Experimental Investigation of Highly Separated Transitional Shockwave Boundary Layer Interactions

Oblique shock wave - boundary layer interactions (OSBLIs) at transitional Reynolds numbers are known to generate low-frequency unsteadiness that can impact aerodynamic efficiency, structural integrity, and system reliability in high-speed aerospace components. The present research examines how variations in Mach number, Reynolds number, and inviscid pressure jump influence the dynamics of these transitional interactions. Experimental campaigns were conducted in the TST-27 transonic–supersonic wind tunnel at TU Delft, utilizing high-speed and spark-light Schlieren imaging to capture flow behavior, followed by digital and spectral post-processing. The results reveal that transitional OSBLIs exhibit low-frequency shock oscillations strongly correlated with the periodic formation and disappearance of a Mach stem, a mechanism referred to as the “dual-domain” phenomenon. Systematic parameter variations showed that even small changes in Mach or Reynolds number substantially altered the presence of this dual domain as well as the amplitude and frequency of the oscillations. Furthermore, the Reynolds number regime previously identified as transitional for flat-plate boundary layers was experimentally validated in the present configuration. A secondary focus of the study was the application of passive flow control by introducing thin two-dimensional steps (≈60 μm) to trip the boundary layer. These perturbations were found to effectively suppress shock oscillations and modify the overall interaction dynamics, with frequency analyses confirming the disappearance of oscillation peaks observed in the baseline cases. A nondimensional analysis demonstrated a consistent Strouhal number convergence around St ≈ 0.33 for cases with strong unsteadiness. Increasing Reynolds number led to reduced laminar separation lengths and higher oscillation frequencies, consistent with expected transition dynamics. Overall, this research identifies the primary parameters governing unsteadiness in transitional OSBLIs, highlights effective suppression through minimal boundary-layer tripping, and provides nondimensional scaling to support predictive modeling. These insights contribute to improved understanding and design of aerospace components subject to shock-induced boundary-layer interactions.