Forced oscillation experiments in supercritical diffuser flows

Low-frequency oscillations induced in ramjet inlets by combustion instabilities were simulated by mechanically modulating the exit area of a two-dimensional, supercritical diffuser at frequencies up to 330 Hz. Boundary layers were attached below a terminal shock Mach number of 1.27, and shock-induced separation occurred above this value up to the experimental limit of 1.35. Shock position histories were obtained and streamwise distributions of static/total dynamic pressures were determined both on the wall and within the flow for various shock strengths and frequencies. Excitation at the natural frequencies of the shock motion produced no obvious resonance effects. For weak shocks, the perturbations and their reflections from the shock are reasonably approximated by one-dimension al, acoustic considerations, but this description fails for strong shocks.