Shock/Turbulent Boundary Layer Interactions: Review of Recent Work on Sources of Unsteadiness

Shock wave / boundary layers interactions occur in wide-range of supersonic internal and external flows, and often these interactions are associated with severe turbulent boundary layer separation. The shock-induced separation is associated with unsteady motions that are characterized by a wide range of frequencies, which include those characteristic of the upstream boundary layer as well as motions whose frequencies are about two orders of magnitude lower. Despite significant work over the past few decades, the source of the low-frequency motions is still an open question and is currently a topic of intense debate. Owing to a flurry of activity over the past decade on this single topic, both by computational and experimental researchers, we are close to developing a comprehensive understanding of the mechanisms that lead to low-frequency unsteadiness. Recent investigations on the source of low-frequency unsteadiness have provided what seems to be contradictory evidence since some indicate that the driving mechanism resides in the upstream boundary layer, whereas others suggest the dominant mechanism is an intrinsic instability of the separated flow. In this paper, based on a review of the evidence, we argue for a view where both processes are present in all shock-induced turbulent separated flows