Although several mechanisms have been suggested as explanations for the low-frequency unsteadiness in shock wave/turbulent boundary layer interactions, questions remain on causes and effects. In this effort, we examine the observed asymmetry in large-scale shock motions to highlight which of the suggested mechanisms is most consistent with shock-speed observations and accompanying separation dynamics. The analysis is based on a flowfield obtained from a validated large eddy simulation of a fully separated interaction. A statistical analysis is used to determine the speed of bubble collapse relative to dilation. The low-pass filtering required to separate upstream from downstream motions in the presence of higher-frequency jitter is accomplished with a relatively new technique, empirical mode decomposition, that is very appropriate for this purpose. The dynamics of bubble dilation versus collapse are then elaborated with conditional dynamic mode decomposition (DMD) analyses on the respective pressure fields. Bubble breathing is shown to have a different structure during dilation than during collapse—larger structures are observed during collapse when fluid is expelled from the bubble. The nature of the DMD mode associated with Kelvin–Helmholtz (K–H) shedding in the mixing layer also differs between dilation and collapse: When the bubble is dilating, the structures at the dominant K–H frequency are larger than when the bubble is collapsing. Additionally, a link is established between the convecting K–H structures and corrugation observed along the reflected shock. Some aspects of the nature of the asymmetry are linked to the ease of eddy formation (K–H structures), which plays an important role in the collapse of the bubble.

中文翻译：

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冲击波/湍流边界层相互作用中分离气泡膨胀和坍塌的动力学

尽管已经提出了几种机制来解释冲击波/湍流边界层相互作用中的低频不稳定，但关于因果关系的问题仍然存在。在这项工作中，我们检查了在大规模冲击运动中观察到的不对称性，以强调哪种建议机制与冲击速度观察和伴随的分离动力学最一致。该分析基于从完全分离的相互作用的经过验证的大涡模拟获得的流场。统计分析用于确定气泡破裂相对于膨胀的速度。在存在较高频率抖动的情况下将上游运动与下游运动分开所需的低通滤波是通过一种相对较新的技术实现的，即经验模式分解，非常适合此目的。然后通过对相应压力场的条件动态模式分解 (DMD) 分析详细阐述气泡膨胀与坍塌的动力学。气泡呼吸在膨胀期间与坍缩期间的结构不同——当流体从气泡中排出时，在坍缩期间观察到更大的结构。与混合层中开尔文-亥姆霍兹 (K-H) 脱落相关的 DMD 模式的性质在膨胀和坍塌之间也有所不同：当气泡膨胀时，主导 K-H 频率的结构比气泡大时崩溃。此外，在对流 K-H 结构和沿反射冲击观察到的波纹之间建立了联系。不对称性质的某些方面与涡流形成的难易有关（K-H 结构），