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  • ASTROPHYSICS  (15)
  • FLUID MECHANICS AND HEAT TRANSFER  (14)
  • 2000-2004
  • 1990-1994  (29)
  • 1940-1944
  • 1992  (29)
  • 1944
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  • 2000-2004
  • 1990-1994  (29)
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  • 1
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    The X-ray variability of NGC 6814 - Power spectrum (1992)
    In:  Other Sources
    Details
    Publikationsdatum: 2011-08-24
    Beschreibung: The existence of the periodic component seen in NGC 6814 with Exosat at 12,000 +/- 100 s is confirmed by a power spectrum and folded light curve analysis of unevenly sampled Ginga data. A comparison of the power spectra produced from simulated light curves with that observed enables the intrinsic shape of the power spectrum of the source to be determined despite the distortions introduced by the window function. The best estimate for the period is 12,132 +/- 3 s, where the error is that derived from simulations. An upper limit to the rate of change of period of about 10 exp -9 is inferred if the light curves are truly phase-coherent, but as this is not required by the data, the conservative upper limit is not greater than 5 x 10 exp -7. The large amount of power in the periodic component and its stability both suggest occultation of the source as its origin.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 400; 1; p. 138-152.
    Format: text
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  • 2
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    The X-ray variability of AGN and the anomalous behavior of NGC 6814 (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The power spectra of the X-ray variable AGN are typically scale invariant with no characteristic timescale. The one strong exception to this is NGC 6814, where the EXOSAT data showed evidence for a periodic component at 12200 +/- 100 seconds. The power spectra of a GINGA lightcurve from this source, found using simulation techniques to account for the uneven sampling, also cannot be well fit by a single power law. A folded light curve analysis of GINGA data shows strong evidence for this periodic component. A second GINGA observation of this source taken one year later is consistent with the phenomena being completely periodic and phase coherent for 7 periods in the range of 12,110-12,145 seconds. Including the (optimistic) limits from the folding selects a period of 12130.39 +/- 0.05 seconds. Phase coherence is not maintained between this and the EXOSAT observations, as the structure of the folded light curves is very different. Thus the periodicity is long lived and stable, but phase coherence is only maintained on timescales of about 1 year.
    Schlagwort(e): ASTROPHYSICS
    Materialart: In: Testing the AGN paradigm; Proceedings of the 2nd Annual Topical Astrophysics Conference, Univ. of Maryland, College Park, Oct. 14-16, 1991 (A93-29801 11-90); p. 83-92.
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  • 3
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    On the molecular complexity of the hot cores in Orion A - Grain surface chemistry as 'The last refuge of the scoundrel' (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: We have modeled the gas phase chemistry of warm molecular material around protostars that is seeded with evaporating grain mantles. We show that the release of simple molecules into the gas drives ion-molecule and neutral chemistries which can account for many of the complex O-bearing and N-bearing molecules observed in hot cores. Initial grain mantle components and secondary product molecules are identified, and the observational consequences are discussed.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X); 399; 1; p. L71-L74.
    Format: text
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  • 4
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    Evidence for an ionized reprocessor in NGC 6814 (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: A detailed spectroscopic analysis of a Ginga observation of the Seyfert galaxy NGC 6814 is reported. It is shown that the X-ray data are consistent with a scenario in which the continuum radiation is reprocessed in a highly ionized medium, which can either be an accretion disk or a shell of absorbing material. Both of these can produce the strong observed iron K-alpha line, which must originate within 200 lt-sec of the continuum source. Significant iron K-beta and nickel K-alpha are predicted, based on the iron K-alpha line strength, and these lines partially conceal the iron edge in this source. The spectral variability below about 4 keV, where the apparent absorption increases as the continuum flux decreases, is probably caused by a warm absorber covering the reprocessing system.
    Schlagwort(e): ASTROPHYSICS
    Materialart: Astrophysical Journal, Part 1 (ISSN 0004-637X); 391; 1, Ma
    Format: text
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  • 5
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    Boundary conditions for direct simulations of compressible viscous flows (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: The present consideration of procedures for the definition of boundary conditions for the Navier-Stokes equations emphasizes the derivation of boundary conditions that are compatible with nondissipative algorithms applicable to direct simulations of turbulent flows. A novel formulation for the Euler equations is derived on the basis of characteristic wave relations through boundaries; this formulation is generalized to the Navier-Stokes equations. The method, which applies to both sub- and supersonic flows, is used in reflecting and nonreflecting boundary-condition treatments. Attention is given to practical implementations involving inlet and outlet boundaries and slip and nonslip walls, as well as the test cases of a ducted shear layer, vortices propagating through boundaries, and Poiseuille flow.
    Schlagwort(e): FLUID MECHANICS AND HEAT TRANSFER
    Materialart: Journal of Computational Physics (ISSN 0021-9991); 101; 1, Ju; 104-129
    Format: text
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  • 6
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    Calculation of a circular jet in crossflow with a multiple-time-scale turbulence model (1992)
    In:  Other Sources
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    Publikationsdatum: 2011-08-24
    Beschreibung: Numerical calculation of a three dimensional turbulent flow of a jet in a crossflow using a multiple time scale turbulence model is presented. The turbulence in the forward region of the jet is in a stronger inequilibrium state than that in the wake region of the jet, while the turbulence level in the wake region is higher than that in the front region. The calculated flow and the concentration fields are in very good agreement with the measured data, and it indicated that the turbulent transport of mass, concentration, and momentum is strongly governed by the inequilibrium turbulence. The capability of the multiple time scale turbulence model to resolve the inequilibrium turbulence field is also discussed.
    Schlagwort(e): FLUID MECHANICS AND HEAT TRANSFER
    Materialart: International Journal of Heat and Mass Transfer (ISSN 0017-9310); 35; 10; p. 2357-2365.
    Format: text
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  • 7
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    Comment on: The present state and future direction of eddy viscosity models (1992)
    In:  CASI
    Details
    Publikationsdatum: 2013-08-29
    Beschreibung: Two turbulence models are compared under various flow conditions. Presented first is a k-omega model which is best suited for wall bounded flows. Presented second is a k-epsilon model which is best suited for free shear flow. Generalizations of the k-omega model are discussed, and various aerodynamic parameters such as skin friction, velocity distribution, and hypersonic cylinder-flare, are compared for the different models. All results are presented in viewgraph format.
    Schlagwort(e): FLUID MECHANICS AND HEAT TRANSFER
    Materialart: NASA. Lewis Research Center, Workshop on Engineering Turbulence Modeling; p 79-86
    Format: text
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  • 8
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    DNS and modeling of the interaction between turbulent premixed flames and walls (1992)
    In:  CASI
    Details
    Publikationsdatum: 2013-08-31
    Beschreibung: The interaction between turbulent premixed flames and walls is studied using a two-dimensional full Navier-Stokes solver with simple chemistry. The effects of wall distance on the local and global flame structure are investigated. Quenching distances and maximum wall heat fluxes during quenching are computed in laminar cases and are found to be comparable to experimental and analytical results. For turbulent cases, it is shown that quenching distances and maximum heat fluxes remain of the same order as for laminar flames. Based on simulation results, a 'law-of-the-wall' model is derived to describe the interaction between a turbulent premixed flame and a wall. This model is constructed to provide reasonable behavior of flame surface density near a wall under the assumption that flame-wall interaction takes place at scales smaller than the computational mesh. It can be implemented in conjunction with any of several recent flamelet models based on a modeled surface density equation, with no additional constraints on mesh size or time step.
    Schlagwort(e): FLUID MECHANICS AND HEAT TRANSFER
    Materialart: Stanford Univ., Studying Turbulence Using Numerical Simulation Databases. 4: Proceedings of the 1992 Summer Program; p 307-323
    Format: application/pdf
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  • 9
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    AGARD WG13 aerodynamics of high speed air intakes: Assessment of CFD results (1992)
    In:  CASI
    Details
    Publikationsdatum: 2017-10-02
    Beschreibung: A brief review of the work accomplished by the numerical subgroup of AGARD Working Group 13 on the aerodynamics of high speed air intakes is presented. This work comprised the selection of test cases for which experimental data were available. The test cases were chosen to range in complexity from normal-shock/boundary-layer interaction to full forebody-inlet combinations. Computations for these test cases were solicited from a large number of organizations and individual researchers within the NATO countries. The computation methods reached from Euler solvers (with and without boundary layer corrections) to full Reynolds averaged Navier-Stokes codes. The group compared these results with the test data available for each test case. A short overview of the CFD methods employed, a description of the test cases selected, and some of the comparisons between CFD solutions and test data are presented. The conclusions and recommendations drawn from this assessment are given.
    Schlagwort(e): FLUID MECHANICS AND HEAT TRANSFER
    Materialart: AGARD, Aerodynamic Engine(Airframe Integration for High Performance Aircraft and Missiles; 24 p
    Format: text
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  • 10
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    Thermocapillary flow about an evaporating meniscus (1992)
    In:  Other Sources
    Details
    Publikationsdatum: 2019-06-28
    Beschreibung: The steady motion and thermal behavior of an evaporating superheated liquid in a small cavity bounded by isothermal sidewalls is examined. Scaling analyses and a two-dimensional finite element model are used to investigate the influence of thermocapillarity, buoyancy, and temperature-dependent mass flux on flowfield, interfacial heat transfer, and meniscus morphology. Numerical investigations indicate the existence of two counter-rotating cells symmetric about the cavity center. Results also show that evaporation tends to counteract this circulation by directing flow toward the hotter sidewalls. Although thermocapillarity and evaporation yield different flowfield distributions, both effects tend to increase interfacial temperature and heat transfer.
    Schlagwort(e): FLUID MECHANICS AND HEAT TRANSFER
    Materialart: AIAA PAPER 92-2863
    Format: text
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