Oscillatory Flows of Rectangular Hypersonic Inlet Unstart Caused by Downstream Mass-Flow Choking

摘要

Unstart flows of a rectangular hypersonic iniet are experimentally studied at a freestream Mach number of 5. With the aid of high speed Schlieren and time-accurate pressure measurements, the unsteady flow processes of the entire inlet, including the shock system motions, the separation bubble transformations, and the surface pressure fluctuations, are recorded and discussed. The relation of the oscillation frequency with the exit throttling ratio is also obtained. Results indicate that the oscillatory flows of hypersonic inlet unstart can be classified into two types, namely, relatively mild "little buzz" and highly violent "big buzz." Because of the presence of a large transient supersonic region in the duct, the upstream propagation of acoustic waves is impeded temporarily in a buzz cycle. As a result, the traditional oscillation mechanism of supersonic inlet buzz, based on an acoustic wave feedback loop, is invalid for hypersonic inlet buzz. Therefore, a new oscillation mechanism is brought forward in which the airflow spillage at the entrance of the inlet is taken as the disturbing source and three communicating ways, namely, convection, shock train motion, and acoustic wave propagation, jointly establish the signal feedback loop. Then, the base frequencies of hypersonic inlet buzz at different throttling ratios are estimated and found in good consistency with those observed in current experiments.