Behavior of Multiple-Jet Interactions in a Hypersonic Boundary Layer

The fundamental flow physics of the interaction between an array of fuel jets and a hypersonic boundary layer is investigated. Hydrogen is injected at jet-to-freestream dynamic pressure ratios ranging from 0.350 to 0.875 on a flat plate into a Mach 4.5 crossflow. The injection array consists of four streamwise-aligned flush circular portholes. Both the streamwise spacing and jet-to-freestream dynamic pressure ratio are varied in a parametric study. The injection was performed completely within the boundary layer, with the intention of application to film-cooling drag reduction and boundary-layer combustion. Numerical simulations of four streamwise-aligned transverse sonic injectors in a fully turbulent hypersonic boundary layer revealed a very complex jet interaction flowfield. Variation of the streamwise injector port spacing, along with the jet-to-freestream dynamic pressure ratio, induced a variety of flow structures in the cases investigated. For all downstream interactions, the associated flow behavior was found to be a direct result of both the various upstream effects and interactions between adjacent injectors. Variations in the jet-to-freestream dynamic pressure ratio had a strong effect on the flow behavior. At low injection mass flow rates, coupling of adjacent injectors was small, whereas high mass flow rates increased the effect of jet-to-jet coupling. Variations in the streamwise jet-to-jet spacing were also found to play a critical role in the flow behavior. At very close spacings, intense interactions coupled the behavior of the individual jets; however, at increased spacings, the larger spatial freedom allowed individual jets to develop more naturally, leading to less jet-to-jet interactions. At the maximum spacing investigated, the jet interactions behaved more like discrete jets in crossflow.