Experimental Investigations on Ramp-induced Large Separation Bubble in a Hypersonic Flow

Abstract

Shockwave Boundary Layer Interactions (SBLIs) are ubiquitous in supersonic/ hypersonic flows and often lead to the separation of the boundary layer. These separations can be broadly classified into small and large separation bubbles based on their lengths compared to the boundary layer thickness. While small bubbles have minimal influence on the outer flow conditions, large bubbles significantly impact the outer flow, causing changes in pressure distribution. These large separation bubbles have been observed to be unsteady and thus can adversely affect the performance of aerodynamic devices. Moreover, these unsteady pressure loads can lead to vibrations and fatigue failure of the structure. Therefore, understanding the flow physics within the separated region and its influence on the outer flow is essential for efficient aerodynamic design. The research gap in the field lies in the accurate prediction of the onset of unsteadiness in Ramp-induced Shockwave Boundary Layer Interactions (R-SBLIs) and the lack of controlled experimental data on unsteady flows with separation occurring at the leading edge. Previous studies have primarily focused on ramp angles below 30°, neglecting higher angles that could lead to detached shock solutions. These gaps in the literature motivate the present study, which aims to investigate the different flow regimes, mechanisms, and sources of low-frequency unsteadiness in R-SBLI. The investigation includes incipient separation, steady separated flow, unsteady separated flow, and the identification of flow topology. A comprehensive approach is proposed for the identification and characterisation of different flow regimes encountered in compression corner flows. The flow regime depends on the pressure ratio imposed by the ramp angle. Shock polar analysis helps identify the nature of shock-shock interactions, which determines the pressure variation along the ramp surface. The oscillation and pulsation modes are identified based on the location of the shear layer impingement on the ramp. A conditional-based algorithm is developed for flow regime identification. This approach provides a systematic understanding of the flow topology for a given set of freestream conditions and test models.