Unstart Detection in a Simplified-Geometry Hypersonic Inlet-Isolator Flow

Unstart detection techniques based on high-frequency pressure measurements made in a hypersonic inlet isolator model are investigated. In this study, data that were acquired in a previous study of backpressure-induced unstart were examined. The data were acquired in a simplified-geometry inlet isolator model that consisted of a 6 deg compression ramp inlet followed by a constant-area duct that was 25.4 mm high by 50.8 mm wide by 227.1 mm long. Fluctuating wall pressures were measured along the length of the model. A downstream flap was used to induce unstart in the model. Beyond a certain flap angle, unstart was induced, and the shock system propagated upstream and out of the inlet. The wall-pressure data, acquired as the flap was raised, were postprocessed for spectral and statistical content to evaluate different unstart detection criteria. Three shock leading-edge detection criteria are examined based on the following observations as the shock system passes over a pressure transducer: 1) a rise in pressure, 2) an increase in standard-deviation of the pressure signal, and 3) an increase in power in the 300-400 Hz frequency band. After calibrating the algorithms based on runs with no active control, a comparison of the times detected for unstart onset and unstart arrest was made based on runs with active control. Results indicate that the power-spectrum-based algorithm implemented close to the isolator exit is more sensitive to the onset of unstart, whereas the pressure-magnitude-change criterion gives earlier detection in many cases. A combination of the two, a pressure-magnitude criterion close to the inlet entrance and a spectral-power criterion near the isolator exit, therefore seems to be the most robust scheme for unstart active control. The appropriate choice of sampling frequency significantly improved the computation speed of the spectral-power-based algorithm without delaying the unstart detection times.