Nonlinear Disturbance Evolution Downstream of a Medium Height Roughness Element

The present study is devoted to nonlinear mode interactions in the wake of a medium height, isolated roughness placed in a laminar, airfoil boundary layer. Upstream of the roughness low-amplitude, fundamental TS-wave bands are excited. Nonlinear interactions in-between the fundamental modes in the near wake lead to the formation of low-frequency, subharmonic-type modes before the fundamental modes can recover linear stability characteristics with the stabilization of the mean flow. For lower roughness heights the low-frequency modes are only weakly distinct and the disturbance evolution is dominated by the fundamental modes in the roughness wake. With increasing mean flow distortion, that is roughness height, the subharmonic-type modes experience a resonant growth stage in the far wake, which provokes the onset of turbulence. For further increased roughness heights, the fundamental and the subsequent interactions modes reach nonlinear amplitudes in the near wake and, thereby, initiate the laminar flow breakdown before the boundary layer can stabilize.