Cross helicity and related dynamo
Cross helicity and related dynamo
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DOI：
10.1080/03091929.2012.754022
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发表时间：
2013-02
影响因子：
1.3
通讯作者：
N. Yokoi
中科院分区：
文献类型：
--
作者：
N. Yokoi
湍流交叉螺旋度与电动势的平均涡度和雷诺应力张量的平均磁场应变的耦合系数直接相关。这表明，在全球不均匀流动和磁场结构存在的情况下，交叉螺旋度效应是重要的。由于这种大尺度结构在地球/天体物理现象中普遍存在，交叉螺旋度效应有望在地球/天体物理流动中发挥重要作用。在湍流交叉螺旋度存在的情况下，平均涡旋运动对湍流电动势有贡献。这种效应产生的磁场被称为交叉螺旋度发电机。介绍了交叉螺旋度发电机的几个特点。平均电流密度J与平均涡度Ω的对准以及平均磁场B与速度U的对准被认为是发电机的特征之一。与螺旋度或α效应的情况不同，在湍流电动势中，J与B对齐，在交叉螺旋度发电机中，我们通常有有限的平均场洛伦兹力J×B。这给出了交叉螺旋度效应的一个显著特征。通过考虑动量方程中交叉螺旋度的影响，我们看到了这种影响的几个有趣的结果。湍流交叉螺旋度与平均磁切变相结合，减小了湍流或涡动粘性的影响。流动诱导是这种效应的一个重要结果。交叉螺旋度发电机的一个关键问题是研究湍流中交叉螺旋度的存在方式和程度。在交叉螺旋度输运方程的基础上，讨论了交叉螺旋度输运的产生机理。还介绍了交叉螺旋度发电机的基本概念在数值验证方面的一些最新进展。
The turbulent cross helicity is directly related to the coupling coefficients for the mean vorticity in the electromotive force and for the mean magnetic-field strain in the Reynolds stress tensor. This suggests that the cross-helicity effects are important in the cases where global inhomogeneous flow and magnetic-field structures are present. Since such large-scale structures are ubiquitous in geo/astrophysical phenomena, the cross-helicity effect is expected to play an important role in geo/astrophysical flows. In the presence of turbulent cross helicity, the mean vortical motion contributes to the turbulent electromotive force. Magnetic-field generation due to this effect is called the cross-helicity dynamo. Several features of the cross-helicity dynamo are introduced. Alignment of the mean electric-current density J with the mean vorticity Ω , as well as the alignment between the mean magnetic field B and velocity U , is supposed to be one of the characteristic features of the dynamo. Unlike the case in the helicity or α effect, where J is aligned with B in the turbulent electromotive force, we in general have a finite mean-field Lorentz force J × B in the cross-helicity dynamo. This gives a distinguished feature of the cross-helicity effect. By considering the effects of cross helicity in the momentum equation, we see several interesting consequences of the effect. Turbulent cross helicity coupled with the mean magnetic shear reduces the effect of turbulent or eddy viscosity. Flow induction is an important consequence of this effect. One key issue in the cross-helicity dynamo is to examine how and how much cross helicity can be present in turbulence. On the basis of the cross-helicity transport equation, its production mechanisms are discussed. Some recent developments in numerical validation of the basic notion of the cross-helicity dynamo are also presented.