The topological structure of an underexpanded Mach 2·5 jet injected into a Mach 0·75 crossflow are studied experimentally and numerically. The structure of the skin friction pattern is examined in detail by localising precisely the topological singularities. Oil-flow visualisations reveal the complex nature of the wall flow and its strong three-dimensionality. Four vortical structures appear in the vicinity of the wall, where two are essentially parallel to the wall (horseshoe vortex and boundary-layer vortices) and the other two are directed perpendicular to the wall (wake vortices). Comparison of computation with experiment shows good agreement for the complex separated-flow surface shear pattern and the pressure distribution. The crossflow Mach number plays an important role in the separation mechanisms. Significant differences in the topology occur with a Mach 0·3 crossflow whereas above Mach 0·6 the mean structure of the interaction is almost conserved, except that the secondary wake vortices disappear.
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