Dynamics of separation bubble dilation and collapse in shock wave/turbulent boundary layer interactions

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.