Spatial organisation of low-frequency unsteadiness in oblique shock wave -- boundary layer interactions

This thesis examines the spatial organisation of low-frequency unsteadiness of incident -- reflected shock -- boundary layer interactions (SBLIs). Such SBLI unsteadiness continues to be an active topic of research due to its detrimental effects on supersonic engine intakes, for example. Recent studies have investigated the influence of duct geometry on the topology of the shock-induced separation bubble. This thesis aims to unite these two topics in order to investigate the inter-dependence of the low-frequency unsteadiness and the three-dimensional separation bubble shape and to contribute to the understanding of the low-frequency dynamics. In this study, experiments have been undertaken on eight test cases in the Imperial College blow-down supersonic wind tunnel with a freestream Mach number of two. Three different interaction strengths were achieved by turning the flow $12\,\degree$, ${10\,\degree}$ and ${8\,\degree}$. Each strength interaction was examined at three different effective aspect ratios ${\AR=1}$, ${\AR=1.3}$ and ${\AR=2}$, with the exception of the $12\,\degree$ for which the latter was not examined. The separation regions were measured by oil flow visualisation and the unsteadiness was inferred from pressure fluctuation measurements under the separation shock across the full span of the wind tunnel floor. The obtained measurements provide the first experimentally-measured two-dimensional maps that capture the unsteady characteristics beneath an SBLI. The maps show that low-frequency motion exists across the entire span of the separation shock and is more prevalent where earlier separation occurs. Similar low-frequency unsteadiness is observed in the corner separations and it appears that the interaction, including the central and corner separation bubbles and the separation shock, moves as a whole entity. This might be driven by the pressure gradient across the incident shock rather than by the upstream boundary layer or separation bubble. The driving force of the incident shock strength appears to dictate the characteristic frequency and the product of the amplitude of shock motion and separation volume.Open Acces