The Magnetohydrodynamic Kelvin-Helmholtz Instability. III. The Role of Sheared Magnetic Field in Planar Flows
The Magnetohydrodynamic Kelvin-Helmholtz Instability. III. The Role of Sheared Magnetic Field in Planar Flows
复制标题
DOI：
10.1086/308259
复制
发表时间：
1999-09
期刊：
影响因子：
--
通讯作者：
Hyunju Jeong;D. Ryu;T. Jones;A. Frank
中科院分区：
文献类型：
--
作者：
Hyunju Jeong;D. Ryu;T. Jones;A. Frank
我们对2.5维可压缩流体的磁流体动力学（MHD）Kelvin-Helmholtz（KH）不稳定性的非线性演化进行了模拟，扩展了Frank等人和Jones等人以前的工作。在目前的工作中，我们模拟了x-y平面中的流动，其中均匀强度的“剪切”磁场从z-方向到x方向，与流场对齐。速度转捩的音速马赫数为1。如果磁场强度足够大，使得阿尔夫维尼克马赫数MA = U 0/cA < 2，那么这种在x方向上包含均匀磁场的流动是线性稳定的。然而，该限制并不直接适用于剪切磁场，因为z场分量对线性稳定性几乎没有影响。因此，如果磁剪切层包含在速度剪切层内，KH不稳定性仍然可能增长，即使当磁场强度相当大时。因此，在这里我们考虑一个广泛的剪切场强度范围，包括阿尔夫维尼克马赫数，MA = 142.9到2。我们专注于动态演化的流体功能，动能耗散，和混合的两层之间的流体，考虑其依赖于磁场强度的这种几何形状。与我们先前在x-y平面中使用均匀磁场进行的模拟有许多不同之处。对于后一种更简单的情况，我们发现了一系列明显的行为，随着场强的增加，从不稳定性演变为几乎稳定且耗散增强的猫眼涡旋的接近流体动力学的流动，到磁场破坏猫眼的流动。一旦猫眼形成，最后，在场线拉伸稳定速度剪切层之前，流动的演变很少。磁剪切的引入可以允许形成猫眼状涡旋，即使当磁场比上面给出的标称线性不稳定性极限更强时。然而，对于强场，涡相对于初始剪切层是不对称的，因此随后的耗散比可比场强度的均匀场情况下增强。事实上，只要磁场在涡流翻转时间内达到一定程度的动力学重要性，通过磁剪切引入的不对称性将增加流动的复杂性，并随之增加耗散和混合。两层之间的流体混合的程度强烈地受磁场强度的影响。当涡旋在瞬时重联期间被磁张力扰乱时，流体的混合最为有效，随后出现局部混乱行为。
We have carried out simulations of the nonlinear evolution of the magnetohydrodynamic (MHD) Kelvin-Helmholtz (KH) instability for compressible fluids in 2.5 dimensions, extending our previous work by Frank et al. and Jones et al. In the present work we have simulated flows in the x-y plane in which a "sheared" magnetic field of uniform strength smoothly rotates across a thin velocity shear layer from the z-direction to the x-direction, aligned with the flow field. The sonic Mach number of the velocity transition is unity. Such flows containing a uniform field in the x-direction are linearly stable if the magnetic field strength is great enough that the Alfvénic Mach number MA = U0/cA < 2. That limit does not apply directly to sheared magnetic fields, however, since the z-field component has almost no influence on the linear stability. Thus, if the magnetic shear layer is contained within the velocity shear layer, the KH instability may still grow, even when the field strength is quite large. So, here we consider a wide range of sheared field strengths covering Alfvénic Mach numbers, MA = 142.9 to 2. We focus on dynamical evolution of fluid features, kinetic energy dissipation, and mixing of the fluid between the two layers, considering their dependence on magnetic field strength for this geometry. There are a number of differences from our earlier simulations with uniform magnetic fields in the x-y plane. For the latter, simpler case we found a clear sequence of behaviors with increasing field strength ranging from nearly hydrodynamic flows in which the instability evolves to an almost steady cat's eye vortex with enhanced dissipation, to flows in which the magnetic field disrupts the cat's eye once it forms, to, finally, flows that evolve very little before field-line stretching stabilizes the velocity shear layer. The introduction of magnetic shear can allow a cat's eye-like vortex to form, even when the field is stronger than the nominal linear instability limit given above. For strong fields that vortex is asymmetric with respect to the preliminary shear layer, however, so the subsequent dissipation is enhanced over the uniform field cases of comparable field strength. In fact, so long as the magnetic field achieves some level of dynamical importance during an eddy turnover time, the asymmetries introduced through the magnetic shear will increase flow complexity and, with that, dissipation and mixing. The degree of the fluid mixing between the two layers is strongly influenced by the magnetic field strength. Mixing of the fluid is most effective when the vortex is disrupted by magnetic tension during transient reconnection, through local chaotic behavior that follows.