Shock wave boundary layer interactions in sonic gas jet injection into supersonic crossflow

Jet in supersonic cross flow is a conceptually simple configuration with applications in fuel injection in combustors of high-speed vehicles. The focus of the present work is on the unsteadiness in this flow configuration caused primarily by Shock-Wave Boundary Layer Interactions that has not received much attention in prior experimental studies, and which can have implications on the combustion and thrust generation from such engines. This is studied in the present work for the case of a sonic gas jet injection in a supersonic (M = 2.5) cross flow using Particle Image Velocimetry (PIV), unsteady pressure measurements and high-speed shadowgraph visualizations of the flow. The studies are done for a range of momentum flux ratios (J) from 1.5 to 3.0. High speed shadowgraphy of the shocks show that the bow, separation and barrel shock formed are unsteady, with wall pressure measurements just upstream of the jet injection location showing a large increase in fluctuations compared to that in the boundary layer far upstream of the jet injection. PIV measurements in the injected jet’s central wall-normal plane show the presence of low/high speed streaks in the incoming boundary layer, and the locations and shape of the separation, bow and barrel shocks. These measurements indicate that the low/high speed streaks in the incoming boundary layer have a significant effect on the instantaneous shock locations and the instantaneous jet penetration, with a high-speed boundary layer streak leading to the shocks being pushed downstream and the jet penetration being reduced, while the opposite effects are seen for a low-speed streak in the incoming boundary layer. Corresponding changes are also seen in the barrel shock depending on the presence of low/high-speed streaks in the boundary layer, with these deformations leading to the ejection and subsequent convection downstream of high-speed pockets. These observations indicate that the source of the unsteady fluctuations seen in the shocks and in the instantaneous jet penetration are related to the low and high-speed streaks in the incoming boundary layer, which can have consequences for the combustion and thrust generation characteristics of high-speed engines where such fuel injection strategies may be adopted.