Surface roughness, such as forward-facing steps (FFS), significantly impacts the laminar-turbulent transition in swept-wing boundary layers. This study employs direct numerical simulations (DNS) to investigate the non-monotonic relationship between FFS height and transition location, complementing recent experimental findings. Two step heights and a baseline clean surface are analyzed under experimentally representative conditions. The simulations confirm a delay in transition for the shallow FFS and premature transition for the higher FFS, reproducing experimental trends while revealing some discrepancies in the abruptness of transition advancement for the larger step. DNS results provide detailed insights into the interaction of crossflow modes with the FFS, particularly near the step, highlighting the spanwise modulation and the formation of reverse flow regions.

This work presents an experimental and numerical investigation jointly conducted by TU Delft and DLR on Tollmien-Schlichting (TS) waves interaction with a Forward-Facing Step (FFS). Experiments are conducted at the TU Delft low-turbulence anechoic wind tunnel on an unswept flat plate model. Single-frequency disturbances are introduced using controlled acoustic excitation. The temporal response of the flow in the vicinity of the step is measured using Hot-Wire Anemometry (HWA). In addition, the global effect of the step on laminar-turbulent transition is captured using Infrared Thermography (IR). Two-dimensional (2-D) Direct Numerical Simulations (DNS) performed at DLR provide detailed information at the step. Experimental and DNS results in clean and step case conditions present very good agreement. Both methods predict large distortion of the TS waves downstream of the step, where DNS results present different growth trends between |û| and |(equation presented)| components of the TS waves. Furthermore, negative and positive regions of the production term are observed to correlate with streamwise positions where the disturbances appear tilted in and against the mean shear, respectively. These findings point towards the presence of different growth mechanisms triggered by the step which could modify the level of amplification of disturbances far downstream.