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  • 1
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    The robustness of seismic moment and magnitudes estimated using spectral analysis (2014)
    Wiley
    Details
    Publication Date: 2014-05-24
    Description: ABSTRACT We present an assessment of how microseismic moment magnitude, , estimates vary with the method and parameters used to calculate seismic moment. This is an important topic for operators and regulators who require good magnitude estimates when monitoring induced seismicity. It is therefore imperative that these parties know and understand what errors exist in given magnitude values, something that is poorly reported. This study concentrates on spectral analysis techniques and compares computed in the time and frequency domains. Using recordings of events at Cotton Valley, east Texas, the maximum discrepancy between estimated using the different methods is 0.6 units, a significant variation. By adjusting parameters in the calculation we find that the radiation pattern correction term can have the most significant effect on , generally up to 0.8 units. Following this investigation we make a series of recommendations for estimating microseismic using spectral methods. Noise should be estimated and removed from recordings and an attenuation correction should be applied. The spectral level can be measured by spectral fitting or taken from the low frequency level. Significant factors in obtaining reliable microseismic estimates include using at least four receivers recording at ⩾1000 Hz and making radiation pattern corrections based on focal mechanism solutions, not average values.
    Print ISSN: 0016-8025
    Electronic ISSN: 1365-2478
    Topics: Geosciences , Physics
    Published by Wiley
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    PAPER CURRENT
  • 2
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    Unsteady aerodynamics of blade rows (1987)
    In:  Other Sources
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    Publication Date: 2011-08-19
    Description: The requirements placed on an unsteady aerodynamic theory intended for turbomachinery aeroelastic applications are discussed along with a brief description of the various theoretical models that are available to address these requirements. The main emphasis is placed on the description of a linearized inviscid theory which fully accounts for the effects of a nonuniform mean or steady flow on unsteady aerodynamic response. Although this theory has been developed primarily for blade flutter prediction, more general equations are presented which account for unsteady excitations due to incident external aerodynamic disturbances as well as those due to blade motions. The resulting equations consist of a system of three field equations along with conditions imposed at blade, wake and shock surfaces and in the far field. These equations can be solved to determine the fluctuations in all fluid dynamic properties throughout the required solution domain. Example solutions are presented to demonstrate several effects associated with nonuniform steady flows on the linearized unsteady aerodynamic response to prescribed blade motions.
    Keywords: AERODYNAMICS
    Format: text
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    NASA TECHNICAL REPORTS
  • 3
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    Response of a rigid aircraft to nonstationary atmospheric turbulence. (1973)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: The plunging response of an aircraft to a type of nonstationary turbulent excitation is considered. The latter consists of stationary Gaussian noise modulated by a well-defined envelope function. The intent of the investigation is to model the excitation experienced by an airplane flying through turbulence of varying intensity and to examine the influence of intensity variations on exceedance frequencies of the gust velocity and the airplane's plunging velocity and acceleration. One analytical advantage of the proposed model is that the Gaussian assumption for the gust excitation is retained. The analysis described herein is developed in terms of an envelope function of arbitrary form; however, numerical calculations are limited to the case of harmonic modulation.
    Keywords: AIRCRAFT
    Type: AIAA Journal; 11; Aug. 197
    Format: text
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    NASA TECHNICAL REPORTS
  • 4
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    On the application of a linearized unsteady potential-flow analysis to fan-tip cascades (1986)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: A linearized potential flow analysis, which accounts for the effects of nonuniform steady flow phenomena on the unsteady response to prescribed blade motions, has been applied to five two-dimensional cascade configurations. These include a flat-plate cascade and three cascades which are representative of the tip sections of current fan designs. Here the blades are closely spaced, highly staggered, and operate at low mean incidence. The fifth configuration is a NASA Lewis cascade of symmetric biconvex airfoils for which experimental measurements are available. Numerical solutions are presented that clearly illustrate the effects and importance of blade geometry and mean blade loading on the linearized unsteady response at high subsonic inlet Mach number and high blade-vibrational frequency. In addition, a good qualitative agreement is shown between the analytical predictions and experimental measurements for the cascade of symmetric biconvex airfoils. Finally, recommendations on the research needed to extend the range of application of linearized unsteady aerodynamic analyses are provided.
    Keywords: AERODYNAMICS
    Type: ASME PAPER 86-GT-87
    Format: text
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    NASA TECHNICAL REPORTS
  • 5
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    Application of a linearized unsteady aerodynamic analysis to standard cascade configurations (1986)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A linearized potential flow analysis, which accounts for the effects of nonuniform steady flow phenomena on the linearized unsteady aerodynamic response to prescribed blade motions, has been applied to five cascade configurations. These include the first, fifth, eighth and ninth standard configurations proposed as a result of the Second International Symposium on Aeroelasticity in Turbomachines and a NASA Lewis flutter cascade. Selected results from this study, including comparisons between analytical predictions and the experimental measurements submitted for three of the foregoing configurations, are described. The correlation between theory and experiment for the first standard configuration (a compressor cascade operating at low Mach number and frequency) is quite good. Moreover, the predictions and measurements for the NASA Lewis cascade of symmetric biconvex airfoils show good qualitative agreement. However, wide discrepancies exist between the theoretical predictions and the experimental measurements for the fifth standard configuration (a subsonic transonic fan tip cascade). These can be partially attributed to conditions being imposed in the experiment which differ from those commonly used in unsteady aerodynamic analyses.
    Keywords: AERODYNAMICS
    Type: NASA-CR-3940 , R85-956896-4 , E-2686 , NAS 1.26:3940
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 6
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    Development of an efficient analysis for high Reynolds number inviscid/viscid interactions in cascades (1992)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: An efficient steady analysis for predicting strong inviscid/viscid interaction phenomena such as viscous-layer separation, shock/boundary-layer interaction, and trailing-edge/near-wake interaction in turbomachinery blade passages is described. It uses an inviscid/viscid interaction approach, wherein the flow in the outer inviscid region is assumed to be potential, and that in the inner or viscous-layer region is governed by Prandtl's equations. The inviscid solution is determined using an implicit, least-squares, finite-difference approximation. The viscous-layer solution is obtained using an inverse, finite-difference, space-marching method which is applied along the blade surfaces and the wake streamline. A semiinverse global iteration procedure permits the prediction of boundary-layer separation and other strong-interaction phenomena. Results are presented for two cascades where a range of inlet flow conditions was considered for one of them including conditions leading to large-scale flow separation.
    Keywords: AERODYNAMICS
    Type: AIAA PAPER 92-3073
    Format: text
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    NASA TECHNICAL REPORTS
  • 7
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    The effects of inlet turbulence and rotor/stator interactions on the aerodynamics and heat transfer of a large-scale rotating turbine model, volume 1 (1987)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A combined experimental and analytical program was conducted to examine the effects of inlet turbulence on airfoil heat transfer. Heat transfer measurements were obtained using low conductivity airfoils with miniature thermocouples welded to a thin, electrically heated surface skin. Heat transfer data were acquired for various combinations of low or high inlet turbulence intensity, flow coefficient (incidence), first-stator/rotor axial spacing, Reynolds number, and relative circumferential position of the first and second stators. Aerodynamic measurements include distributions of the mean and fluctuating velocities at the turbine inlet and, for each airfoil row, midspan airfoil surface pressures and circumferential distributions of the downstream steady state pressures and fluctuating velocities. Analytical results include airfoil heat transfer predictions and a examination of solutions of the unstead boundary layer equipment.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: NASA-CR-4079 , E-3536 , NAS 1.26:4079 , UTRC-R86-956480-VOL-1
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 8
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    Analysis of unsteady compressible viscous layers (1990)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: The development of an analysis to predict the unsteady compressible flows in blade boundary layers and wakes is presented. The equations that govern the flows in these regions are transformed using an unsteady turbulent generalization of the Levy-Lees transformation. The transformed equations are solved using a finite difference technique in which the solution proceeds by marching in time and in the streamwise direction. Both laminar and turbulent flows are studied, the latter using algebraic turbulence and transition models. Laminar solutions for a flat plate are shown to approach classical asymptotic results for both high and low frequency unsteady motions. Turbulent flat-plate results are in qualitative agreement with previous predictions and measurements. Finally, the numerical technique is also applied to the stator and rotor of a low-speed turbine stage to determine unsteady effects on surface heating. The results compare reasonably well with measured heat transfer data and indicate that nonlinear effects have minimal impact on the mean and unsteady components of the flow.
    Keywords: AERODYNAMICS
    Type: ASME PAPER 90-GT-14
    Format: text
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    NASA TECHNICAL REPORTS
  • 9
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    A linear aerodynamic analysis for unsteady transonic cascades (1984)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A potential flow analysis to predict unsteady airloads produced by the vibrations of turbomachinery blades operating at transonic Mach numbers is presented. The unsteady aerodynamic model includes the effects of blade geometry, finite mean pressure variation across the blade row, high frequency blade motion, and shock motion within the framework of a linearized, frequency domain formulation. The unsteady equations are solved implicit, least squares, finite difference approximation which is applicable on arbitrary grids. A numerical solution for the entire unsteady field is determined by matching a solution determined on a rectilinear type cascade mesh, which covers an extended blade passage region, to a solution determined on a detailed polar type local mesh, which covers and extends well beyond the supersonic region(s) adjacent to a blade surface. Cascades of double circular arc and flat plate blades demonstrate the unsteady analysis, and partially illustrate the effects of blade geometry, inlet Mach number, blade vibration frequency and shock motion on unsteady response.
    Keywords: AERODYNAMICS
    Type: NASA-CR-3833 , E-2202 , NAS 1.26:3833 , R84-956393-8
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 10
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    Development of an unsteady aerodynamic analysis for finite-deflection subsonic cascades (1981)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: An unsteady potential flow analysis, which accounts for the effects of blade geometry and steady turning, was developed to predict aerodynamic forces and moments associated with free vibration or flutter phenomena in the fan, compressor, or turbine stages of modern jet engines. Based on the assumption of small amplitude blade motions, the unsteady flow is governed by linear equations with variable coefficients which depend on the underlying steady low. These equations were approximated using difference expressions determined from an implicit least squares development and applicable on arbitrary grids. The resulting linear system of algebraic equations is block tridiagonal, which permits an efficient, direct (i.e., noniterative) solution. The solution procedure was extended to treat blades with rounded or blunt edges at incidence relative to the inlet flow.
    Keywords: AERODYNAMICS
    Type: NASA-CR-3455 , R81-914777-7
    Format: application/pdf
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