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首页> 外文期刊>Applied Mathematical Modelling >Direct numerical simulation as a tool for understanding MHD liquid metal turbulence
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Direct numerical simulation as a tool for understanding MHD liquid metal turbulence

机译：直接数值模拟作为了解MHD液态金属湍流的工具

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摘要

The method of direct numerical simulation (DNS) is applied to investigate the most general properties of turbulent flows of liquid metals in the presence of a constant magnetic field. Various aspects of the flow transformation into an anisotropic state are thoroughly examined. The flow is assumed to be homogeneous and the problem is reduced to the classical case of a turbulent flow in a 3D box with periodic boundary conditions. In the framework of this formulation, three specific types of the flow are considered, which are the forced flow, thermal convection, and freely decaying flow. To investigate the long-time evolution of an initially isotropic flow a large-scale forcing is applied to maintain the flow energy at a statistically steady level. The evolution is found to depend strongly on the magnetic interaction parameter (Stuart number). In the case of small Stuart number, the flow remains three-dimensional, turbulent, and approximately isotropic. At large Stuart number (strong magnetic field) the turbulence is suppressed rapidly and the flow becomes two-dimensional and laminar. Very interesting is the intermittent flow evolution detected at moderate Stuart number. Long periods of almost two-dimensional, laminar behaviour are interrupted by strong turbulent three-dimensional bursts. The influence of a constant magnetic field on scalar transport properties of liquid metal turbulence is investigated using the simplified formulation of a homogeneous flow driven by an imposed mean temperature gradient. The flow structure is dominated by two turbulent antiparallel jets providing an effective mechanism of heat transfer. It is shown that the magnetic field parallel to the mean temperature gradient stabilizes the jets and, thus, enhances heat transfer considerably. In the third part, freely decaying MHD turbulence is considered. Numerical simulations are applied to verify the theoretical model proposed in [J. Fluid Mech. 336 (1997) 123]. In particular, it is confirmed that the structure of viscous dissipation and evolution of perpendicular length scale are affected only slightly by the magnetic field. A simple approximation for the mean Joule dissipation is proposed.
机译：直接数值模拟（DNS）方法用于研究在恒定磁场存在下液态金属湍流的最一般特性。彻底研究了流体转化为各向异性状态的各个方面。假定流动是均匀的，并且问题被简化为具有周期性边界条件的3D盒子中湍流的经典情况。在此公式的框架中，考虑了三种特定类型的流，即强制流，热对流和自由衰减流。为了研究初始各向同性流的长期演变，可以应用大规模强迫将流能保持在统计稳定水平。发现其演化在很大程度上取决于磁相互作用参数（斯图尔特数）。在小斯图尔特数的情况下，流动保持三维，湍流和近似各向同性。在大斯图尔特数（强磁场）下，湍流被迅速抑制，流动变成二维和层流。非常有趣的是在中等Stuart数下检测到间歇性流动演变。长时间的几乎二维的层流行为被强力的湍流三维突发中断。使用由施加的平均温度梯度驱动的均质流的简化公式，研究了恒定磁场对液态金属湍流标量传输特性的影响。流动结构由两个湍流的反平行射流控制，提供了有效的传热机制。结果表明，平行于平均温度梯度的磁场使射流稳定，从而显着增强了热传递。在第三部分中，考虑了自由衰减的MHD湍流。数值模拟被应用来验证[J．流体机械。 336（1997）123]。特别地，已经证实，粘性耗散的结构和垂直长度尺度的演变仅受到磁场的轻微影响。提出了平均焦耳耗散的简单近似。

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