We investigate flow instability created by an oblique shock wave impinging on a Mach 5.92 laminar boundary layer at a transitional Reynolds number. The adverse pressure gradient of the oblique shock causes the boundary layer to separate from the wall, resulting in the formation of a recirculation bubble. For sufficiently large oblique shock angles, the recirculation bubble is unstable to three-dimensional perturbations and the flow bifurcates from its original laminar state. We utilize Direct Numerical Simulation (DNS) and Global Stability Analysis (GSA) to show that this first occurs at a critical shock angle of θ = 12.9• . At bifurcation, the least stable global mode is nonoscillatory, and it takes place at a spanwise wavenumber β = 0.25, in good agreement with DNS results. Examination of the critical global mode reveals that it originates from an interaction between small spanwise corrugations at the base of the incident shock, streamwise vortices inside the recirculation bubble, and spanwise modulation of the bubble strength. The global mode drives the formation of long streamwise streaks downstream of the bubble. While the streaks may be amplified by either the lift-up effect or by Görtler instability, we show that centrifugal instability plays no role in the upstream self-sustaining mechanism of the global mode. We employ an adjoint solver to corroborate our physical interpretation by showing that the critical global mode is most sensitive to base flow modifications that are entirely contained inside the recirculation bubble.

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We examine the unsteadiness generated by an oblique shock impinging on a hypersonic boundary layer. Earlier studies found that a M = 2.15 boundary layer undergoes a three-dimensional (3D) bifurcation to linear instability. Using the US3D compressible flow solver, we investigate whether this bifurcation remains 3D in the case of an oblique shock impinging on a laminar boundary layer with M = 5.92. We characterize the frequency and spanwise wavenumber selected by this bifurcation using Direct Numerical Simulation (DNS), Sparsity-Promoting Dynamic Mode Decomposition (SPDMD), and global stability analysis. DNS reveals that the hypersonic boundary layer remains laminar for an oblique shock angle of ✓ = 12 , but becomes unstable and transitions to turbulence once the shock angle increases to ✓ = 14. In the second case, spanwise Fourier velocity spectra as well as SPDMD of the DNS data show that the flow selects a particular spanwise wavenumber and frequency. We investigate the origin of these length and time scales via global stability analysis about steady two-dimensional (2D) base flows obtained from DNS.

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