The mechanism behind jet entrainment into the upstream boundary layer for multi-regime mixing of a jet in hypersonic crossflow

Existing methods for improving the mixing of jets in supersonic and hypersonic crossflows focus on the addition of obstructions such as cavities, steps, and wedges, which serve to create re-circulation zones and increase the residence time of the fuel–air mixing. Recent literature has shown that, under certain conditions, the jet stream can pass into the boundary layer upstream of the jet, where low-velocity high-residence time mixing can occur. To develop a fundamental understanding of the entrainment mechanism of the jet fluid to the forward boundary layer (J-FBL), an implicit large eddy simulation is employed for a Mach 5 hypersonic crossflow and a momentum flux ratio of 5.18 between the jet and crossflow. It is observed that the jet fluid entrainment occurs through a thin channel stemming from the barrel shock, close to the bow shock and near-wall shear layer. By measuring the flow through this channel, it is shown that the J-FBL entrainment flux varies over time. It is observed that the entrainment channel from the jet to the boundary layer varies in size, shape, and direction with the deformation of the barrel shock by the formation and shedding of the barrel shock shear layer (BSL) vortices. From this, it is determined that the driving mechanism for the J-FBL entrainment is the size and shape of the barrel shock. It is concluded that any flow control schemes that alter the shape of the barrel shock may be employed for utilizing the J-FBL entrainment phenomena and thus near-wall mixing.