Surface imperfections on laminar aircraft surfaces, such as panel joints and discontinuities, can strongly modify boundary-layer stability. Sharp-edged rectangular bumps (SERBs) combine two widely studied surface features, the Forward-Facing Step (FFS) and a Backward-Facing Step (BFS), and provide a useful geometry for examining how step-induced mean-flow distortion and separation affect the development of Tollmien-Schlichting (TS) waves. However, previous studies on similar bump shapes have mostly considered a single incoming TS-wave amplitude, leaving the dependence of the SERB-induced boundary-layer response on incoming TS-wave amplitude insufficiently resolved.

To investigate how the boundary-layer response to a SERB depends on the amplitude of an incoming forced TS wave, this work combined numerical stability analysis with experiments performed on an unswept flat plate in the low-turbulence environment of the anechoic wind tunnel (A-tunnel) at TU Delft. Dielectric-Barrier Discharge (DBD) plasma actuators were used to force single-frequency TS-wave disturbances while allowing for easy amplitude variation. Surface microphones and Hot-Wire Anemometry (HWA) measurement systems were used to collect data to analyse the harmonic content of the disturbance, its wall-normal structure at the forcing frequency, and its downstream growth, comparing across the clean flat plate with and without the SERB installed over a range of forcing amplitudes. Independent monitoring of forcing consistency was shown to be important in long-duration plasma-forced experiments, where maintaining a consistent forcing amplitude is required. Simultaneous microphone measurements were necessary to distinguish genuine flow-induced amplification from transient variations in actuator output.

The results show that the influence of the SERB is strongly region-dependent: the forced TS-wave behaves like the clean case upstream of the bump and undergoes local amplification in the regions immediately upstream of the FFS and in the region just downstream of the FFS lip, a reduction in growth rate and stabilisation over the bump, and greater amplification downstream of the BFS. Greater forcing amplitudes are associated with a broader redistribution of spectral energy in the wall-normal direction and an earlier loss of a distinct shear-layer instability signature introduced in the recirculation region behind the BFS. The SERB, therefore, does not behave as a simple amplifier of TS waves, but instead produces a region-dependent response, with local amplification ahead of the FFS, reduced growth over the bump, and stronger re-amplification downstream of the BFS, where the response becomes increasingly sensitive to the amplitude of the incoming disturbance.