突扩分叉流动的数值研究：发散和几何非对称性的影响
Numerical Study of Bifurcating Flow through Sudden Expansions: Effect of divergence and geometric asymmetry
摘要：
采用传统计算流体力学和格子玻尔兹曼方法，对增大截面通道中对称和轻微非对称突扩（突扩比为1：3）的层流流动进行了数值研究。发散的通道壁的效果，突然膨胀后，对流动和再循环的对称性进行了研究，为各种雷诺数。可以看出，壁的角度在破坏流动的对称性方面起着积极的作用。对于非平行壁，对称性破缺分叉现象不再存在，对称性的丧失是一个渐进的过程。还通过考虑两种类型的不对称性来研究几何不对称性对流动的影响--第一种类型是在膨胀平面上，两侧的台阶高度不等，而第二种类型涉及通道壁的不同倾斜度与流入方向。本研究揭示了小的不对称性阻碍了急剧的转变，从而平滑了干草叉分叉。增加的不对称性最终导致控制方程的唯一解，表明完全失去了分叉模式。
英文摘要：
A numerical study of laminar flow through symmetric and slightly asymmetric sudden expansion, of expansion ratio 1:3, in channels with increasing cross section, is carried out using two different approaches - Conventional CFD and Lattice Boltzmann Method. The effect of divergence of walls of the channel, after a sudden expansion, on the symmetry of flow and recirculation is studied for various Reynolds numbers. It is seen that the angles of the walls play an active role in disrupting the symmetry of flow. For non-parallel walls, the symmetry breaking bifurcation phenomenon no longer exists and the loss of symmetry is a gradual process. The effect of asymmetry of geometry on flow is also studied by considering two types of asymmetry - First type is at the plane of expansion where the steps on either side are of unequal heights, while the second one deals the walls of the channel are at a different inclination with the direction of inflow. The present study reveals that small asymmetry impedes the sharp transition and thus smoothens the pitchfork bifurcation. Increased asymmetry finally leads to the unique solution of the governing Equations, indicating complete loss of bifurcation pattern.
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