fluorescence (PLIF) of a low-temperature sublimating ablator (naphthalene) to enable visualization of the ablation products in a hypersonic flow is applied. Although high-temperature ablation is difficult and expensive to recreate in a laboratory environment, low-temperature sublimation creates a limited physics problem that can be used to explore ablation-product transport in a hypersonic flow-field. In the current work, a subscale capsule reentry vehicle model with a solid naphthalene heat shield has been tested in a Mach 5 wind tunnel. The PLIF technique provides images of the spatial distribution of sublimated naphthalene in the heat-shield boundary layer, separated shear layer, and backshell recirculation region. Visualizations of the capsule shear layer using both naphthalene PLIF and Schlieren imaging compared favorably. PLIF images have shown high concentrations of naphthalene in the capsule separated flow region, intermittent turbulent structures on the heat shield surface, and interesting details of the capsule shear layer structure. It was shown that, in general, the capsule shear layer appears to be more unsteady at lower angels of attack. The PLIF images demonstrated that during a wind tunnel run, as the model heated up, the rate of naphthalene ablation increased, since the PLIF signal increased steadily over the course of a run. Additionally, the shear layer became increasingly unsteady over the course of a wind tunnel run, likely because of increased surface roughness but also possibly because of the increased blowing. Regions with a relatively low concentration of naphthalene were also identified in the capsule backshell recirculation region and are most likely the result of cross-flow-induced vortices on the capsule afterbody.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: AIAA Paper-2014-1152 , NF1676L-16749 , AIAA Aerospace Sciences Meeting; Jan 13, 2014 - Jan 17, 2014; National Harbor, MD; United States
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    Object-Oriented/Data-Oriented Design of a Direct Simulation Monte Carlo Algorithm (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Over the past decade, there has been much progress towards improved phenomenological modeling and algorithmic updates for the direct simulation Monte Carlo (DSMC) method, which provides a probabilistic physical simulation of gas Rows. These improvements have largely been based on the work of the originator of the DSMC method, Graeme Bird. Of primary importance are improved chemistry, internal energy, and physics modeling and a reduction in time to solution. These allow for an expanded range of possible solutions In altitude and velocity space. NASA's current production code, the DSMC Analysis Code (DAC), is well-established and based on Bird's 1994 algorithms written in Fortran 77 and has proven difficult to upgrade. A new DSMC code is being developed in the C++ programming language using object-oriented and data-oriented design paradigms to facilitate the inclusion of the recent improvements and future development activities. The development efforts on the new code, the Multiphysics Algorithm with Particles (MAP), are described, and performance comparisons are made with DAC.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: AIAA Paper 2014-2546 , NF1676L-17733 , AIAA/ASME Joint Thermophysics and Heat Transfer Conference; Jun 16, 2014 - Jun 20, 2014; Atlanta, GA; United States
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    High Order Numerical Methods for LES of Turbulent Flows with Shocks (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Simulation of turbulent flows with shocks employing explicit subgrid-scale (SGS) filtering may encounter a loss of accuracy in the vicinity of a shock. In this work we perform a comparative study of different approaches to reduce this loss of accuracy within the framework of the dynamic Germano SGS model. One of the possible approaches is to apply Harten's subcell resolution procedure to locate and sharpen the shock, and to use a one-sided test filter at the grid points adjacent to the exact shock location. The other considered approach is local disabling of the SGS terms in the vicinity of the shock location. In this study we use a canonical shock-turbulence interaction problem for comparison of the considered modifications of the SGS filtering procedure. For the considered test case both approaches show a similar improvement in the accuracy near the shock.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: ARC-E-DAA-TN17606 , International Conference on Computational Fluid Dynamics (ICCFD8); Jul 14, 2014 - Jul 18, 2014; Chengdu; China
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    Gravity and Heater Size Effects on Pool Boiling Heat Transfer (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: The current work is based on observations of boiling heat transfer over a continuous range of gravity levels between 0g to 1.8g and varying heater sizes with a fluorinert as the test liquid (FC-72/n-perfluorohexane). Variable gravity pool boiling heat transfer measurements over a wide range of gravity levels were made during parabolic flight campaigns as well as onboard the International Space Station. For large heaters and-or higher gravity conditions, buoyancy dominated boiling and heat transfer results were heater size independent. The power law coefficient for gravity in the heat transfer equation was found to be a function of wall temperature under these conditions. Under low gravity conditions and-or for smaller heaters, surface tension forces dominated and heat transfer results were heater size dependent. A pool boiling regime map differentiating buoyancy and surface tension dominated regimes was developed along with a unified framework that allowed for scaling of pool boiling over a wide range of gravity levels and heater sizes. The scaling laws developed in this study are expected to allow performance quantification of phase change based technologies under variable gravity environments eventually leading to their implementation in space based applications.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: NASA/CR-2014-216672 , E-18879 , GRC-E-DAA-TN13259
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    Test and Recommendation of Flight-forward Resistive Temperature Detector for Resource Prospector Mission (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: The Resource Prospector Mission (RPM) is an in-situ resource utilization (ISRU) technology demonstration mission planned to launch in 2018. The mission will use the Regolith and Environment Science & Oxygen and Lunar Volatile Extraction (RESOLVE) Payload to prospect for lunar volatiles such as water, oxygen, and carbon dioxide. These compounds will validate ISRU capability. The payload, particularly the Lunar Advanced Volatile Analysis (LAVA) subsystem, requires numerous temperature measurements to accurately control on-board heaters that keep the volatiles in the vapor phase to allow quantification and prevent the clogging of delivery lines. Previous spaceflight missions have proven that Resistive Temperature Detector (RTD) failure impedes mission success. The research resulted in a recommendation for a flight-forward RTD. The recommendation was based on accuracy, consistency, and ease of installation of RTDs procured from IST, QTI, and Honeywell.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: KSC-E-DAA-TN14467
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    System Testing of Ground Cooling System Components (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: This internship focused primarily upon software unit testing of Ground Cooling System (GCS) components, one of the three types of tests (unit, integrated, and COTS/regression) utilized in software verification. Unit tests are used to test the software of necessary components before it is implemented into the hardware. A unit test determines that the control data, usage procedures, and operating procedures of a particular component are tested to determine if the program is fit for use. Three different files are used to make and complete an efficient unit test. These files include the following: Model Test file (.mdl), Simulink SystemTest (.test), and autotest (.m). The Model Test file includes the component that is being tested with the appropriate Discrete Physical Interface (DPI) for testing. The Simulink SystemTest is a program used to test all of the requirements of the component. The autotest tests that the component passes Model Advisor and System Testing, and puts the results into proper files. Once unit testing is completed on the GCS components they can then be implemented into the GCS Schematic and the software of the GCS model as a whole can be tested using integrated testing. Unit testing is a critical part of software verification; it allows for the testing of more basic components before a model of higher fidelity is tested, making the process of testing flow in an orderly manner.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: KSC-E-DAA-TN14359
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    Development of a Higher Fidelity Model for the Cascade Distillation Subsystem (CDS) (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: Significant improvements have been made to the ACM model of the CDS, enabling accurate predictions of dynamic operations with fewer assumptions. The model has been utilized to predict how CDS performance would be impacted by changing operating parameters, revealing performance trade-offs and possibilities for improvement. CDS efficiency is driven by the THP coefficient of performance, which in turn is dependent on heat transfer within the system. Based on the remaining limitations of the simulation, priorities for further model development include: center dot Relaxing the assumption of total condensation center dot Incorporating dynamic simulation capability for the buildup of dissolved inert gasses in condensers center dot Examining CDS operation with more complex feeds center dot Extending heat transfer analysis to all surfaces
    Keywords: Fluid Mechanics and Thermodynamics
    Type: JSC-CN-30991
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    Study of Aerothermodynamic Modeling Issues Relevant to High-Speed Sample Return Vehicles (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: This paper examines the application of state-of-the-art coupled ablation and radiation simulations to highspeed sample return vehicles, such as those returning from Mars or an asteroid. A defining characteristic of these entries is that the surface recession rates and temperatures are driven by nonequilibrium convective and radiative heating through a boundary layer with significant surface blowing and ablation products. Measurements relevant to validating the simulation of these phenomena are reviewed and the Stardust entry is identified as providing the best relevant measurements. A coupled ablation and radiation flowfield analysis is presented that implements a finite-rate surface chemistry model. Comparisons between this finite-rate model and a equilibrium ablation model show that, while good agreement is seen for diffusion-limited oxidation cases, the finite-rate model predicts up to 50% lower char rates than the equilibrium model at sublimation conditions. Both the equilibrium and finite rate models predict significant negative mass flux at the surface due to sublimation of atomic carbon. A sensitivity analysis to flowfield and surface chemistry rates show that, for a sample return capsule at 10, 12, and 14 km/s, the sublimation rates for C and C3 provide the largest changes to the convective flux, radiative flux, and char rate. A parametric uncertainty analysis of the radiative heating due to radiation modeling parameters indicates uncertainties ranging from 27% at 10 km/s to 36% at 14 km/s. Applying the developed coupled analysis to the Stardust entry results in temperatures within 10% of those inferred from observations, and final recession values within 20% of measurements, which improves upon the 60% over-prediction at the stagnation point obtained through an uncoupled analysis. Emission from CN Violet is shown to be over-predicted by nearly and order-of-magnitude, which is consistent with the results of previous independent analyses. Finally, the coupled analysis is applied to a 14 km/s Earth entry representative of a Mars sample return. Although the radiative heating provides a larger fraction of the total heating, the influence of ablation and radiation on the flowfield are shown to be similar to Stardust.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: NF1676L-18312 , Von Karman Institute (VKI) Lecture Series; Champaign, Il; United States
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    Dissertation Defense Computational Fluid Dynamics Uncertainty Analysis for Payload Fairing Spacecraft Environmental Control Systems (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: Spacecraft thermal protection systems are at risk of being damaged due to airflow produced from Environmental Control Systems. There are inherent uncertainties and errors associated with using Computational Fluid Dynamics to predict the airflow field around a spacecraft from the Environmental Control System. This paper describes an approach to quantify the uncertainty in using Computational Fluid Dynamics to predict airflow speeds around an encapsulated spacecraft without the use of test data. Quantifying the uncertainty in analytical predictions is imperative to the success of any simulation-based product. The method could provide an alternative to traditional "validation by test only" mentality. This method could be extended to other disciplines and has potential to provide uncertainty for any numerical simulation, thus lowering the cost of performing these verifications while increasing the confidence in those predictions. Spacecraft requirements can include a maximum airflow speed to protect delicate instruments during ground processing. Computational Fluid Dynamics can be used to verify these requirements; however, the model must be validated by test data. This research includes the following three objectives and methods. Objective one is develop, model, and perform a Computational Fluid Dynamics analysis of three (3) generic, non-proprietary, environmental control systems and spacecraft configurations. Several commercially available and open source solvers have the capability to model the turbulent, highly three-dimensional, incompressible flow regime. The proposed method uses FLUENT, STARCCM+, and OPENFOAM. Objective two is to perform an uncertainty analysis of the Computational Fluid Dynamics model using the methodology found in "Comprehensive Approach to Verification and Validation of Computational Fluid Dynamics Simulations". This method requires three separate grids and solutions, which quantify the error bars around Computational Fluid Dynamics predictions. The method accounts for all uncertainty terms from both numerical and input variables. Objective three is to compile a table of uncertainty parameters that could be used to estimate the error in a Computational Fluid Dynamics model of the Environmental Control System /spacecraft system. Previous studies have looked at the uncertainty in a Computational Fluid Dynamics model for a single output variable at a single point, for example the re-attachment length of a backward facing step. For the flow regime being analyzed (turbulent, three-dimensional, incompressible), the error at a single point can propagate into the solution both via flow physics and numerical methods. Calculating the uncertainty in using Computational Fluid Dynamics to accurately predict airflow speeds around encapsulated spacecraft in is imperative to the success of future missions.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: KSC-E-DAA-TN13123
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    Extension of a Kinetic Approach to Chemical Reactions to Electronic Energy Levels and Reactions Involving Charged Species with Application to DSMC Simulations (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: The ability to compute rarefied, ionized hypersonic flows is becoming more important as missions such as Earth reentry, landing high mass payloads on Mars, and the exploration of the outer planets and their satellites are being considered. Recently introduced molecular-level chemistry models that predict equilibrium and nonequilibrium reaction rates using only kinetic theory and fundamental molecular properties are extended in the current work to include electronic energy level transitions and reactions involving charged particles. These extensions are shown to agree favorably with reported transition and reaction rates from the literature for near-equilibrium conditions. Also, the extensions are applied to the second flight of the Project FIRE flight experiment at 1634 seconds with a Knudsen number of 0.001 at an altitude of 76.4 km. In order to accomplish this, NASA's direct simulation Monte Carlo code DAC was rewritten to include the ability to simulate charge-neutral ionized flows, take advantage of the recently introduced chemistry model, and to include the extensions presented in this work. The 1634 second data point was chosen for comparisons to be made in order to include a CFD solution. The Knudsen number at this point in time is such that the DSMC simulations are still tractable and the CFD computations are at the edge of what is considered valid because, although near-transitional, the flow is still considered to be continuum. It is shown that the inclusion of electronic energy levels in the DSMC simulation is necessary for flows of this nature and is required for comparison to the CFD solution. The flow field solutions are also post-processed by the nonequilibrium radiation code HARA to compute the radiative portion.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: NASA/TP-2014-218254 , L-20376
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    Dissertation Defense: Computational Fluid Dynamics Uncertainty Analysis for Payload Fairing Spacecraft Environmental Control Systems (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: Spacecraft thermal protection systems are at risk of being damaged due to airflow produced from Environmental Control Systems. There are inherent uncertainties and errors associated with using Computational Fluid Dynamics to predict the airflow field around a spacecraft from the Environmental Control System. This paper describes an approach to quantify the uncertainty in using Computational Fluid Dynamics to predict airflow speeds around an encapsulated spacecraft without the use of test data. Quantifying the uncertainty in analytical predictions is imperative to the success of any simulation-based product. The method could provide an alternative to traditional validation by test only mentality. This method could be extended to other disciplines and has potential to provide uncertainty for any numerical simulation, thus lowering the cost of performing these verifications while increasing the confidence in those predictions.Spacecraft requirements can include a maximum airflow speed to protect delicate instruments during ground processing. Computational Fluid Dynamics can be used to verify these requirements; however, the model must be validated by test data. This research includes the following three objectives and methods. Objective one is develop, model, and perform a Computational Fluid Dynamics analysis of three (3) generic, non-proprietary, environmental control systems and spacecraft configurations. Several commercially available and open source solvers have the capability to model the turbulent, highly three-dimensional, incompressible flow regime. The proposed method uses FLUENT, STARCCM+, and OPENFOAM. Objective two is to perform an uncertainty analysis of the Computational Fluid Dynamics model using the methodology found in Comprehensive Approach to Verification and Validation of Computational Fluid Dynamics Simulations. This method requires three separate grids and solutions, which quantify the error bars around Computational Fluid Dynamics predictions. The method accounts for all uncertainty terms from both numerical and input variables. Objective three is to compile a table of uncertainty parameters that could be used to estimate the error in a Computational Fluid Dynamics model of the Environmental Control System spacecraft system.Previous studies have looked at the uncertainty in a Computational Fluid Dynamics model for a single output variable at a single point, for example the re-attachment length of a backward facing step. For the flow regime being analyzed (turbulent, three-dimensional, incompressible), the error at a single point can propagate into the solution both via flow physics and numerical methods. Calculating the uncertainty in using Computational Fluid Dynamics to accurately predict airflow speeds around encapsulated spacecraft in is imperative to the success of future missions.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: KSC-E-DAA-TN13127
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    Supersonic Retropropulsion Test 1853 in NASA LaRC Unitary Plan Wind Tunnel Test Section 2 (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: A supersonic retropropulsion experiment was conducted in the Langley Research Center Unitary Plan Wind Tunnel Test Section 2 at Mach numbers of 2.4, 3.5, and 4.6. Intended as a code validation effort, this study used pretest computations to size and refine the model such that tunnel blockage and internal flow separations were minimized. A 5-in diameter 70 degree sphere-cone forebody, which can accommodate up to four 4:1 area ratio nozzles, followed by a 9.55 inches long cylindrical aft body was selected for this test after computational maturation. The primary measurements for this experiment were high spatial-density surface pressures. In addition, high speed schlieren video and internal pressures and temperatures were acquired. The test included parametric variations in the number of nozzles utilized, thrust coefficients (roughly 0 to 4), and angles of attack (-8 to 20 degrees). The run matrix was developed to also allow quantification of various sources of experimental uncertainty, such as random errors due to run-to-run variations and systematic errors due to flowfield or model misalignments. To accommodate the uncertainty assessment, many runs and replicates were conducted with the model at various locations within the tunnel and with model roll angles of 0, 60, 120, and 180 degrees. This test report provides operational details of the experiment, contains a review of trends, and provides all schlieren and pressure results within appendices.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: NASA/TP-2014-218256 , L-20392 , NF1676L-18610
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    NASA Physical Sciences - Presentation to Annual Two Phase Heat Transfer International Topical Team Meeting (2014)
    In:  CASI
    Details
    Publication Date: 2019-08-14
    Description: The Two-phase Heat Transfer International Topical Team consists of researchers and members from various space agencies including ESA, JAXA, CSA, and RSA. This presentation included descriptions various fluid experiments either being conducted by or planned by NASA for the International Space Station in the areas of two-phase flow, flow boiling, capillary flow, and crygenic fluid storage.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: GRC-E-DAA-TN20188 , International Conference on Two-Phase Systems for Ground and Space Applications; Sep 22, 2014 - Sep 26, 2014; Baltimore, MD; United States
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  • 188
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    Arbitrary Shape Deformation in CFD Design (2014)
    In:  CASI
    Details
    Publication Date: 2019-08-13
    Description: Sculptor(R) is a commercially available software tool, based on an Arbitrary Shape Design (ASD), which allows the user to perform shape optimization for computational fluid dynamics (CFD) design. The developed software tool provides important advances in the state-of-the-art of automatic CFD shape deformations and optimization software. CFD is an analysis tool that is used by engineering designers to help gain a greater understanding of the fluid flow phenomena involved in the components being designed. The next step in the engineering design process is to then modify, the design to improve the components' performance. This step has traditionally been performed manually via trial and error. Two major problems that have, in the past, hindered the development of an automated CFD shape optimization are (1) inadequate shape parameterization algorithms, and (2) inadequate algorithms for CFD grid modification. The ASD that has been developed as part of the Sculptor(R) software tool is a major advancement in solving these two issues. First, the ASD allows the CFD designer to freely create his own shape parameters, thereby eliminating the restriction of only being able to use the CAD model parameters. Then, the software performs a smooth volumetric deformation, which eliminates the extremely costly process of having to remesh the grid for every shape change (which is how this process had previously been achieved). Sculptor(R) can be used to optimize shapes for aerodynamic and structural design of spacecraft, aircraft, watercraft, ducts, and other objects that affect and are affected by flows of fluids and heat. Sculptor(R) makes it possible to perform, in real time, a design change that would manually take hours or days if remeshing were needed.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: SSC-00290 , NASA Tech Briefs, January 2014; 19
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  • 189
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    Additive Manufacturing Thermal Performance Testing of Single Channel GRCop-84 SLM Components (2014)
    In:  CASI
    Details
    Publication Date: 2019-08-13
    Description: The surface finish found on components manufactured by sinter laser manufacturing (SLM) is rougher (0.013 - 0.0006 inches) than parts made using traditional fabrication methods. Internal features and passages built into SLM components do not readily allow for roughness reduction processes. Alternatively, engineering literature suggests that the roughness of a surface can enhance thermal performance within a pressure drop regime. To further investigate the thermal performance of SLM fabricated pieces, several GRCop-84 SLM single channel components were tested using a thermal conduction rig at MSFC. A 20 kW power source running at 25% duty cycle and 25% power level applied heat to each component while varying water flow rates between 2.1 - 6.2 gallons/min (GPM) at a supply pressure of 550 to 700 psi. Each test was allowed to reach quasi-steady state conditions where pressure, temperature, and thermal imaging data were recorded. Presented in this work are the heat transfer responses compared to a traditional machined OHFC Copper test section. An analytical thermal model was constructed to anchor theoretical models with the empirical data.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: M14-3811 , JANNAF Additive Manufacturing for Propulsion Applications Technical Interchange Meeting; 3-5 Sept. 2014; Huntsville, AL; United States
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  • 190
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    Vortex Generators to Control Boundary Layer Interactions (2014)
    In:  CASI
    Details
    Publication Date: 2019-08-24
    Description: Devices for generating streamwise vorticity in a boundary includes various forms of vortex generators. One form of a split-ramp vortex generator includes a first ramp element and a second ramp element with front ends and back ends, ramp surfaces extending between the front ends and the back ends, and vertical surfaces extending between the front ends and the back ends adjacent the ramp surfaces. A flow channel is between the first ramp element and the second ramp element. The back ends of the ramp elements have a height greater than a height of the front ends, and the front ends of the ramp elements have a width greater than a width of the back ends.
    Keywords: Fluid Mechanics and Thermodynamics
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  • 191
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    Space Shuttle Boundary Layer Transition Flight Experiment Ground Testing Overview (2014)
    In:  CASI
    Details
    Publication Date: 2019-08-13
    Description: In support of the Boundary Layer Transition (BLT) Flight Experiment (FE) Project in which a manufactured protuberance tile was installed on the port wing of Space Shuttle Orbiter Discovery for STS-119, STS- 128, STS-131 and STS-133 as well as Space Shuttle Orbiter Endeavour for STS-134, a significant ground test campaign was completed. The primary goals of the test campaign were to provide ground test data to support the planning and safety certification efforts required to fly the flight experiment as well as validation for the collected flight data. These test included Arcjet testing of the tile protuberance, aerothermal testing to determine the boundary layer transition behavior and resultant surface heating and planar laser induced fluorescence (PLIF) testing in order to gain a better understanding of the flow field characteristics associated with the flight experiment. This paper provides an overview of the BLT FE Project ground testing. High-level overviews of the facilities, models, test techniques and data are presented, along with a summary of the insights gained from each test.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: NF1676L-18703 , Thermal and Fluids Analysis Workshop (TFAWS) 2014; Aug 04, 2014 - Aug 08, 2014; Cleveland, OH; United States
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  • 192
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    Pre-Test CFD for the Design and Execution of the Enhanced Injection and Mixing Project at NASA Langley Research Center (2014)
    In:  CASI
    Details
    Publication Date: 2019-08-13
    Description: With the increasing costs of physics experiments and simultaneous increase in availability and maturity of computational tools it is not surprising that computational fluid dynamics (CFD) is playing an increasingly important role, not only in post-test investigations, but also in the early stages of experimental planning. This paper describes a CFD-based effort executed in close collaboration between computational fluid dynamicists and experimentalists to develop a virtual experiment during the early planning stages of the Enhanced Injection and Mixing project at NASA Langley Research Center. This projects aims to investigate supersonic combustion ramjet (scramjet) fuel injection and mixing physics, improve the understanding of underlying physical processes, and develop enhancement strategies and functional relationships relevant to flight Mach numbers greater than 8. The purpose of the virtual experiment was to provide flow field data to aid in the design of the experimental apparatus and the in-stream rake probes, to verify the nonintrusive measurements based on NO-PLIF, and to perform pre-test analysis of quantities obtainable from the experiment and CFD. The approach also allowed for the joint team to develop common data processing and analysis tools, and to test research ideas. The virtual experiment consisted of a series of Reynolds-averaged simulations (RAS). These simulations included the facility nozzle, the experimental apparatus with a baseline strut injector, and the test cabin. Pure helium and helium-air mixtures were used to determine the efficacy of different inert gases to model hydrogen injection. The results of the simulations were analyzed by computing mixing efficiency, total pressure recovery, and stream thrust potential. As the experimental effort progresses, the simulation results will be compared with the experimental data to calibrate the modeling constants present in the CFD and validate simulation fidelity. CFD will also be used to investigate different injector concepts, improve understanding of the flow structure and flow physics, and develop functional relationships. Both RAS and large eddy simulations (LES) are planned for post-test analysis of the experimental data.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: NF1676L-19299 , Combustion; Dec 08, 2014 - Dec 11, 2014; Albuquerque, NM; United States|Propulsion Systems Hazards; Dec 08, 2014 - Dec 11, 2014; Albuquerque, NM; United States|Exhaust Plume and Signatures; Dec 08, 2014 - Dec 11, 2014; Albuquerque, NM; United States|Airbreathing Propulsion; Dec 08, 2014 - Dec 11, 2014; Albuquerque, NM; United States|JANNAF Joint Subcommittee Meeting; Dec 08, 2014 - Dec 11, 2014; Albuquerque, NM; United States
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  • 193
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    One Dimensional Analysis Model of a Condensing Spray Chamber Including Rocket Exhaust Using SINDA/FLUINT and CEA (2014)
    In:  CASI
    Details
    Publication Date: 2019-08-13
    Description: Modeling droplet condensation via CFD codes can be very tedious, time consuming, and inaccurate. CFD codes may be tedious and time consuming in terms of using Lagrangian particle tracking approaches or particle sizing bins. Also since many codes ignore conduction through the droplet and or the degradating effect of heat and mass transfer if noncondensible species are present, the solutions may be inaccurate. The modeling of a condensing spray chamber where the significant size of the water droplets and the time and distance these droplets take to fall, can make the effect of droplet conduction a physical factor that needs to be considered in the model. Furthermore the presence of even a relatively small amount of noncondensible has been shown to reduce the amount of condensation [Ref 1]. It is desirable then to create a modeling tool that addresses these issues. The path taken to create such a tool is illustrated. The application of this tool and subsequent results are based on the spray chamber in the Spacecraft Propulsion Research Facility (B2) located at NASA's Plum Brook Station that tested an RL-10 engine. The platform upon which the condensation physics is modeled is SINDAFLUINT. The use of SINDAFLUINT enables the ability to model various aspects of the entire testing facility, including the rocket exhaust duct flow and heat transfer to the exhaust duct wall. The ejector pumping system of the spray chamber is also easily implemented via SINDAFLUINT. The goal is to create a transient one dimensional flow and heat transfer model beginning at the rocket, continuing through the condensing spray chamber, and finally ending with the ejector pumping system. However the model of the condensing spray chamber may be run independently of the rocket and ejector systems detail, with only appropriate mass flow boundary conditions placed at the entrance and exit of the condensing spray chamber model. The model of the condensing spray chamber takes into account droplet conduction as well as the degrading effect of mass and heat transfer due to the presence of noncondensibles. The one dimension model of the condensing spray chamber makes no presupposition on the pressure profile within the chamber, allowing the implemented droplet physics of heat and mass transfer coupled to the SINDAFLUINT solver to determine a transient pressure profile of the condensing spray chamber. Model results compare well to the RL-10 engine pressure test data.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: GRC-E-DAA-TN17011 , Thermal and Fluids Analysis Workshop 2014; Aug 03, 2014 - Aug 07, 2014; Cleveland, OH; United States
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  • 194
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    One Dimensional Analysis Model of a Condensing Spray Chamber Including Rocket Exhaust Using SINDA/FLUINT and CEA (2014)
    In:  CASI
    Details
    Publication Date: 2019-08-13
    Description: Modeling droplet condensation via CFD codes can be very tedious, time consuming, and inaccurate. CFD codes may be tedious and time consuming in terms of using Lagrangian particle tracking approaches or particle sizing bins. Also since many codes ignore conduction through the droplet and or the degradating effect of heat and mass transfer if noncondensible species are present, the solutions may be inaccurate. The modeling of a condensing spray chamber where the significant size of the water droplets and the time and distance these droplets take to fall, can make the effect of droplet conduction a physical factor that needs to be considered in the model. Furthermore the presence of even a relatively small amount of noncondensible has been shown to reduce the amount of condensation. It is desirable then to create a modeling tool that addresses these issues. The path taken to create such a tool is illustrated. The application of this tool and subsequent results are based on the spray chamber in the Spacecraft Propulsion Research Facility (B2) located at NASA's Plum Brook Station that tested an RL-10 engine. The platform upon which the condensation physics is modeled is SINDAFLUINT. The use of SINDAFLUINT enables the ability to model various aspects of the entire testing facility, including the rocket exhaust duct flow and heat transfer to the exhaust duct wall. The ejector pumping system of the spray chamber is also easily implemented via SINDAFLUINT. The goal is to create a transient one dimensional flow and heat transfer model beginning at the rocket, continuing through the condensing spray chamber, and finally ending with the ejector pumping system. However the model of the condensing spray chamber may be run independently of the rocket and ejector systems detail, with only appropriate mass flow boundary conditions placed at the entrance and exit of the condensing spray chamber model. The model of the condensing spray chamber takes into account droplet conduction as well as the degrading effect of mass and heat transfer due to the presence of noncondensibles. The one dimension model of the condensing spray chamber makes no presupposition on the pressure profile within the chamber, allowing the implemented droplet physics of heat and mass transfer coupled to the SINDAFLUINT solver to determine a transient pressure profile of the condensing spray chamber. Model results compare well to the RL-10 engine pressure test data.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: GRC-E-DAA-TN17009 , Thermal and Fluids Analysis Workshop 2014; Aug 03, 2015 - Aug 07, 2015; Cleveland, OH; United States
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    Simulations of Turbulent Momentum and Scalar Transport in Confined Swirling Coaxial Jets (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: This paper presents the numerical simulations of confined three dimensional coaxial water jets. The objectives are to validate the newly proposed nonlinear turbulence models of momentum and scalar transport, and to evaluate the newly introduced scalar APDF and DWFDF equation along with its Eulerian implementation in the National Combustion Code (NCC). Simulations conducted include the steady RANS, the unsteady RANS (URANS), and the time-filtered Navier-Stokes (TFNS) with and without invoking the APDF or DWFDF equation. When the APDF or DWFDF equation is invoked, the simulations are of a hybrid nature, i.e., the transport equations of energy and species are replaced by the APDF or DWFDF equation. Results of simulations are compared with the available experimental data. Some positive impacts of the nonlinear turbulence models and the Eulerian scalar APDF and DWFDF approach are observed.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: NASA/TM-2014-218134 , E-18930
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  • 196
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    On Parametric Sensitivity of Reynolds-Averaged Navier-Stokes SST Turbulence Model: 2D Hypersonic Shock-Wave Boundary Layer Interactions (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: Examined is sensitivity of separation extent, wall pressure and heating to variation of primary input flow parameters, such as Mach and Reynolds numbers and shock strength, for 2D and Axisymmetric Hypersonic Shock Wave Turbulent Boundary Layer interactions obtained by Navier-Stokes methods using the SST turbulence model. Baseline parametric sensitivity response is provided in part by comparison with vetted experiments, and in part through updated correlations based on free interaction theory concepts. A recent database compilation of hypersonic 2D shock-wave/turbulent boundary layer experiments extensively used in a prior related uncertainty analysis provides the foundation for this updated correlation approach, as well as for more conventional validation. The primary CFD method for this work is DPLR, one of NASA's real-gas aerothermodynamic production RANS codes. Comparisons are also made with CFL3D, one of NASA's mature perfect-gas RANS codes. Deficiencies in predicted separation response of RANS/SST solutions to parametric variations of test conditions are summarized, along with recommendations as to future turbulence approach.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: NASA/TM-2014-218353 , ARC-E-DAA-TN15380
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  • 197
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    Flow Induced Vibration of Metal Bellows (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: No abstract available
    Keywords: Fluid Mechanics and Thermodynamics
    Type: M15-4194
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  • 198
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    Generating Inviscid and Viscous Fluid Flow Simulations over a Surface Using a Quasi-simultaneous Technique (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: A fluid-flow simulation over a computer-generated surface is generated using a quasi-simultaneous technique. The simulation includes a fluid-flow mesh of inviscid and boundary-layer fluid cells. An initial fluid property for an inviscid fluid cell is determined using an inviscid fluid simulation that does not simulate fluid viscous effects. An initial boundary-layer fluid property a boundary-layer fluid cell is determined using the initial fluid property and a viscous fluid simulation that simulates fluid viscous effects. An updated boundary-layer fluid property is determined for the boundary-layer fluid cell using the initial fluid property, initial boundary-layer fluid property, and an interaction law. The interaction law approximates the inviscid fluid simulation using a matrix of aerodynamic influence coefficients computed using a two-dimensional surface panel technique and a fluid-property vector. An updated fluid property is determined for the inviscid fluid cell using the updated boundary-layer fluid property.
    Keywords: Fluid Mechanics and Thermodynamics
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  • 199
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    Propulsion System Simulation Using the Toolbox for the Modeling and Analysis of Thermodynamic System T-MATS (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A simulation toolbox has been developed for the creation of both steady-state and dynamic thermodynamic software models. This paper describes the Toolbox for the Modeling and Analysis of Thermodynamic Systems (T-MATS), which combines generic thermodynamic and controls modeling libraries with a numerical iterative solver to create a framework for the development of thermodynamic system simulations, such as gas turbine engines. The objective of this paper is to present an overview of T-MATS, the theory used in the creation of the module sets, and a possible propulsion simulation architecture. A model comparison was conducted by matching steady-state performance results from a T-MATS developed gas turbine simulation to a well-documented steady-state simulation. Transient modeling capabilities are then demonstrated when the steady-state T-MATS model is updated to run dynamically.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: GRC-E-DAA-TN16305 , AIAA/ASME/SAE/ASEE Joint Propulsion Conference; Jul 28, 2014 - Jul 30, 2014; Cleveland, OH; United States
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  • 200
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    Active Flow Separation Control on a NACA 0015 Wing Using Fluidic Actuators (2014)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Results are presented from a recent set of wind tunnel experiments using sweeping jet actuators to control ow separation on the 30% chord trailing edge ap of a 30 deg. swept wing model with an aspect ratio (AR) of 4.35. Two sweeping jet actuator locations were examined, one on the flap shoulder and one on the trailing edge flap. The parameters that were varied included actuator momentum, freestream velocity, and trailing edge flap deflection (Delta f ) angle. The primary focus of this set of experiments was to determine the mass flow and momentum requirements for controlling separation on the flap, especially at large flap deflection angles which would be characteristic of a high lift system. Surface pressure data, force and moment data, and stereoscopic particle image velocimetry (PIV) data were acquired to evaluate the performance benefits due to applying active flow control. Improvements in lift over the majority of the wing span were obtained using sweeping jet actuator control. High momentum coefficient, Cu, levels were needed when using the actuators on the ap because they were located downstream of separation. Actuators on the flap shoulder performed slightly better but actuator size, orientation, and spacing still need to be optimized.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: AIAA Paper 2014-2364 , NF1676L-17839 , AVIATION 2014 (The Aviation and Aeronautics Forum and Exposition; Jun 16, 2014 - Jun 20, 2014; Atllanta, GA; United States
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