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  • 11
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 4 (1992), S. 2887-2899 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Direct simulations of homogeneous turbulence have, in recent years, come into widespread use for the evaluation of models for the pressure–strain correlation of turbulence. While work in this area has been beneficial, the increasingly common practice of testing the slow and rapid parts of these models separately in uniformly strained turbulent flows is shown in this paper to be unsound. For such flows, the decomposition of models for the total pressure–strain correlation into slow and rapid parts is ambiguous. Consequently, when tested in this manner, misleading conclusions can be drawn about the performance of pressure–strain models. This point is amplified by illustrative calculations of homogeneous shear flow where other pitfalls in the evaluation of models are also uncovered. More meaningful measures for testing the performance of pressure–strain models in uniformly strained turbulent flows are proposed and the implications for turbulence modeling are discussed.
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  • 12
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 3 (1991), S. 2278-2281 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In a recent paper [Phys. Fluids A 2, 1678 (1990)], the authors compared the performance of a variety of turbulence models including the K-ε model and the second-order closure model derived by Yakhot and Orszag based on renormalization group (RNG) methods. The performance of these RNG models in homogeneous turbulent shear flow was found to be quite poor, apparently due to the value of the constant Cε1 in the modeled dissipation rate equation, which was substantially lower than its traditional value. However, recently a correction has been made in the RNG-based calculation of Cε1. It is shown herein that, with the new value of Cε1, the performance of the RNG K-ε model is substantially improved. On the other hand, while the predictions of the revised RNG second-order closure model are better, some lingering problems still remain that can be remedied by the addition of higher-order terms.
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  • 13
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 30 (1987), S. 1564-1566 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: By adjusting the electrostatic potential on the end plates of a mirror plasma, the plasma's magnetohydrodynamic (MHD) stability can be altered. Furthermore, the plasma's MHD response to low-frequency modulation of the end plates can be used to pump unwanted ions that collect in a thermal barrier. Here, precise boundary conditions are derived for the MHD equations of motion to model an end plate separated from the plasma by a sheath.
    Type of Medium: Electronic Resource
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  • 14
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 8 (1996), S. 1945-1953 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We present the results of a study of the structure of a parallel compressible mixing layer in a binary mixture of gases. The gases included in this study are hydrogen (H2), helium (He), nitrogen (N2), oxygen (O2), neon (Ne) and argon (Ar). Profiles of the variation of the Lewis and Prandtl numbers across the mixing layer for all 30 combinations of gases are given. It is shown that the Lewis number can vary by as much as a factor of 8 and the Prandtl number by a factor of 2 across the mixing layer. Thus assuming constant values for the Lewis and Prandtl numbers of a binary gas mixture in the shear layer, as is done in many theoretical studies, is a poor approximation. We also present profiles of the velocity, mass fraction, temperature and density for representative binary gas mixtures at zero and supersonic Mach numbers. We show that the shape of these profiles is strongly dependent on which gases are in the mixture as well as on whether the denser gas is in the fast stream or the slow stream. © 1996 American Institute of Physics.
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  • 15
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Fluids 6 (1994), S. 662-673 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Some characteristic features of supersonic/supersonic, laminar, variable-density shear layers are studied by examining the self-similar behavior of five nitrogen/hydrogen streams. With the Levy–Lees transformation, the flow-field variables, which include the transverse velocity and dilatation, are obtained through the solution of the coupled set of nonlinear conservation equations. The issue of the appropriate "third boundary condition,'' first given for the supersonic/supersonic case by Ting [J. Math. Phys. 28, 153 (1959)], is addressed and implemented in the present formulation. Expressions for the thermal conductivity, viscosity, specific heat, and binary diffusion coefficients of an arbitrary mixture are utilized so that the Prandtl and Lewis numbers and the Chapman–Rubesin parameter can vary freely across the shear layer. In the particular cases considered, these three quantities varied by factors of approximately 3, 7, and 22, respectively. The region of high vorticity moves toward the less-dense hydrogen stream for large density ratios (∼9:1), and becomes nearly decoupled from the density profile. Because the vorticity is responsible for the kinematic mixing of the two streams, this mixing of the two laminar streams is likely to be inhibited. Even though laminar flows are considered here, this effect is consistent with the experimental observation that as density ratios become very large, further increases in the density ratio have no effect on the turbulent shear layer growth rate. The density and hydrogen mass-fraction profiles are quite elongated in the transverse direction. An increase in the velocity ratio exaggerates both of these effects. Results obtained in this work are compatible with earlier work on incompressible, variable-density flows; more importantly, these results qualitatively resemble those from experiments of compressible and incompressible turbulent flows.
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  • 16
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 9 (2002), S. 1721-1733 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Measurements of equation of state (EOS) points on the principal Hugoniots of Cu, Au, Pb and the plastics Parylene-C and brominated CH at multimegabar pressures have been made using the 1 TW HELEN laser at AWE. The aim was 1% accuracy in shock velocity measurement (3%–4% in pressure) in order to compare with data from gas-gun and nuclear underground test experiments and the theoretical EOS's based on, or supported by, these data. Experiments comprised a hohlraum heated by two 500 J, 0.53 μm wavelength, 1 ns Gaussian laser pulses generating an x-ray flux which drove a shock into a target consisting of a base, with steps of a known EOS material and of the material of unknown EOS. Shock breakout from base and steps was detected by monitoring light emission from the target with optical streak cameras and shock velocities were derived from the transit times across the known-height steps. © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 17
    Electronic Resource
    Electronic Resource
    New York, NY : American Institute of Physics (AIP)
    Physics of Fluids 2 (1990), S. 1678-1684 
    ISSN: 1089-7666
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A variety of turbulence models, including five second-order closures and four two-equation models, are tested for the problem of homogeneous turbulent shear flow in a rotating frame. The model predictions for the time evolution of the turbulent kinetic energy and dissipation rate, as well as those for the equilibrium states, are compared with the results of physical and numerical experiments. Most of the two-equation models predict the same results for all rotation rates, in which there is an exponential time growth of the turbulent kinetic energy and dissipation rate. The second-order closures are qualitatively superior since, consistent with physical and numerical experiments, they only predict this type of unstable flow for intermediate rotation rates in the range −0.1≤Ω/S≤0.6. For rotation rates outside this range, there is an exchange of stabilities with a solution whose kinetic energy and dissipation rate decay with time. Although the second-order closures are superior to the two-equation models, there are still problems with the quantitative accuracy of their predictions.
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  • 18
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Ultra-short-pulse reflectometry is studied by means of the numerical integration of one- and two-dimensional full-wave equations for ordinary and extraordinary modes propagating in a plasma. The numerical calculations illustrate the use of the reflection of ultra-short-pulse microwaves as an effective probe of the density or magnetic profile in the presence of density or magnetic fluctuations in the plasma. Bragg resonance effects can be identified in the reflected signals, which give information on fluctuations. It is also demonstrated that ultra-short-pulse reflectometry can be used to perform correlation reflectometry measurements in which correlation lengths for density fluctuations are deduced from the observed cross-correlation function of phase shifts as a function of frequency. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 19
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 61 (1990), S. 385-388 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Sources producing beams of spin-polarized atoms and ions are now widely used in nuclear physics. Modern systems are increasingly expensive and complex. They provide a variety of species and beam polarizations with intensities often sufficient for experimental needs. The laboratory seeking such a device for its program must evaluate a variety of options and match them to its accelerator, experimental program, and expected resources. The major systems and techniques used are surveyed and criteria dictating the choice of source for particular experimental applications are discussed briefly.
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  • 20
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 68 (1997), S. 1238-1243 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Ultra-short-pulse reflectometry is studied by means of the numerical integration of one- and two-dimensional full-wave equations for ordinary and extraordinary modes propagating in a plasma. The numerical calculations illustrate the use of the reflection of ultra-short-pulse microwaves as an effective probe of the density or magnetic profile in the presence of density or magnetic fluctuations in the plasma. Bragg resonance effects can be identified in the reflected signals, which give information on fluctuations. It is also demonstrated that ultra-short-pulse reflectometry can be used to perform correlation reflectometry measurements in which correlation lengths for density fluctuations are deduced from the observed cross-correlation function of phase shifts as a function of frequency. © 1997 American Institute of Physics.
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