Swept Impinging Oblique Shock Boundary Layer Interactions

Abstract

The focus of this dissertation is on understanding the underlying physics of swept impinging oblique shock boundary layer interactions (SBLIs) induced by shock generators with varying sweep angles ($\psi$). Experiments were conducted at the University of Arizona's In-draft Supersonic Wind Tunnel (ISWT) operating at Mach 2.28, with a fully turbulent incoming boundary layer. The study employed oil flow visualization, mean pressure measurements, and high-bandwidth pressure transducers to examine both the mean and unsteady features of these interactions. The findings reveal the presence of large-scale separation in all cases and conical similarity (3D SBLI) at higher sweep angles. Mean pressure rise near the onset of separation for all $\psi$ angles is independent of span, agreeing with free-interaction theory. However, mean pressures at reattachment for higher sweep angles exhibit mild spanwise dependence, suggesting the overall mean flow topology of the interactions is conical. To verify the conical topology and identify key 3D SBLI structural features such as strong cross-flow and open separation, numerical simulations using compressible Reynolds-averaged Navier-Stokes (RANS) approach were performed. This comprehensive investigation provides valuable insights into the complex mean flow associated with swept impinging oblique SBLIs. In order to understand the fundamental flow similarity in swept shock boundary layer interactions, numerical studies looking at both laminar and turbulent SBLIs are performed. The hypothesis is that conical similarity (3D open separation) is due to an inviscid shock detachment, while cylindrical similarity (2D closed separation) results in cases of an attached shock. Both the laminar and turbulent SBLIs in this study are induced by swept impinging shocks and swept compression ramps. While in the laminar case, mean flow features show that all cases below the detachment limit still display conical behavior, turbulent simulations show cylindrical interactions are possible for weakly separated flows. The latter case is also consistent with experiments. The results suggest that inviscid detachment is not the only mechanism behind the flow similarity and the dynamics of open separation itself could dictate the mean scaling of the interaction. The present study also discusses open questions about whether the cylindrical observations in swept interactions are influenced by the limited aspect ratio in typical university-scale wind tunnels. Finally, unsteady pressure measurements beneath the separation shock foot show clear low-frequency unsteadiness, orders of magnitude below that of the incoming boundary layer. An increase in $\psi$ leads to a corresponding increase in the frequency of the separation shock motion. A specific analysis of the $\psi = 30.0^\circ$ configuration showed that the shock foot frequencies remain unaffected spanwise, despite changes in the local interaction length. Along the separation and reattachment line, locally accelerating convective structures in the cross-flow direction are observed. These structures are also coherent in the same low-frequency band as the separation shock motion and their local wavelength increases along the span. A minimal influence of the incoming turbulent boundary layer pressure fluctuations on the low-frequency unsteadiness of the separation shock is observed. Instead, significant coherence is noted in low frequencies at the reattachment line. Phase analysis indicates that the reattachment line leads the separation shock motion, suggesting a downstream unsteadiness mechanism. Additionally, cross-correlation analysis in the low-frequency band identifies an upstream propagating pressure disturbance from the reattachment line influencing the separation shock motion