The interaction between an isolated roughness element and free-stream turbulence

Control and delay of the laminar-turbulent transition is a key parameter in reducing skin friction and drag. The flow characteristics, surface roughness, and environmental noise can affect the onset of transition. The present work investigates, numerically and experimentally, the interaction of the free-stream turbulence (FST) and an isolated cylindrical roughness element, and the resulting impact on the transition onset in a flat-plate boundary layer. High-fidelity direct numerical simulations (DNS) are performed for a roughness element immersed in the boundary layer over a flat plate with an asymmetrical leading edge, with and without FST. The numerical results are compared to hot-wire anemometry measurements performed in the Minimum Turbulence Level wind tunnel at KTH. The initial numerical and experimental results show that in the absence of FST, for the chosen flow parameters, highand low-speed streaks are generated downstream of the roughness element while the flow remains laminar and globally stable. When FST is added, the spanwise spacing of the streaky structures changes and the transition location of the boundary layer moves upstream. It was found that the aspect ratio of the streaky structures does not vary significantly