Influence of air-jet stability on the flow-control effectiveness of air-jet vortex generators*

Shock-wave / turbulent boundary layer interaction are complex flow-fields, commonly encountered in many air and space transportation applications. A common flow control technique to alleviate its adverse effects and to mitigate separation is the application of air-jet vortex generators (AJVGs). AJVGs induce a system of streamwise vortices downstream of jet injection and enhance the distribution of momentum in the boundary layer. Consequently, the boundary layer is more resistant to separation and the shock-induced separation region decreases. However, the nature of the jet-in-crossflow interaction and the jet-induced vortical structures can be affected by the stability of the injected jet, which can have a direct impact on flow-control effectiveness; even under similar cross-flow and jet stagnation conditions. With a combined experimental-numerical effort, we study the influence of the stability of the flow in the jet-injection pipe: we compare a stable and fully developed injection-pipe flow with a partially-developed case and discuss their influence on flow-control effectiveness for an array of air-jet vortex generators on a fully-separated 24^o compression ramp-interaction.

LESs of single spanwise-inclined jets in to a supersonic cross-flow were performed for two injection-pipe lengths – (1) shorter pipe with a jet-development length of 4.24 d_jet and a longer pipe with a length of 18.3 d_jet, to assess the influence of jet-stability on the jet-induced vortical structures. To quantify the effects of a jet-array on shock-induced separation, experiments were conducted on a 24^o compression-ramp model under similar jet and cross-flow conditions. Oil-flow visualisations and PIV at multiple planes were utilized to quantify the separation behavior and turbulent statistics under the two different jet-stability conditions.