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LARGE-EDDY SIMULATION OF ACCELERATING BOUNDARY LAYERS OVER ROUGH SURFACES

Junlin Yuan
Department of Mechanical and Materials Engineering Queen's University Kingston, ON, K7L 3N6, Canada

Ugo Piomelli
Dept. of Mechanical Engineering University of Maryland College Park, MD 20742, USA; Department of Mechanical and Materials Engineering Queen's University, Kingston (ON) K7L 3N6, Canada

Sinopsis

Large-eddy simulations (LES) are carried out to study the combined effects of roughness and pressure gradient in boundary layer flows, where the high acceleration facilitates flow reversion to the quasi-laminar state. First, validations of the immersed-boundary scheme and the LES resolution are carried out on rough-wall open-channel flows. Then the roughness model is applied to boundary layers subject to strong acceleration, leading to relaminarization and retransition. The roughness (represented by the roughness Reynolds number k+) counteracts with the favourable pressure gradients (FPGs): while flow acceleration tends to stabilize the flow by significantly damping turbulent motions in the vertical and spanwise directions and produces one-dimensional turbulence after the high-acceleration region, the roughness elements increase the turbulent mixing in the wall layer, reducing the stability of the low-speed streaks, and leading to earlier retransition to turbulence.