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An unsteady viscous numerical simulation is performed to study the hypersonic inlet pulse-starting in a shock tunnel under the condition of inflow Mach number higher than the design value with a simplified nozzle startup wave system. A local unstart of hypersonic inlet is obtained with the initial backpressure and temperature of test section equal to the hypersonic freestream static pressure and static temperature, respectively. During the hypersonic inlet pulse-starting process, the nozzle startup wave system (including the primary shock, contact discontinuity and secondary shock) propagates downstream and interacts with the hypersonic inlet, the secondary shock induces boundary layer separation on the compression ramp. A low velocity flow induced by the separation bubble enters into the internal duct with the passage of time. It thickens the boundary layer and then the cowl shock induces a large separation bubble on the compression side. The shock induced by the large separation bubble impinges on the cowl and induces a small separation bubble, which moves upstream and induces the reflection of the external compression shock at the cowl transiting from regular reflection to Mach reflection. Mach reflection results in the shock propagating forward to cause a shock detachment at the cowl lip. The hypersonic inlet is not fully started and this phenomenon is referred to as “local unstart of inlet”. Moreover, it is found that the hypersonic inlet starts with lower initial backpressure or higher initial temperature of test section, and the hypersonic inlet unstarts with higher initial backpressure. The effect mechanism is analyzed with the help of numerical simulations and unsteady one-dimensional shock theory. As there are three operation modes of hypersonic inlet at high Mach number: start, local unstart and unstart. The results of the hypersonic inlet pulse-starting in a shock tunnel should be carefully treated. [ABSTRACT FROM AUTHOR]

For a better understanding of the flow instability in TBCC inlet, the unstart/restart characteristics have been investigated. Firstly, the flow field of TBCC inlet at different transition stages are analyzed. As the splitter rotates from upside to downside (transition from low-speed inlet to high-speed inlet) with a fixed backpressure, the shock train moves downstream. The maximum backpressure that the inlet can tolerate also rises. Secondly, according to the unstart boundary (namely maximum backpressure) and restart boundary, it can be found that if the inlet is propelled to transit from started state to unstarted state, inlet restart cannot be achieved along the same control route. Unstart/restart hysteresis loops in each transition phase are discussed according to the simulation results. Besides the hysteresis behavior under backpressure, the same phenomenon is also observed during the inlet transition process. Finally, the distinctiveness of the hysteresis caused by inlet mode transition is also discussed. [ABSTRACT FROM AUTHOR]