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 Superscript 6 5.8 × 10 6 $5.8\times 10^6$ to 12.3 times 10 Superscript 6 12.3 × 10 6 $12.3\times 10^6$ m Superscript negative 1 − 1 $^{-1}$ . The model comprised a 7 Superscript ring 7 ∘ $7^\circ$ half-angle circular cone, a 20 Superscript ring 20 ∘ $20^\circ$ 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. The flow exhibits both convective and global instabilities, with the latter giving rise to stationary and oscillatory three-dimensional disturbances 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 the recirculation bubble’s global instability is the primary driver of the reattachment streaks in this flow, providing direct experimental evidence of this mechanism.