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Shiyi Chen - One of the best experts on this subject based on the ideXlab platform.
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Compressibility Effect in hypersonic boundary layer with isothermal wall condition
Physical Review Fluids, 2021Co-Authors: Jianchun Wang, Minping Wan, Shiyi ChenAbstract:Helmholtz decomposition is applied to investigate the Compressibility Effect in hypersonic turbulent boundary layers. The dilatational component of flow plays an important role in the near-wall region, while the solenoidal component is dominant far from the wall. The cold wall condition significantly enhances the Compressibility, and especially enhances compression motions near the wall.
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Compressibility Effect on coherent structures energy transfer and scaling in magnetohydrodynamic turbulence
Physics of Fluids, 2017Co-Authors: Minping Wan, Shiyi Chen, W H Matthaeus, Yan Yang, Yipeng ShiAbstract:Compressible magnetohydrodynamic (MHD) turbulence, a model often used to study space and astrophysical plasmas, differs from incompressible magnetohydrodynamic and hydrodynamic (HD) turbulence in many ways. Here direct numerical simulations of mechanically forced compressible MHD turbulence are used to study the degree to which some turbulence theories proposed in the incompressible case remain applicable in the compressible one. Several aspects of compressible turbulence are studied: (i) Intermittency in the compressible case is studied by addressing flows driven with varying forcing mechanisms; these display different features, such as compression and coherent structures. The more compressive simulation is characterized by sheet-like current density structures and shocks, which lead to saturated scaling exponents of high order structure functions of density and compressive velocity. (ii) Further investigations employing conditional averages of different energy transfer fluxes reveal that the influence o...
W H Matthaeus - One of the best experts on this subject based on the ideXlab platform.
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Compressibility Effect on coherent structures energy transfer and scaling in magnetohydrodynamic turbulence
Physics of Fluids, 2017Co-Authors: Minping Wan, Shiyi Chen, W H Matthaeus, Yan Yang, Yipeng ShiAbstract:Compressible magnetohydrodynamic (MHD) turbulence, a model often used to study space and astrophysical plasmas, differs from incompressible magnetohydrodynamic and hydrodynamic (HD) turbulence in many ways. Here direct numerical simulations of mechanically forced compressible MHD turbulence are used to study the degree to which some turbulence theories proposed in the incompressible case remain applicable in the compressible one. Several aspects of compressible turbulence are studied: (i) Intermittency in the compressible case is studied by addressing flows driven with varying forcing mechanisms; these display different features, such as compression and coherent structures. The more compressive simulation is characterized by sheet-like current density structures and shocks, which lead to saturated scaling exponents of high order structure functions of density and compressive velocity. (ii) Further investigations employing conditional averages of different energy transfer fluxes reveal that the influence o...
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on the Compressibility Effect in test particle acceleration by magnetohydrodynamic turbulence
Physics of Plasmas, 2016Co-Authors: C A Gonzalez, Pablo Dmitruk, P D Mininni, W H MatthaeusAbstract:The Effect of Compressibility in a charged particle energization by magnetohydrodynamic (MHD) fields is studied in the context of test particle simulations. This problem is relevant to the solar wind and the solar corona due to the compressible nature of the flow in those astrophysical scenarios. We consider turbulent electromagnetic fields obtained from direct numerical simulations of the MHD equations with a strong background magnetic field. In order to explore the flow Compressibility Effect over the particle dynamics, we performed different numerical experiments: an incompressible case and two weak compressible cases with Mach number M = 0.1 and M = 0.25. We analyze the behavior of protons and electrons in those turbulent fields, which are well known to form aligned current sheets in the direction of the guide magnetic field. What we call protons and electrons are test particles with scales comparable to (for protons) and much smaller than (for electrons) the dissipative scale of MHD turbulence, maint...
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on the Compressibility Effect in test particle acceleration by magnetohydrodynamic turbulence
arXiv: Plasma Physics, 2016Co-Authors: C A Gonzalez, Pablo Dmitruk, P D Mininni, W H MatthaeusAbstract:The Effect of Compressibility in charged particle energization by magnetohydrodynamic (MHD) fields is studiedin the context of test particle simulations. This problem is relevant to the solar wind and the solar corona due to the compressible nature of the flow in those astrophysical scenarios. We consider turbulent electromagnetic fields obtained from direct numerical simulations of the MHD equations with a strong background magnetic field. In order to explore the flow compressibilty Effect over the particle dynamics we performed different numerical experiments: an incompressible case, and two weak compressible cases with Mach number M = 0.1 and M = 0.25. We analyze the behavior of protons and electrons in those turbulent fields, which are well known to form aligned current sheets in the direction of the guide magnetic field. What we call protons and electrons are test particles with scales comparable to (for protons) and much smaller than (for electrons) the dissipative scale of MHD turbulence, maintaining the correct mass ratio me /mi. For these test particles we show that Compressibility enhances the efficiency of proton acceleration, and that the energization is caused by perpendicular electric fields generated between currents sheets. On the other hand, electrons remain magnetized and display an almost adiabatic motion, with no Effect of Compressibility observed. Another set of numerical experiments takes into account two fluid modifications, namely electric field due to Hall Effect and electron pressure gradient. We show that the electron pressure has an important contribution to electron acceleration allowing highly parallel energization. In contrast, no significant Effect of these additional terms is observed for the protons.
Katya M Casper - One of the best experts on this subject based on the ideXlab platform.
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Compressibility Effects in the shear layer over a rectangular cavity
Journal of Fluid Mechanics, 2016Co-Authors: Steven J Beresh, Justin L Wagner, Katya M CasperAbstract:The influence of Compressibility on the shear layer over a rectangular cavity of variable width has been studied in a free stream Mach number range of 0.6–2.5 using particle image velocimetry data in the streamwise centre plane. As the Mach number increases, the vertical component of the turbulence intensity diminishes modestly in the widest cavity, but the two narrower cavities show a more substantial drop in all three components as well as the turbulent shear stress. This contrasts with canonical free shear layers, which show significant reductions in only the vertical component and the turbulent shear stress due to Compressibility. The vorticity thickness of the cavity shear layer grows rapidly as it initially develops, then transitions to a slower growth rate once its instability saturates. When normalized by their estimated incompressible values, the growth rates prior to saturation display the classic Compressibility Effect of suppression as the convective Mach number rises, in excellent agreement with comparable free shear layer data. The specific trend of the reduction in growth rate due to Compressibility is modified by the cavity width.
M S Ruderman - One of the best experts on this subject based on the ideXlab platform.
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Compressibility Effect on the rayleigh taylor instability with sheared magnetic fields
Solar Physics, 2017Co-Authors: M S RudermanAbstract:We study the Effect of plasma Compressibility on the Rayleigh–Taylor instability of a magnetic interface with a sheared magnetic field. We assume that the plasma is ideal and the equilibrium quantities are constant above and below the interface. We derive the dispersion equation. Written in dimensionless variables, it contains seven dimensionless parameters: the ratio of plasma densities above and below the interface $\zeta$ , the ratio of magnetic field magnitude squared $\chi$ , the shear angle $\alpha$ , the plasma beta above and below the interface, $\beta_{2}$ and $\beta_{1}$ , the angle between the perturbation wave number and the magnetic field direction above the interface $\phi$ , and the dimensionless wave number $\kappa$ . Only six of these parameters are independent because $\chi$ , $\beta_{1}$ , and $\beta_{2}$ are related by the condition of total pressure continuity at the interface. Only perturbations with the wave number smaller than the critical wave number are unstable. The critical wave number depends on $\phi$ , but it is independent of $\beta_{1}$ and $\beta_{2}$ , and is the same as that in the incompressible plasma approximation. The dispersion equation is solved numerically with $\zeta= 100$ , $\chi= 1$ , and $\beta_{1} = \beta_{2} = \beta$ . We obtain the following results. When $\beta$ decreases, so does the maximum instability increment. However, the Effect is very moderate. It is more pronounced for high values of $\alpha$ . We also calculate the dependence on $\phi$ of the maximum instability increment with respect to $\kappa$ . The instability increment takes its maximum at $\phi= \phi_{\mathrm{m}}$ . Again, the decrease of $\beta$ results in the reduction of the instability increment. This reduction is more pronounced for high values of $|\phi- \phi_{\mathrm{m}}|$ . When both $\alpha$ and $|\phi- \phi_{\mathrm{m}}|$ are small, the reduction Effect is practically negligible. The theoretical results are applied to the magnetic Rayleigh–Taylor instability of prominence threads in the solar atmosphere.
Gary N Coleman - One of the best experts on this subject based on the ideXlab platform.
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a numerical study of turbulent supersonic isothermal wall channel flow
Journal of Fluid Mechanics, 1995Co-Authors: Gary N Coleman, John Kim, Robert D MoserAbstract:A study of compressible supersonic turbulent flow in a plane channel with isothermal walls has been performed using direct numerical simulation. Mach numbers, based on the bulk velocity and sound speed at the walls, of 1.5 and 3 are considered; Reynolds numbers, defined in terms of the centreline velocity and channel half-width, are of the order of 3000. Because of the relatively low Reynolds number, all of the relevant scales of motion can be captured, and no subgrid-scale or turbulence model is needed. The isothermal boundary conditions give rise to a flow that is strongly influenced by wall-normal gradients of mean density and temperature. These gradients are found to cause an enhanced streamwise coherence of the near-wall streaks, but not to seriously invalidate Morkovin's hypothesis : the magnitude of fluctuations of total temperature and especially pressure are much less than their mean values, and consequently the dominant Compressibility Effect is that due to mean property variations. The Van Driest transformation is found to be very successful at both Mach numbers, and when properly scaled, statistics are found to agree well with data from incompressible channel flow results.