Effect of Two-Scale Roughness on Boundary Layer Transition Over a Heated Flat Plate: Part 2—Boundary Layer Structure

Velocity and temperature measurements made within transitional boundary layers over a surface roughened by two roughness scales were examined. The variation of intermittency through the transition region was shown to be consistent with a smooth-wall model of transition, but the prediction of transition onset was not well represented by momentum thickness for cases with strong surface disturbances. During transition, distributed roughness was shown to reduce the growth of velocity fluctuation, possibly through stronger dissipation, and to enhance wallward transport of momentum significantly without a corresponding increase in thermal transport. The step change between the two roughness scales was shown to notably affect boundary layer behavior. Spectral analysis supported the hypothesis in Part 1 that the separated region downstream of the step shed vortices into the downstream flow. The wavelet analysis suggested that transition over rough surfaces may be in bypass mode because the disturbances were shown to be amplified in a broad spectral band. [S0889-504X(00)00302-0]