PIV Measurements of Unstart of an Inlet-Isolator Model in a Mach 5 Flow

Abstract

The dynamics of unstart and unstarted flows in a floor-mounted inlet-isolator model in a Mach 5 flow are investigated experimentally using PIV, fast response wall pressures and 8 kHz schlieren imaging. The inlet compression is obtained with a 6-degree ramp and the isolator is a rectangular straight duct that is 25.4 mm high by 50.8 mm wide by 242.3 mm long. Unstart is initiated by deflecting a motorized flap at the downstream end of the isolator. PIV measurements are made in a streamwise-transverse plane as well as a streamwise-spanwise plane. With the flap fully-down, the PIV data of the started flow capture the characteristics of the isolator boundary layers and are consistent with shockexpansion theory. It is observed that the unstart process is associated with shock-induced separation, which leads to large reverse flow velocities up to about 300 m/s and the formation of strong shear layers. Furthermore, unstart is clearly three-dimensional as separation of the sidewall boundary layers is observed. In addition, the results indicate that the inlet geometry and the resulting initial shock system play a critical role in the flow structure of the unstart process. Once the model unstarts, a high-amplitude oscillatory unstarted flow with a dominant frequency of 124 Hz ensues. At times during this oscillatory unstarted flow when the isolator pressure is decreasing, the flow velocities in the isolator are relatively low. However, during cycles times when the isolator pressure is increasing the flow is associated with propagating shock systems and the isolator entrance flow is supersonic.