Instabilities in separation and reattachment induced by a hypersonic shock-wave/boundary-layer interaction

Mean flow and instability measurements of a nominally axisymmetric, transitional separation bubble arising from a flare-induced shock-wave/boundary-layer interaction were conducted on a cone–flare geometry in hypersonic flow. Simultaneous infrared thermography and high-speed, high-resolution background-oriented schlieren measurements provided a global, time-resolved characterisation of both the mean and unsteady flow. Experiments were performed in the AFOSR–Notre Dame Large Mach-6 Quiet Tunnel under conventional noise at free-stream unit Reynolds numbers ranging from $5.8\times 10^6$ to $12.3\times 10^6$ m –1 . The model comprised a $7^\circ$ half-angle circular cone, a $20^\circ$ half-angle flare and three nose radii. Complementary laminar and turbulent base flows, convective and global stability analyses (GSAs) and direct numerical simulations (DNSs) were carried out at the experimental conditions. With inflow forcing of disturbances, the DNS reproduces the experimentally observed flare-heating trends. While the flow is convectively unstable, global instability arises specifically from stationary and oscillatory three-dimensional disturbances localised within the recirculation bubble. Thermal streaks observed in global heating measurements provide experimental evidence of these instabilities, which is further corroborated by the onset of three-dimensionality in DNS. Quantitative agreement of streak wavelengths and their spatial prevalence across experiments, GSAs and DNSs confirms that the recirculation bubble’s global instability is the primary driver of the reattachment streaks in this flow, providing the first direct experimental evidence of this mechanism.