Transitional shock boundary layer interactions on a compression ramp at mach 4

Strong laminar/transitional shock boundary layer interactions (SBLIs) have been investigated in a Mach 4 vacuum-driven wind tunnel with supporting computational fluid dynamics analysis. Such flows are extremely susceptible to large-scale separation, unsteadiness, and high surface heat flux, which can limit control authority on high-speed vehicles. The research community has heavily focused on turbulent interactions, leaving little understanding of how laminar/ transitional cases behave with varying external parameters. Four compression ramp angles of 15, 18, 22, and 28° have been tested on a flat plate (unit Reynolds number Re₁ 4.56 × 10⁶ 1∕m) with a range of external length scales (flat plate Reynolds number Re_x varies between 1.2 × 10⁵ and 2.5 × 10⁵ at the ramp corner). Laminar separation was observed at boundary-layer thickness Reynolds number Re_δ0 between 2700 and 4300, with the centerline length of separation L∕δ₀ scaling mildly with Reynolds number, increasing proportionally with Re^0.4 δ₀ . Transitional reattachment is observed for strong SBLIs at higher values of Re_x . Analysis of high-speed schlieren images shows that the separation shock exhibits low-frequency unsteadiness at a Strouhal number near St_L 0.025, consistent with turbulent SBLIs. Detailed phase analysis of schlieren data shows no evidence of upstream influence. Instead, motion of the shear layer and reattachment shock precedes motion of the separation shock foot, suggesting a downstream unsteadiness mechanism. In addition, extensive analysis of subtle features within the interaction identified a slow-moving density disturbance within the bubble that convects toward the shock foot and directly leads separation shock motion. Negligible coherence was observed between low-frequency separation shock motion and acoustic content within the separation bubble.