EXPERIMENTAL INVESTIGATION OF SPANWISE TANDEM LIQUID INJECTION IN SUPERSONIC CROSSFLOW

This study investigates the penetration and flow separation characteristics of a pressurized liquid jet injected transversely into a Mach 2.1 supersonic crossflow, using both single and spanwise tandem injection configurations with varying injector spacing (4 mm, 8 mm, and 12 mm). High-speed back-lit imaging was employed to capture the unsteady nature of jet breakup, which was further analyzed using proper orthogonal decomposition (POD) to identify coherent instability modes. The breakup of the liquid jet was primarily governed by Rayleigh-Taylor and Kelvin-Helmholtz instabilities, leading to the formation of surface waves, large liquid clumps, and a fine droplet spray. Time-averaged Schlieren visualization and image processing techniques were used to quantify jet penetration for single and tandem injection cases. Results show that the single injection configuration achieved the highest penetration, while tandem jets exhibited reduced penetration. Among the tandem cases, the 4-mm spacing shows enhanced penetration due to strong jet-jet interaction near the injection location, whereas larger spacings show reduced penetration due to weaker interaction. Oil flow visualizations reveal the formation of a separation shock region and the development of a liquid entrainment region upstream of the injector. For larger injector spacings, two distinct entrainment zones and minimal jet-jet interaction are observed, indicating that each entrainment region evolves independently. The findings provide new insights into how injector spacing affects jet dynamics, penetration, and interaction in supersonic crossflow.