Onset of separation unsteadiness in hypersonic shock–boundary-layer interaction on a cone-step

Shock–boundary-layer interactions on hypersonic cone-step flows exhibit a range of intrinsic unsteady behaviours, from shear-layer oscillations to large-scale pulsations. This work investigates the unsteadiness in a cone-step geometry at Mach 6 under quiet-flow conditions at different free-stream Reynolds numbers using time-resolved schlieren imaging and spectral proper orthogonal decomposition. Experimental results are compared with high-fidelity axisymmetric and three-dimensional simulations. Results demonstrate regime transition in the parameter space, across the unsteadiness boundary, all the way from shear-layer breakdown to shock system oscillations and ultimately to large-amplitude pulsations. The dominant mode in the experiments and the simulations corresponds to a Strouhal number St $\approx 0.17$ for small oscillations reducing to St $ \approx 0.13$ for large pulsations. A detailed description of the unsteady shock dynamics and an analysis of the nonlinear limit cycle is presented.