Propagating shock-boundary layer interaction with application to unstart

Unstart is a feared failure mode for scramjet engines, characterized by the upstream motion of a shock train in the isolator. In many cases this is surprisingly slow, typically less than 10% of the flow speed. It is also a nearly constant speed in a long channel (isolator), suggesting little dependence on conditions far downstream that drive the back pressure: combustion, blowing, geometry, etc. We investigate how the propagating shock-boundary layer interaction sets the propagation speed. Although a triple-deck structure is anticipated, which can characterize flow structure and separation in the interaction region, it does not directly provide a complete description due to ambiguity in the downstream boundary condition and the unsteadiness in this configuration. Compressible Navier-Stokes simulations are used to reproduce shock-driven unsteady separation, both in the lab frame as a shock travels through a channel, and in the shock frame, where a parametric evaluation of imposed propagation speeds highlights changes in flow structure. The upstream motion is reproduced in model problems, and described with a combination of asymptotic analysis and a global balance to understand the mechanisms that underlie the slow propagation speed.