Selective upstream influence on the unsteadiness of a separated turbulent compression ramp flow

A study of the shock-wave/boundary-layer interaction induced by a compression ramp was carried out using high-fidelity simulations. The objective was to investigate the influence of upstream disturbances on low-frequency separation unsteadiness. Two computations were performed for a 24° compression ramp at Mach 2.25, one highly resolved case and one reduced-resolution case. The reduced-resolution case was run for an extended duration to capture many cycles of low-frequency unsteadiness. Basic flow characteristics, including statistics on the boundary layer, wall pressure, and skin friction, were computed. Frequency spectra were calculated to confirm the presence of low-frequency unsteadiness. The influence of upstream disturbances on large-scale separation unsteadiness was investigated using correlations, filtering, and conditional averaging based on the position of the primary separation shock. Low-frequency unsteadiness was found to be related to structures near the wall (y/δ < 0.5) with a time scale greater than 20δ/U∞, whereas higher frequency separation motion could be attributed to turbulent boundary layer structures with a time scale on the order of δ/U∞. The finding that the separation region responds selectively to certain large-scale, near-wall perturbations in the incoming flow supports a model of separation unsteadiness in which external forcing by certain components of boundary layer turbulence drives a weakly damped global mode of the separation bubble. This contrasts with suggestions that have been made in the literature that the separation region oscillates on its own, as in an amplified global mode.