磁流体湍流中的结构和能量传输

磁流体湍流中存在多种相干结构,包括电流和涡旋结构。文章主要对近几年有关磁流体湍流相干结构及其相关的能量传输的一些工作进行了介绍。不同于中性流体湍流中的管状结构,磁流体中的相干结构多呈现为片状,强电流片附近也存在强涡片结构。磁流体湍流中的能量级串使能量跨尺度从大尺度传递至小尺度;与中性流体湍流能量传输主要集中在相近尺度上不同,磁流体湍流除了动能传输,还有磁能传输,以及动能与磁能之间的跨尺度互相转换。研究表明,磁流体湍流中的能量传输及耗散具有强间歇性,集中在较小区域范围,且与相干结构存在关联。在存在背景磁场的情况下,磁流体湍流中的相干结构在背景磁场方向上被拉长,而同样方向上的能量传输受到抑制。在高马赫数的情况下,激波的产生会使能量传输明显增强,同时带来很强的间歇性,结构函数标度律远远偏离线性标度关系,并且随着阶数的增加,标度指数会达到饱和值。

Structures and energy transfer in MHD turbulence

There are several types of coherent structures in magnetohydrodynamic(MHD)turbulence,including current and vortex structures.The article reviews some of the recent works on the study of coherent structures and the related energy transfer in MHD turbulence.Different from the tubular structures in hydrodynamic(HD)turbulence,the coherent structures in MHD are mostly sheet-like structures,with strong current sheets accompanied by sheet-like vortex structures.Unlike the energy cascade in HD turbulence,where kinetic energy is transferred from large scales to small scales locally,the energy cascade in MHD is more complicated.In addition to the local transfer of kinetic energy,there is also a local transfer of magnetic energies across scales,as well as a non-local conversion between kinetic and magnetic energies.Studies have shown that energy transfer and dissipation in MHD turbulence are strongly intermittent,concentrated in a small area,and associated with coherent structures.In the presence of a strong background magnetic field,the coherent structures in MHD turbulence are elongated in the direction of the background magnetic field,while energy transfer in the same direction is suppressed.In the case of compressible MHD turbulence,the generation of shocks significantly enhances the energy transfer,and at the same time brings a strong intermittency.The scaling of density structure functions is found far from a linear scale,and scaling exponent saturates with the increase of the order of structure functions.