Unstart Process of a Rectangular Hypersonic Inlet at Different Mach Numbers

A rectangular hypersonic inlet is tested at the low-speed mode (Ma 5), the design mode (Ma 6), and the overspeed mode (Ma7) to enrich the understanding of hypersonic-inlet unstart. A flow plug is placed at the duct exit to simulate the combustion-induced high pressure and 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 results indicate that the unstart processes are basically the same for the low-speed mode and the design mode. An unstart phenomenon dominated by the ramp-side flow separation, which exhibits violent fluctuations along the entire duct with a low frequency, is observed at these two operating modes. While operating at the overspeed mode, the strong shear layer, induced by the Mach reflection of the coalesced external oblique shock, divides the internal flow into two parts. The cowl side is filled by fully subsonic flow after the compression of the detached normal shock, and the ramp side mainly contains supersonic flow downstream of the shear-layer-induced oblique shock. Therefore, a real-time adjustment of the detached cowl shock to the downstream high pressure is formed, keeping the entire flowfield of the inlet from oscillation in a much wider range of throttling ratio. With a further increase of the throttling ratio, an oscillation dominated by the cowl-side reversed flow occurs. The oscillation phenomenon exhibits relatively gentle fluctuations, and the characteristic frequencies are mainly determined by the shock train in front of the throat section.