Unsteady shock-driven separation simulations with application to unstart

Scramjets can famously fail by unstart, which involves the upstream travel of a compression structure in the isolator. A notable feature of this is that the speed by which this occurs is significantly slower than obvious flow time scales: for example, it is typically well less than 10 percent of the flow speed. It can also be nearly steady in a long channel, suggesting that it does not significantly depend on downstream conditions, despite becoming subsonic. Compressible Navier-Stokes simulation models are used to study the shock-driven unsteady separation propagation in a model flow, particularly dependence on shock and boundary layer characteristics, seeking to identify their role in the slow time scales associated with motion. Adaptive mesh refinement affords excellent resolution of boundary and separated shear layers and localizes the shocks, which in turn allows broad parametric evaluation of slow-time-scale mechanisms. A central result concerns the accelerating influence of a thicker boundary layer on upstream propagation speed. It is not a simple scaling (and might not be expected to be simple). For the configuration simulated, it delays the formation of a steady upstream separation zone. Implications for unstart speed will be discussed. The slower the propagation, the more time to avoid unstart.