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Experiments have been conducted on a swept wing model in a low-turbulence wind tunnel at chord Reynolds number of $2.17 \\times 10^{6}$ to investigate the unsteady interaction of a forward-facing step (FFS) with incoming stationary crossflow (CF) vortices. The impact of varying the FFS height on the development and growth of primary and secondary CF disturbances and the ensuing laminar–turbulent transition is quantified through detailed hot-wire anemometry and infrared thermography measurements. The presence of the FFS results in either a critical (i.e. moderate transition advancement) or a supercritical behaviour (i.e. transition advancing abruptly to the FFS location). The arrival of the forced stationary CF vortices at the step is accompanied by their amplification. Unsteady analysis for the critical cases indicates temporal velocity fluctuations following closely the development of the baseline configuration (i.e. agreeing with the development of secondary instabilities). Consequently, laminar breakdown originates from the outer side of the upwelling region of the CF vortices. In contrast, for the supercritical FFS, the laminar breakdown unexpectedly originates from the inner side of the upwelling region. Evidence points to an unsteady mechanism possibly supported by locally enhanced spanwise-modulated shears and the recirculation region downstream of the FFS edge. This mechanism appears to govern the abrupt tripping of the flow in supercritical step cases. The findings in this work provide insight into the unsteady FFS–CF vortex interaction, which is pivotal to understanding the influence of an FFS on the laminar–turbulent boundary-layer transition in swept aerodynamic surfaces.

The impact of a forward-facing step (FFS) on the development of stationary crossflow instability is investigated on a swept wing model in a low-turbulence wind tunnel at chord Reynolds number of $2.3 \\times 10^{6}$. Infrared thermography and particle image velocimetry measurements are used to quantify the transition location and growth of the crossflow instability under the influence of FFSs with different heights. Forced monochromatic stationary crossflow vortices experience an abrupt change in their trajectory as they interact with the step geometry. As the boundary layer intercepts the step an increase in the vertical velocity component and an amplification of the crossflow vortices is observed. Near the step, the vortices reach maximum amplification, while dampening downstream. The smaller FFS cases, show a local stabilising effect on the primary stationary mode and its harmonics, while in the higher step cases transition occurs. The analysis of the temporal velocity fluctuations shows a reduction in the region associated with the type-III travelling crossflow modes downstream of the step. In contrast, the velocity fluctuations in the region associated with type-I secondary instabilities increase past the FFS edge. Nonetheless, in the shortest FFS cases, these velocity fluctuations eventually decay below the clean configuration (i.e. without an FFS) levels. This behaviour is linked to a novel transition delay effect for the shortest step height investigated. The findings highlight new physical aspects driving the interaction between an amplified stationary crossflow vortex and an FFS and provide insight into possible transition delay mechanisms using such geometries.