On shock-wave unsteadiness in separated flows

Separated external flows in the high-speed (compressible) regime present interesting fluid-dynamical features. An aspect of particular interest, which forms the broad theme of this article, is the temporal unsteadiness that is inherent to such flows, with the unsteadiness being driven by interaction of shock waves with flow regions of high shear and/or separation. In certain cases the unsteadiness is primarily characterized by periodic shock wave motion with large spatial amplitudes, which presents the flow with a visually spectacular character in addition to a rich set of dynamics. Historically, a class of separated external flows have been studied through canonical problems involving geometries with a slender fore-body and a blunt aft-body, and also double-cone and double-wedge geometries. This article provides a brief review of related literature, and presents a perspective on these canonical flows which emphasizes the underlying mechanisms that give rise to, and sustain, unsteadiness.