Local structure of boundary layer transition in experiments with a single streamwise vortex

•Response of subcritical boundary layer to isolated, embedded, streamwise vortex.•Non-inclined high shear layer rolls up and vortices tear away during transition.•Vortex passing frequency scales with square of freestream velocity.•Large ejection events in outer layer.•Some features common to FST-induced, and roughness-induced transition identified. Transition induced by an isolated streamwise vortex embedded in a flat plate boundary layer was studied experimentally. The vortex was created by a gentle hill with a Gaussian profile that spanned on half of the width of a flat plate mounted in a low turbulence wind tunnel. PIV and hot-wire anemometry data were taken. Transition occurs as a non-inclined shear layer breaks up into a sequence of vortices, close to the boundary layer edge. The passing frequency of these vortices scales with square of the freestream velocity, similar to that in single-roughness induced transition. Quadrant analysis of streamwise and wall-normal velocity fluctuations show large ejection events in the outer layer.