论文标题
探测可压缩的MHD湍流中耗散的性质
Probing the nature of dissipation in compressible MHD turbulence
论文作者
论文摘要
语境。湍流的一个必不可少的方面是级联能量的时空间歇性，导致高消散的连贯结构。目标。在这项工作中，我们试图在衰减等温磁流体动力学（MHD）湍流中系统地研究强烈耗散区域的物理性质。方法。我们使用基于网格的可压缩等温衰减MHD湍流的模拟探测湍流耗散。我们在解决和控制耗散方面接受前所未有的护理：我们设计了由于数值方案而在局部恢复耗散的方法。我们在局部研究流体状态变量梯度的几何形状。我们开发了一种评估模拟中最大梯度的物理性质并估算其行进速度的方法。最后，我们研究了他们的统计数据。结果。我们发现，强烈的耗散区域主要对应于床单：局部，密度，速度和磁场主要在一个方向上有所不同。我们将这些高度耗散的区域确定为快速/慢速冲击或alfv {é} n不连续性（帕克床单或旋转不连续性）。在这些结构上，我们发现与1D平面稳态的主要偏差是结构平面的质量损失。我们研究了初始条件的影响，这些条件在早期产生不同的烙印对这四个类别之间的相对分布。但是，当这些差异大约一个失误时间后，这些差异被弱化的alfv {é} n不连续性所支配时。我们表明，磁性PRANDTL数量对这些不连续性的统计数据几乎没有影响，但它控制着其中的欧姆与粘性加热率。最后，我们发现结构的入口特征（例如入口速度和磁压）密切相关。结论。这些新方法允许将开发的可压缩湍流视为强烈耗散结构的统计收集。这可用于使用详细的1D模型的后处理3D湍流，可与观测值进行比较。它还可以揭示作为制定湍流的新动力学特性的框架。
Context. An essential facet of turbulence is the space-time intermittency of the cascade of energy that leads to coherent structures of high dissipation. Aims. In this work, we attempt to investigate systematically the physical nature of the intense dissipation regions in decaying isothermal magnetohydrodynamical (MHD) turbulence. Methods. We probe the turbulent dissipation with grid based simulations of compressible isothermal decaying MHD turbulence. We take unprecedented care at resolving and controlling dissipation: we design methods to locally recover the dissipation due to the numerical scheme. We locally investigate the geometry of the gradients of the fluid state variables. We develop a method to assess the physical nature of the largest gradients in simulations and to estimate their travelling velocity. Finally we investigate their statistics. Results. We find that intense dissipation regions mainly correspond to sheets: locally, density, velocity and magnetic fields vary primarily across one direction. We identify these highly dissipative regions as fast/slow shocks or Alfv{é}n discontinuities (Parker sheets or rotational discontinuities). On these structures, we find the main deviation from 1D planar steady-state is mass loss in the plane of the structure. We investigate the effect of initial conditions which yield different imprints at early time on the relative distributions between these four categories. However, these differences fade out after about one turnover time, when they become dominated by weakly compressible Alfv{é}n discontinuities. We show that the magnetic Prandtl number has little influence on the statistics of these discontinuities, but it controls the Ohmic vs viscous heating rates within them. Finally, we find the entrance characteristics of the structures (such as entrance velocity and magnetic pressure) are strongly correlated. Conclusions. These new methods allow to consider developed compressible turbulence as a statistical collection of intense dissipation structures. This can be used to post-process 3D turbulence with detailed 1D models apt for comparison with observations. It could also reveal useful as a framework to formulate new dynamical properties of turbulence.
