Influence of a forward-facing step surface irregularity on swept wing transition

An experimental study has been carried out to determine the influence of a forward facing-step (FFS) on the laminar–turbulent transition of a swept wing boundary layer. Wind tunnel experiments were conducted at a fixed 3 deg angle of attack and varying Reynolds number between 2.5 and 4.5 million. Moreover, the FFS influence was investigated under unforced (i.e., smooth leading edge) and forced conditions (i.e., using discrete roughness elements). For each test case infrared thermography was used to determine the transition location and spatial organization of the crossflow vortices. In addition, the change in amplification factor was calculated using linear stability theory. Results reveal the importance of considering multiple parameters when estimating the critical FFS height. The unforced cases indicate that one-parameter correlations (i.e., based on the crossflow vortex core height or boundary-layer displacement thickness) might not be sufficient to universally capture the dynamics of these complex flows. Analysis of the forced cases shows that in addition to local parameters (i.e., step height and vortex core height), the FFS influence on transition depends on the stability characteristics of the incoming instability mode. These findings suggest a complex nonlinear interaction between instabilities and surface irregularities, which highlight the need for multiparameter correlations for accurate transition prediction.