Experimental investigations of hypersonic inlet unstart/restart process and hysteresis phenomenon caused by angle of attack

Abstract

In this study, the unstart/restart process of a hypersonic inlet caused by the angle of attack (AOA) is experimentally performed at a Mach number of 5.0, and the unsteady flow characteristics during the test are investigated. The flow characteristics are simultaneously recorded using high-frequency pressure measurement and high-speed schlieren imaging. The results show that the inlet enters the unstarting state at AOA = 8.4° but restarts at AOA = 0.3°, exhibiting an obvious hysteresis phenomenon. During the unstart process, the forebody shock gradually approaches the body and then impinges on the inside of the cowl with the increasing angle of attack, indicating that the inlet works under an overrated mode. As the angle of attack increases further, the airflow chokes, and a high-pressure zone is first formed in the exit of the duct. Then, the high-pressure signal propagates upstream in the duct, leading to the emergence of an inlet unstart. When the inlet enters the unstarted pattern, a typical local unstarted flow field is formed. Low-amplitude oscillations occur in the internal contraction region, but further propagation to the isolator is weak. During the restart process, when the angle of attack decreases from 3.2° to 0.3°, two kinds of big buzz patterns, namely, intermittent high-amplitude oscillation and periodic high-amplitude oscillation, appear successively. The intermittent high-amplitude oscillation is characterized by the intermittent occurrence of a high-amplitude oscillation after the low-amplitude oscillations for a random length of time. In the periodic high-amplitude oscillation, the separation bubble is repeatedly disgorged and swallowed, resulting in a dominant fluctuation frequency in the entire duct. Furthermore, as the angle of attack decreases, the dominant frequency gradually rises from 280 Hz to 339 Hz. This work provides new insights into the flow evolutions and unsteady behaviors of hypersonic inlet unstart/restart owing to the AOA and provides potential applications on the detection and prediction of hypersonic inlet unstart/restart.