On the Mechanics and Control of Boundary Layer Transition induced by Discrete Roughness Elements
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2017 by Saikishan Suryanarayanan, David B. Goldstein, Garry L. Brown, Alexandre R. Berger and Edward B. White. Published by the American Institute of Aeronautics and Astronautics, Inc. We examine in detail the flow past a single and two discrete roughness elements in a boundary layer using immersed boundary direct numerical simulations combined with matched wind-tunnel experiments. Discrete roughness elements are a simplified representation of either unavoidable design features such as fasteners or environmental roughness that accumulates during aircraft operation. The present work follows the earlier study of flow past single and two discrete elements presented by Sharma et al (AIAA 2014-0235, 2014) and the unpublished results of Sharma & Goldstein on 'cancellation' of the transition caused by the single roughness element by a second downstream element. Current simulations and wind tunnel experiments performed at Rek 230 (Reynolds number based on height of the roughness k and the velocity at y = k in the undisturbed boundary layer) show that the single roughness element causes instability and transition to turbulence within about 100k downstream. Addition of an appropriate second discrete roughness element downstream of the first is observed to entirely suppress the transition in both CFD and experiment. A series of simulations and experiments show that the suppression is relatively robust to small changes in Reynolds numbers, to some geometrical details and to variations in the location of the second DRE in relation to the first. The effectiveness of the cancellation is highest at the lowest streamwise distance between the DREs. These results suggest a means of passive control of transition caused by known roughness elements, such as fasteners or other design elements, near, say, the leading edge of aircraft wings. With the objective of generalizing the concept and design of inverse roughness elements, we seek understanding of the observed transition and cancellation via detailed analysis of the flow evolution downstream of the roughness elements, such as budgeting production and dissipation of different components of enstrophy. Results show that the dominant mechanism that triggers transition is the amplification of the wall normal vorticity (that dominates the evolution) by tilting of spanwise vorticity of the boundary layer by streamwise vortices.
