A review of the shock-dominated flow in a hypersonic inlet/isolator

A hypersonic inlet/isolator acts as the "compressor" for scramjet engines through a series of shocks, which induces complex internal flows. This paper comprehensively reviews the recent research achievements, focusing on the shock-dominated internal flow of an inlet/isolator. Considering the specific geometrical feature of the hypersonic inlet, the shock wave/boundary layer interactions (SWBLIs) are characterized by multiple successive shocks. Three types of couplings have been observed between adjacent interaction regions. Moreover, shock and expansion waves, which are induced by the SWBLIs and named "background wave", are reflected in an isolator, forming a background wave/shock train interaction flow. The shock train behavior significantly differs from that in direct-connect facilities under uniform incoming flow conditions, and energy-level-transition-like phenomenon is observed when the shock train intersects with the background wave. Four types of quasi-steady background wave/shock train interactions have been reported, and three types of dynamic transitions have been observed when the shock train passes across the reflection point of the background shock. After the shock train is expelled from the internal duct, the inlet/isolator falls into unstart, and the unsteady shock-dominated flow with violent low-frequency shock oscillation occurs. A typical unstart period contains several stages, including the motion of the shock train in the isolator, large-scale separation in the inlet, and shock oscillation at the external part of the inlet. The flow mechanics of the hypersonic inlet/isolator unstart differs from that of a supersonic inlet. An unstart loop for a hypersonic inlet/isolator has been proposed, including convection wave, shock train, and acoustic wave. Once the induced factor of the unstart is removed, the unstarted shock retreats and the inlet experiences restart with the rebuilding of the supersonic flow. The restart process is highly dependent on the initial flow state and the historical effect. An instantaneous buzz arises before the unstarted shock retreats into the internal duct. Finally, the related passive (e.g., micro-vortex generator, bump, boundary layer bleed and selfcirculation secondary flow control method) and active flow control methods (e.g., air jet vortex generator, plasma jet flow control, and solid-particle injection) for weakening the unfavorable impact of these shockdominated flows are reviewed. Furthermore, the control mechanics and control effects of these flow control methods are analyzed.