Passive control of shock-wave/turbulent boundary-layer interaction

Abstract

Shock-wave/turbulent boundary-layer interaction (STBLI) is a ubiquitous phenomenon in high-speed aerodynamic applications, such as rocket nozzles, engine inlets, airplane wings and control surfaces. The interaction between a shock wave and a turbulent boundary layer often leads to flow separation, severe wall-pressure fluctuations, and unsteady thermal loads, which can significantly degrade aerodynamic performance and structural integrity. Mitigating these adverse effects remains a long-standing challenge in compressible flow research. While numerous active and passive control strategies have been proposed, many of them suffer from practical limitations, including high energy consumption, geometric complexity, or strong sensitivity to installation location.

This dissertation investigates the potential of spanwise-heterogeneous roughness as a purely passive control strategy for STBLI, with a particular focus on convergent–divergent riblets and streamwise-homogeneous ridge-type roughness. Using wall-resolved large eddy simulations combined with an immersed boundary method, a Mach 2.0 impinging shock-wave/turbulent boundary-layer interaction is systematically studied over smooth and rough walls. The numerical framework enables a detailed analysis of both the mean flow and unsteady characteristics of the interaction, as well as the underlying physical mechanisms governing roughness-induced flow modification.

The first part of the study examines the control effects of convergent-divergent riblet patches. It is shown that the riblets induce organized secondary flows in the form of counter-rotating streamwise vortices, which significantly modify the incoming turbulent boundary layer prior to the interaction. These secondary flows lead to a spanwise redistribution of momentum, resulting in a corrugated separation topology and an attenuation of wall-pressure fluctuations near the separation shock foot, while simultaneously causing an upstream shift of the interaction onset and an enlargement of the interaction and separation regions. Owing to the localized nature of the induced vortices, whose influence is expected to decay in the streamwise direction, the overall control authority remains inherently limited, while an additional pressure-drag penalty is inevitably introduced.

Motivated by these limitations, the second part of the dissertation focuses on streamwise-homogeneous ridge-type roughness, which offers greater robustness and reduced sensitivity to installation location. The results demonstrate that ridge-type roughness induces Prandtl’s secondary flows of the second kind, leading to systematic modifications of the upstream turbulent boundary layer. When the ridge spacing is comparable to the boundary-layer thickness, strong downwash motions locally energize the turbulent boundary layer, thereby suppressing flow separation while simultaneously increasing wall-pressure fluctuations. For smaller ridge spacings, a pronounced subsonic region forms within the incoming boundary layer, resulting in a less-full velocity profile. This modification weakens the streamwise wall-pressure gradient and smears the separation shock foot, leading to a substantial attenuation of wall-pressure fluctuations over a broad frequency range, albeit at the cost of an enlarged separation region. Parametric studies further reveal that increasing the ridge height amplifies the attenuation of wall-pressure fluctuations by enhancing the roughness-induced modification of the upstream boundary layer.

Finally, the influence of Reynolds number on the control performance is examined. The results show that wall-pressure fluctuations near the separation shock foot comprise a low-frequency component associated with shock motion and a high-frequency component associated with shear-layer turbulence, with their relative contributions strongly dependent on the Reynolds number. At low Reynolds numbers, the high-frequency component dominates, whereas at higher Reynolds numbers the low-frequency component becomes prevalent. In this high-Reynolds-number regime, where low-frequency shock unsteadiness governs the interaction dynamics, ridge-type roughness remains effective and yields an even stronger attenuation, with peak wall-pressure fluctuations reduced by up to 27%. Spectral analysis and cross-correlation studies support a downstream-influence mechanism for the low-frequency unsteadiness, while dynamic mode decomposition reveals the presence of large-scale Görtler-like vortices downstream of the interaction region.

Overall, this dissertation demonstrates that spanwise-heterogeneous roughness, if properly designed, can serve as a robust and practical passive control strategy for mitigating STBLI unsteadiness in high-speed flows, albeit at the cost of a moderate increase in skin-friction drag. The findings provide new physical insights into the interplay between roughness-induced secondary flows, Reynolds number effects, and low-frequency STBLI dynamics, and offer guidance for the design of roughness-based flow control concepts in future high-speed aerodynamic applications.