Numerical and experimental study of tandem dual-jet injection into a supersonic cross-flow

To minimise the required length of supersonic-combustion ramjets (scramjets), injected fuel should mix rapidly with the supersonic cross-flow. Tandem dual-jet injection has shown improved mixing performance over single-jet injection. The present study comprises experimental work of tandem dual-jet injection, using schlieren flow visualisation in a supersonic wind tunnel, as well as numerical simulations of this flow inside the wind tunnel channel, both at Mach number 1.6. The numerical simulations are based on the Reynolds-averaged Navier–Stokes (RANS) equations for the time-averaged flow, and the time-resolved hybrid unsteady RANS-Large-Eddy Simulation, i.e. Delayed Detached Eddy Simulations (DDES), to assess the need of time-resolved solutions for capturing the time-averaged behaviour of the jets. From the wind-tunnel schlieren images, the time-averaged location in the cross-flow of the upper boundary of the main jet plume is found to obey an empirical similarity relation, providing for a given jet-to-cross-flow momentum flux ratio upper J J $J$ the value of the dimensionless distance upper S S $S$ between the jets for which plume penetration into the cross-flow is maximal. Numerical simulations facilitated the detailed analysis of the time-dependent flow, and numerical schlieren images in the midplane are used for comparison with wind-tunnel schlieren images. The numerical results, also for other cross-sections, are used to increase insight into the flow phenomena that occur in the interaction of the jet plume with the supersonic cross-flow, i.e. the experiments are complemented by numerics. These numerical results also validate the empirical similarity relation for the penetration depth of the jet plume determined from wind-tunnel schlieren images.