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中国物理学会期刊

不同磁场构型下Richtmyer-Meshkov不稳定性的数值研究及动态模态分解

Numerical investigations of Richtmyer-Meshkov instability in different magnetic field configurations and the corresponding dynamic mode decomposition

CSTR: 32037.14.aps.68.20190410
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  • 基于磁流体动力学, 本文通过数值模拟对不同磁场构型下轻质气柱界面Richtmyer-Meshkov不稳定性的演化过程进行了研究. 结果显示: 磁场对波系演化影响甚微, 但能抑制界面不稳定性发展, 且横向磁场抑制效果更好. 无磁场时, 界面形成涡串, SF6射流穿过下游界面; 有磁场时, 界面光滑无涡串. 其中, 横向磁场下界面更光滑, SF6射流不再穿过界面. 此外, 由于Richtmyer-Meshkov不稳定性的作用, 磁力线在气柱界面发生扭曲, 且上游界面处磁力线扭曲程度更大, 产生强洛伦兹力, 使涡量分层明显; 下游界面处, 纵向磁场产生的洛伦兹力较横向磁场更小, 涡层之间相互干扰. 最后, 本文将动态模态分解用于界面不稳定性研究, 发现: 磁场作用下界面仍存在小涡, 且纵向磁场下扰动更多. 第一模态的稳定涡结构能反映主要流场信息, 第二到第四模态下的小涡频率依次增加, 且无磁场、纵向和横向磁场的同一模态下, 小涡频率依次减小. 因而磁场能抑制小涡频率, 且横向磁场抑制效果更好.

     

    Based on magnetohydrodynamics(MHD), the evolution of the Richtmyer-Meshkov instability in different magnetic field configurations are studied. To ensure the zero magnetic divergence, an unsplit integration algorithm is adopted by combining corner transport upwind and constrained transport (CTU+CT) algorithm. The second order Godunov flux is obtained by using piecewise parabolic method(PPM) to construct conserved variables. The numerical results show that the evolution of complex wave patterns is not affected by magnetic fields, but the interface instability is compressed by magnetic field, especially in the case of transverse magnetic fields. Specifically, whether there exists magnetic field or not, irregular reflections occur outside the cylinder. Meanwhile, the central part of incident shock wave interacts with the density interface and generates the transmitted shock wave. Subsequently, the transmitted shock wave oscillates back and forth inside the cylinder, forming a transmission-reflection structure multiple times. Besides, in the absence of magnetic field, the density interface rolls up with a series of vortex sequences and an SF6 jet surrounded by vortex pairs appears. Then the SF6 jet passes through the downstream interface. In a longitudinal magnetic field, although density interface is smooth, a few vortex sequences still exist in the downstream interface and SF6 jet can still pass through downstream interface. However, in the case of transverse magnetic field, the interface is much smoother than in the other cases and the SF6 jet cannot pass through the downstream interface. The quantitative study also indicate that the increase of characteristic sizes is suppressed by the magnetic field. In addition, because of the influence of Richtmyer-Meshkov instability, magnetic lines are distorted near density interfaces. More distortions can be observed in the upstream interfaces, resulting in strong Lorentz forces in that area, which leads to the long distance between two vortex sheets distributed along two sides of the interface. In the downstream interfaces Lorentz forces are rather small, but the forces are even smaller in the longitudinal magnetic field, as a result vortex sheets interact with each other in that area. Furthermore, the dynamic mode decomposition(DMD) is primarily used in this paper and the results illustrate that even controlled by magnetic fields, vortex sequences can still exist, especially in the case of longitudinal magnetic field. For all cases, the first DMD modes all illustrate that a stable mode is the dominated feature of fluid field, and the following second to fourth mode show that the strength of vortex sequences decreases while their frequencies increase continually. Besides, for the same modes, the frequency of vortex sequences is reduced by magnetic fields, especially by the transverse magnetic field.

     

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