Unstart of a Hypersonic Inlet with Side Compression Caused by Downstream Choking

A hypersonic inlet with side compression has been tested at a freestream Mach number of 6.0 to enrich the understandings of hypersonic inlet unstart. A flow plug is placed at the duct exit to simulate the combustion induced high pressure and to initiate the inlet unstart. High-speed schlieren imaging and time-resolved pressure measurements are used simultaneously to record the unsteady flow structures and surface pressures of the unstart process. The inlet operates in a big buzz mode with a base frequency of 30Hz and exhibits a series of unsteady flow patterns similar to those of rectangular hypersonic inlets in a buzz cycle when the throttling ratio is 87.4%. During the upstream propagating process of the unstart shock system, the propagation velocity in the two ends of the duct is higher than that in the middle section of the duct, with a minimum value around the isolator. Once the separation bubble induced oblique shock is expelled over the cowl lip, a supersonic reverse flow with a Mach number of 1.56 is observed in the contracting part of the inlet, which acts as an inverted supersonic nozzle. When the external unstart shock system moves back toward the cowl lip, two kinds of secondary oscillations with high dominant frequencies of about 360Hz and 900-1300Hz are observed in turn by the time-accurate pressure system. The two mentioned secondary oscillations are both acoustic resonance modes formed in different parts of the duct.