Mechanics of Distributed Roughness Shielding for Suppression of Roughness Induced Boundary Layer Transition
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2017, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved. We revisit in greater detail the experiments by Kuester and White (J.Fluid Mech. 777, 2015) who demonstrated the roughness-shielding mechanism proposed by Drews et al. (AIAA paper 2011-0564, 2011). The roughness-shielding concept consists of reducing the receptivity of large-amplitude discrete roughness elements (DREs) by surrounding DREs with subcritical distributed roughness. The present work studies the effects of upstream and downstream distributed roughness using DNS and wind tunnel experiments. Our distributed roughness shape for this study has been chosen to enable matched, simple DNS and experimental setups. The roughness elements are incorporated in the DNS using an immersed boundary method (Goldstein et al, J.Comp. Phys. 105, 1993, Goldstein et al, J.Fluid Mech. 302, 1995) as done in several of our earlier studies, and detailed evolution of the different components of the vorticity field are examined. Experiments are performed at the KlebanoffSaric Wind Tunnel (KSWT) at Texas A&M University and involve naphthalene shear-stress visualization. Both DNS and experimental results show that placing the distributed roughness either only upstream or only downstream of the DRE suppresses transition. Replacing the distributed roughness with a flat strip also appears effective for transition control, suggesting that such a strip may capture some of effects of the distributed roughness and offers a simpler practical implementation in some cases. Detailed mechanisms responsible for transition control by upstream and downstream distributed roughness patches are investigated.
