The impact of magnetic fields on momentum transport and saturation of shear-flow instability by stable modes
Autor: | Ellen G. Zweibel, M. J. Pueschel, J. M. Schroeder, A. E. Fraser, Paul Terry |
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Přispěvatelé: | Science and Technology of Nuclear Fusion |
Jazyk: | angličtina |
Rok vydání: | 2021 |
Předmět: |
Physics
Field (physics) Fluid Dynamics (physics.flu-dyn) FOS: Physical sciences Field strength Reynolds stress Mechanics Physics - Fluid Dynamics Dissipation Condensed Matter Physics 01 natural sciences Instability Physics - Plasma Physics 010305 fluids & plasmas Momentum Plasma Physics (physics.plasm-ph) Astrophysics - Solar and Stellar Astrophysics 0103 physical sciences Magnetohydrodynamics Shear flow 010303 astronomy & astrophysics Solar and Stellar Astrophysics (astro-ph.SR) |
Zdroj: | Physics of Plasmas, 28(2):022309. American Institute of Physics Physics of Plasmas, 28, 022309 |
ISSN: | 1070-664X |
Popis: | The Kelvin-Helmholtz (KH) instability of a shear layer with an initially-uniform magnetic field in the direction of flow is studied in the framework of 2D incompressible magnetohydrodynamics with finite resistivity and viscosity using direct numerical simulations. The shear layer evolves freely, with no external forcing, and thus broadens in time as turbulent stresses transport momentum across it. As with KH-unstable flows in hydrodynamics, the instability here features a conjugate stable mode for every unstable mode in the absence of dissipation. Stable modes are shown to transport momentum up its gradient, shrinking the layer width whenever they exceed unstable modes in amplitude. In simulations with weak magnetic fields, the linear instability is minimally affected by the magnetic field, but enhanced small-scale fluctuations relative to the hydrodynamic case are observed. These enhanced fluctuations coincide with increased energy dissipation and faster layer broadening, with these features more pronounced in simulations with stronger fields. These trends result from the magnetic field reducing the effects of stable modes relative to the transfer of energy to small scales. As field strength increases, stable modes become less excited and thus transport less momentum against its gradient. Furthermore, the energy that would otherwise transfer back to the driving shear due to stable modes is instead allowed to cascade to small scales, where it is lost to dissipation. Approximations of the turbulent state in terms of a reduced set of modes are explored. While the Reynolds stress is well-described using just two modes per wavenumber at large scales, the Maxwell stress is not. 39 pages, 17 figures, preprint format |
Databáze: | OpenAIRE |
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