The impact of magnetic fields on momentum transport and saturation of shear-flow instability by stable modes
The impact of magnetic fields on momentum transport and saturation of shear-flow instability by stable modes
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DOI：
10.1063/5.0034575
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发表时间：
2020-10
影响因子：
2.2
通讯作者：
A. Fraser;P. Terry;E. Zweibel;M. Pueschel;J. Schroeder
中科院分区：
文献类型：
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作者：
A. Fraser;P. Terry;E. Zweibel;M. Pueschel;J. Schroeder
在二维不可压缩磁流体力学框架下，利用直接数值模拟方法研究了初始磁场沿流动方向均匀分布的剪切层的Kelvin-Helmholtz（KH）不稳定性.剪切层在没有外力作用的情况下自由演化，并随着湍流应力传递动量而在时间上变宽。与流体力学中的KH-不稳定流一样，在没有耗散的情况下，这里的不稳定性的特点是每个不稳定模式都有一个共轭稳定模式。稳定的模式被证明是运输动量的梯度，收缩层的宽度，每当他们超过不稳定的模式的振幅。在弱磁场的模拟中，线性不稳定性受磁场的影响最小，但观察到相对于流体动力学情况的增强的小尺度波动。这些增强的波动与增加的能量耗散和更快的层加宽相一致，这些特征在具有更强场的模拟中更加明显。这些趋势是由于磁场减少了稳定模式相对于能量转移到小尺度的影响。随着场强的增加，稳定模式变得不那么受激发，从而逆着其梯度传输更少的动量。此外，由于稳定模式，否则将转移回驱动剪切的能量反而被允许级联到小尺度，在那里它被耗散掉。近似的湍流状态方面的一组减少的模式进行了探索。在大尺度下，每个波数只用两个模就可以很好地描述雷诺应力，而麦克斯韦应力则不然。
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.