Flow physics of supersonic crossflow with transversely injected jets in streamwise tandem configuration

This study investigates the flow field generated by two tandem jets injected transversely into a Mach 2.1 supersonic crossflow, aligned in the streamwise direction. Such injection configurations are widely utilized in scramjet combustors for enhanced fuel–air mixing. Experimental observations using planar laser-induced Mie scattering and computational mass fraction contours demonstrate improved jet penetration compared to a single jet, attributed to the shielding effect of the first jet on the second and the splitting of the first jet induced by counter-rotating vortex pairs. Experimental and numerical Schlieren images reveal the formation of an additional bow shock wave upstream of the second jet, with the second bow shock wave becoming more prominent as the inter-jet spacing increases. Surface oil flow visualizations and streamline plots highlight multiple vortex structures, including horseshoe and hanging vortices upstream of the first jet, a herringbone-shaped separation region downstream of the second jet formed by collision shocks, and unsteady vortices in the inter-jet region. The study further identifies significant jet mass accumulation in the inter-jet vortex region, with periodic mass shedding into the jet boundary, leading to oscillatory behavior. Using mass fraction stacked plots and dynamic mode decomposition analysis, two distinct oscillation modes of the tandem jets are characterized, providing insight into the unsteady dynamics of this complex flow field.