Vortex–instability coupling in a Mach 6 wing–body junction flow revealed by Lagrangian analysis

Laminar–turbulent transition in hypersonic flows is strongly influenced by three-dimensional geometries, particularly in wing–body junctions. Here, we investigate a Mach 6 junction flow using wind tunnel experiments and direct numerical simulations with Lagrangian timeline analysis. Two dominant vortices are identified: a body-side horseshoe vortex (HV) and a wing-side leading-edge vortex. Both induce strong boundary layer redistribution with local thinning near the junction and thickening on their outer flanks. Timeline visualizations reveal persistent small-scale helical deformations within the HV core, with scales comparable to second-mode instabilities. These results provide direct evidence of coupling between vortices and high-frequency instability waves.