Flow structure and mixing characteristics of a transverse jet in supersonic crossflow under varied boundary layers

Efficient fuel-air mixing within milliseconds is critical for scramjet performance, yet the flow physics of a transverse jet in supersonic crossflow remains insufficiently quantified. Planar Rayleigh/Mie scattering and stereoscopic particle-image velocimetry were applied to a Mach 2.68 crossflow. Jets with a 2 mm orifice diameter were injected at three dynamic-pressure ratios (1.64, 4.92, and 8.19) under two incoming boundary layer thicknesses (1 and 4 mm). Instantaneous imaging captured the bow shock, barrel shock, Mach disk, slip line, recirculation zone, and counterrotating vortex pair (CVP). Vorticity fields revealed streamwise vortices forming beside the barrel shock, merging 20 mm downstream from the jet orifice, and persisting as a typical CVP that entrained freestream fluid. Boundary layer thickness systematically enhanced jet penetration and modulated near-field breakup patterns without altering far-field mixing limits. The penetration depth was fitted using a modified correlation, indicating an approximately 10%–20% increase when the boundary layer thickness increased from 1 to 4 mm. Lateral diffusion reached 15 mm at the orifice and plateaued at 20 mm beyond 20 mm downstream from the jet orifice. Fractal analysis partitioned the jet plume edges into three regions. A thicker boundary layer elevated the initial fractal dimension in Region I but suppressed its growth in Region II, whereas Region III exhibited consistent rapid fragmentation. The quantitative datasets and key parameters established benchmarks for validating computational fluid dynamics simulations and scramjet engine designs.