James Threadgill

Unsteadiness in Shock Wave Boundary Layer Interactions across Multiple Interaction Configurations

The effects of interaction strength and configuration generality are the subject of an experimental investigation to assess the low frequency unsteadiness typically observed in Shock Wave Boundary Layer Interactions (SWBLIs). The core dominant mechanisms responsible remain disputed due to the lack of a broad approach for understanding what drives each mechanism across a range of interaction types. This paper forms the first part of an investigation to observe various interactions in a well characterized baseline environment. Experiments have been conducted in the Imperial College London supersonic wind tunnel where three configurations of SWBLI have been tested, each with a strength of interaction designed to result in incipient separation: a) 14◦ compression ramp interaction (Mach 2.0), b) 8◦ incident shock reflection interaction (Mach 2.0), and c) normal shock interaction (Mach 1.4). Analysis has been conducted using high-speed planar two-component particle image velocimetry and schlieren photography to characterize the baseline flow and inspect shock unsteadiness. Each configuration exhibits similar levels of incipient separation with probabilities of reversed flow between 12% and 26% in each frame. Significant energy content has been observed within the interactions at frequencies far lower than typically witnessed in a supersonic boundary layer, providing agreement with results in literature.

Shock Wave Boundary Layer Interaction Unsteadiness: The Effects of Configuration and Strength

Unsteadiness associated with shock wave boundary layer interactions across a variety of environments is the focus of the experimental study reported herein, recorded via synchronized high-speed measurement of planar velocity fields and wall-pressures. Such unsteadiness has been witnessed to present at frequencies much lower than that found in the incoming boundary layer, and has been linked with a range of damaging local flow phenomena. Despite significant research, there remains considerable disagreement as to the core mechanisms involved. Recent work suggests upstream fluctuations are significant in lowstrength interactions, while downstream effects appear dominant in high-strength interactions. The situation for intermediate strength interaction, i.e. incipiently separated flow, is less clear and appears to display a mixture of these influences. In this study we analyze the interaction with two different configuration types, each with multiple cases varying interaction strength, from incipiently separated interactions to those featuring fully separated flow. 14◦ and 20◦ compression ramp interactions are observed in the Mach 2.0 facility, along with oblique shock reflection interactions induced by 7◦, 8◦, 9◦, and 10◦ shock generators. Resultant unsteadiness beneath the interaction is recorded with a series of fast-response wall-pressure transducers. The approach is unique in that these interactions have been tested in a single facility, with common inflow conditions, limiting other external influences. Scaling of the mean interaction structures by length and imposed interaction strength collapse well to recent models proposed in literature. Unsteady wall pressure spectra agree with similar trends observed in other facilities.