Large-scale unsteadiness in a compression ramp flow confined by sidewalls

Conventional shock-wave-turbulent boundary-layer interactions are often treated as quasi-two dimensional for a tractable analysis. In practical applications such as high-speed engine inlets and combustors, the additional presence of sidewalls leads to dramatic changes in the baseline flow structure by enhancing the inherent three dimensionality of the interaction, in addition to modifying its unsteadiness. This study investigates the flow field generated by a 24 degrees compression ramp in presence of sidewalls, with a confinement ratio delta/omega = 0.12. The free-stream unit Reynolds number and Mach number are 2.5 x 10(7) m(-1) and 2.25, respectively. Statistical analysis of results obtained from high-fidelity simulations carried out by Poggie and Porter [Phys. Rev. Fluids 4, 024602 (2019)] is performed to explore the large-scale unsteadiness of this flow field. The mean and instantaneous flow fields displayed strong three dimensionality due to influence of sidewalls. Fourier analysis of wall-pressure data revealed frequency bands typically associated with lowfrequency shock oscillations, St approximate to O(0.01), as well as vortex shedding occurring at mid frequencies, St approximate to O(0.1). The spectra of shock oscillations and separation bubble breathing indicated a dominant low- and mid-frequency component, respectively, akin to a quasi-two-dimensional shock-induced separation. The centerline shock oscillations were well correlated with the breathing motion of the centerline separation bubble, as well as turbulence in the upstream boundary layer. Effects of sidewalls on the overall unsteadiness were investigated by estimating coherence and correlations between various quantities. Plots of coherence between shock motion and pressure fluctuations in the domain indicated asymmetric motion of the interaction, possibly caused by alternating breathing motion of the separated zones on the sidewalls. Space-time correlations on the floor suggested a strong influence of centerline separation on the corner separations and vice versa. Similar correlations with left and right sidewalls confirmed the asymmetric motion of the interaction with a frequency of St approximate to 0.026.