标题： 非模态增长和磁流体动力学剪切流中的最优扰动 显示英文标题

标题： Nonmodal growth and optimal perturbations in magnetohydrodynamic shear flows

摘要： 在天体物理剪切流中，当存在足够强的顺流磁场时，Kelvin-Helmholtz (KH) 不稳定性通常会被磁张力抑制。 这通常用于根据是否观察到由剪切驱动的波动来推断系统中磁场强度的上限（或下限），其依据是如果没有线性不稳定性，波动无法增长。 相反，通过计算这种系统中小幅度扰动在有限时间内可以达到的最大增长率，我们表明即使流动是亚阿尔芬的，扰动的能量也可以增长几个数量级，这表明即使在强磁场存在的情况下，由剪切驱动的湍流也是可能的，并对基于观测到的或未观测到的剪切驱动波动的存在与否所做出的推断提出了挑战。 我们进一步表明，与流体动力学情况相比，磁场引入了额外的非模态增长机制，并且二维模拟忽略了这些增长机制的关键方面。 显示英文摘要

摘要： In astrophysical shear flows, the Kelvin-Helmholtz (KH) instability is generally suppressed by magnetic tension provided a sufficiently strong streamwise magnetic field. This is often used to infer upper (or lower) bounds on field strengths in systems where shear-driven fluctuations are (or are not) observed, on the basis that fluctuations cannot grow in the absence of linear instability. On the contrary, by calculating the maximum growth that small-amplitude perturbations can achieve in finite time for such a system, we show that perturbations can grow in energy by orders of magnitude even when the flow is sub-Alfv\'enic, suggesting that shear-driven turbulence is possible even in the presence of strong magnetic fields, and challenging inferences from the observed presence or absence of shear-driven fluctuations. We further show that magnetic fields introduce additional nonmodal growth mechanisms relative to the hydrodynamic case, and that 2D simulations miss key aspects of these growth mechanisms.