Direct Numerical Simulation of Boundary Layer Transition Induced by Roughness Elements in Supersonic Flow

Current research on the transition mechanisms induced by moderate-height roughness elements remains insufficiently explored. Hence, direct numerical simulation (DNS) and BiGlobal stability analysis are employed in this study to investigate boundary layer transition from laminar to turbulent flow induced by moderate-height isolated roughness elements and roughness strips under a supersonic freestream at Mach 3.5. Analysis of DNS results reveals that the isolated roughness element induces transition within the boundary layer, characterized by two high-speed streaks in the wake. This transition is attributed to the coupling between the separated shear layer at the roughness apex and the downstream counter-rotating vortex pair (CVP). BiGlobal stability analysis further identifies that symmetric eigenmodes dominate the transition process in the wake, actively promoting flow destabilization. Conversely, the roughness strip configuration suppresses transition, with only attenuated high-speed streaks persisting in the near wake before complete dissipation. The wake flow exhibits multiple CVPs and adjacent horseshoe vortex pairs interacting with the shear layer, with antisymmetric modes dominating this process. These findings provide technical foundations and theoretical frameworks for predicting and controlling roughness-induced transition.