Generation of Upper Trailing Counter-Rotating Vortices of a Sonic Jet in a Supersonic Crossflow

Direct numerical simulations were conducted to investigate the physical structures of a transverse sonic air jet injected into a supersonic air crossflow at a Mach number of 2.7. Simulations were run for two different jet-to-crossflow momentum flux ratios J of 1.85 and 5.5. The main averaged flow features around the transverse jet, such as major counter-rotating vortices and trailing counter-rotating vortices, were captured. The major counter-rotating vortices formed in the lateral portion of the jet plume and absorbed other induced trailing vortices downstream of the jet plume. Upper trailing counter-rotating vortices formed above the major counter-rotating vortices downstream of the jet barrel shock. The streamline analysis indicated that the upper trailing counter-rotating vortices were related to the Mach disk. As the streamlines penetrated the lateral side of Mach disk where a strong shear condition existed, the baroclinic torque induced upper vorticities in the opposite rotating direction against the major counter-rotating vortices. Downstream of the Mach disk, the baroclinic vorticity production along the streamlines tended to approach zero, which means no more torque was pumped into the far field and the upper trailing counter-rotating vortices were merged into the major counter-rotating vortices by their suction.