三维方腔介电液体电对流的数值模拟研究

本文对封闭方腔内介电液体电对流进行了三维数值模拟研究.方腔的6个边界为固壁;4个侧边界为电绝缘边界;上下界面为两个电极.直流电场作用在从底部电极注入的自由电荷上,从而对液体施加库伦体积力并驱动流体流动形成电对流.为了求解这一物理问题,发展了一种二阶精度的有限体积法来求解完整的控制方程,包括Navier-Stokes方程和一组简化的Maxwell方程.考虑到电荷密度方程的强对流占优特性,采用了全逆差递减格式来求解该方程,获得了准确有界的解.通过研究发现,该流动在有限振幅区内的分叉类型为亚临界,即系统存在一个线性和非线性临界值,分别对应流动的开始和终止.由于非线性临界值比线性值小,因此两个临界值之间有一个迟滞回线.与无限大域中的自由对流相比,侧壁施加的额外约束改变了流场结构,使这两个临界值均有所增大.此外,还讨论了电荷密度和速度场的空间分布特征,发现电荷密度分布中存在电荷空白区.最后对更小空间尺寸情况计算结果表明,流动的线性分叉类型为超临界.本文的结果拓展了已有的二维有限空间内电对流的研究,并为三维电对流的线性和弱非线性理论分析提供参考. 

NUMERICAL ANALYSIS OF THREE-DIMENSIONAL ELECTRO-CONVECTION OF DIELECTRIC LIQUIDS IN A CUBICAL CAVITY1)

A full three-dimensional numerical study on the electro-convection of dielectric liquids contained in a cubical cavity is reported. All boundaries are solid walls. The four lateral sides are electrically insulating and the top and bottom walls are planar electrodes. The flow motion is driven by the volumetric Coulomb force exerting on the free charge carriers introduced by a strong unipolar injection from the bottom electrode. The charge injection takes place due to the electro-chemical reaction at the interface between liquid and electrode. The unsteady Navier-Stokes equations and a reduced set of Maxwell's equations in the limit of electroquasistatics are solved using an efficient finite volume method with 2$^{rm nd}$ order accuracy in space and time. Considering the strong convection-dominating nature of the charge conservation equation, a total variation diminishing scheme is specially used to solve this equation in order to obtain physically-bounded and accurate solution. It is found that the flow is characterized by a subcritical bifurcation in the finite amplitude regime. A linear stability criterion and a nonlinear one, which correspond respectively to the onset and stop of the flow motion, are numerically determined. Since the nonlinear criterion is smaller than the linear one, there exists a hysteresis loop. Compared to the free convection in the infinitely large domain case, the constraint imposed by the lateral walls dramatically changes the flow structure and increases the two criteria. In addition, the spatial features of charge density distribution and velocity field are discussed in details. A central region free of charges is observed. This void region is formed due to the competition between the fluid velocity and the drift velocity, and it is closely related to the subcritical bifurcation feature of the flow. In addition, computations are also performed with a case with smaller domain sizes, and the results show that the linear bifurcation of the flow is supercritical. Once the system losses its linear stability, a steady convection state without charge void region is reached. The present results extend previous research on the two-dimensional electro-convection in confined cavities, and they provide reference for the three-dimensional theoretical analysis of the linear and weakly nonlinear stability.