Impact of a forward-facing step on the development of crossflow instability

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. 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.