Roughness induced transition

Surface roughness has numerous different effects on boundary layer stability. For roughness amplitudes lower than a critical level, the velocity disturbances roughness creates in laminar boundary layers experience a transient algebraic growth followed by exponential decay without leading to transition. When the critical level is exceeded unsteady disturbances are amplified and the flow rapidly transitions to turbulence. Whether the flow remains laminar or becomes turbulent is solely determined by the receptivity of the initial disturbance. Two experiments are performed that address two fundamental problems related to roughness induced disturbances. One examines the disturbances produced by subcritical elements. The other identifies the instability mechanism responsible for breakdown in the wake of supercritical roughness elements. For subcritical conditions, multiple component velocity measurements are obtained in a flat plate boundary layer in the wake of an array of subcritical cylindrical elements. These data reveal the topology of the steady horseshoe vortex that exists in the wake of roughness elements and leads to transient growth of streamwise and spanwise velocity disturbances. A theoretical technique that quantifies the receptivity of velocity disturbances to surface roughness requires the complete velocity field as input and the data obtained here are suitable for this purpose. For supercritical conditions, velocity measurements are obtained above and below the critical roughness height Reynolds number, Rek. The steady disturbance field reveals local shear layers in the wall-normal and spanwise directions and the unsteady disturbance field reveals evidence of hairpin vortices observed in flow visualization studies. The locations of maximum fluctuation intensity correspond to the locations of local inflection points of the steady velocity field suggesting that the fluctuations result from a Kelvin-Helmholz-type instability. Rapid transition takes place at Rek = 334 but not below this value. The disturbances' energy growth rates indicate that the transition scenario can be understood as a competition between the unsteady disturbances' growth and the rapid relaxation of the steady flow that tends to stabilize these disturbances.