Propagation of shock-wave/boundary-layer interaction unsteadiness in attached and separated flows

The origin and propagation of low-frequency shock oscillation unsteadiness in the attached and separated flows are investigated. Wind tunnel experiments are performed in an isolator at Mach 1.85 and 2.7 with three types of upstream wedges, generating weak and strong background waves. High-speed schlieren imaging and high-resonance frequency pressure measurements are used to capture the flow features. In the attached flow with weak background waves, the impingement of the reflected shocks along the flow strengthens the original instability waves from the shock oscillation, resulting in the correlation drop and time-delay rise with the original instability waves. In the attached flow with strong background waves, two-point correlation analyses show that the shock oscillations propagate along the shock structure and convection of the boundary layer structures, which enhances the turbulence pulsation in the boundary layer. The correlation and coherence results for pressure indicate that the incident points of two independent background waves move in opposite directions, while the incident points of two merged background waves move in the same direction. Using downstream throttling, the shock train in the separated flow is introduced. Based on the phase analysis of schlieren images, the feedback mechanism of the shock train oscillation is described, which is related to the acoustic wave propagation and the duct volume effect. Power spectra of the pressure in the upstream attached and downstream separated flows of the shock train show that the perturbation pathways in the attached and separated flows do not affect each other.