Shock-Wave Oscillations in a Transonic Diffuser Flow

Low-frequency, self-excited oscillations involving a normal shock wave and the subsonic flow behind it were investigated experimentally in a two-dimensional diffuser. The preshock supersonic flow was uniform and steady, and the exit pressure was constant. Static wall pressure fluctuations at numerous streamwise locations were recorded and analyzed statistically for flow conditions ranging from subsonic to a shock Mach number of 1.38. Root-mean-square fluctuation intensities, power spectra, and space-time correlation maps were computed. The pressure fluctuations correlated well with the shock motion over streamwise distances comparable to the diffuser length. Both downstream-convected and upstream-propagating pressure disturbances were instrumental in sustaining the oscillations. The amplitudes of upstream-moving perturbations diminished with increasing shock strength, while the opposite was true of the downstream-moving disturbances.