超声速剪切层的混合问题

Issues on the mixing of supersonic shearing layer

用数值模拟和物理分析相结合的方法，研究了入口速度剖面具有阶跃型的超声速混合层的混合机理和特征。指出在上游起始阶段，混合层产生的展向涡是平行发展的，近似二维情况，涡的运动先是稳定的，后经非线性不稳定性和分叉，并进一步发展为两涡合并或两合并涡的再合并。随着向下游的增加，由于展向出现物理量的三维效应，展向涡要弯曲，并且沿其轴向要分叉演化，产生一个或多个极限环，这就开始改变二维涡合并的发展规律，产生流向和法向的旋涡。如果进一步走向下游，展向涡有的要产生螺旋型和泡型破裂，并且破裂涡与上游来的涡要混掺、缠绕，形成中间夹有小涡的新的大涡相干结构，相应流向、法向涡进一步非线性增强，流动完全改变了二维混合规律，变成真正的三维混合。由于涡的破裂带有随机和间歇性，这种混合过程也具带有随机和间歇性。通过本文对概率密度分布、分数维和间歇因子的计算，证明在流动出口处，基本上已达到了湍流。文中还给出了转捩发生的特征和机理。

The character and mechanism of supersonic mixing layer with entrance velocity in step-profile are studiedthrough numerical simulation and physical analysis. In the forepart of the 3D flow field, the evolution of the spanwise vortexes in the shearing layer is similar to the evolution of vortexes of 2D flow field, which firstly move downstream stably and then merge bifurcately. When moving downstream, because of the evolvement of flow parameters, vortex will evolve along its axis line and be transfigured, bent, twisted and combined. Hopf bifurcation will show in vortex structures also. So the rules of vortex mergence of 2D flow are invalidated and some of the vortexes will burst. The burst vortex and its neighboring vortexes will blend and twist. As a result, coherent structure with intermittence and randomicity are formed, which is made up of many small vortexes and will lead to the transition of the mixing flow. Physical analysis show that transition and turbulence can be observed at the outflow boundary. The character and mechanism of transition are also studied in this paper.