Velocity field measurements by means of PIV are used in this work to characterize the flow in a shock wave–boundary layer interaction. For a free-stream Mach number of \(M_\infty =2.56\) , the flow over a flat plate model is deflected by a \(16^\circ \) wedge. For these flow conditions, an unsteady dual-state solution is observed where the shock switches between a regular reflection and a Mach reflection. This non-periodic mode switching is atypical for a shock wave–boundary layer interaction and causes significant changes in the flow field. The PIV measurements enable the Mach number and the flow direction to be determined from the measured velocity. In this way, both the position of the shocks and the flow deflection across the shocks can be reliably identified. Our analysis shows that regular reflection rarely occurs and that Mach reflection with varying Mach stem height is present for about \(85\%\) of the measurement time. We provide evidence here that the transition to regular reflection is related to a temporarily thickening of the boundary layer ahead of the shock interaction, which is caused by the breathing of the separation bubble below the shock interaction. This phenomenon results in compression waves that alter the Mach number and flow direction in the region upstream of the shock system, enabling a momentary transition to a regular reflection.
