Matched DNS and experiments of roughness induced transition over realistic airfoil*

Experiments and direct numerical simulations (DNS) have suggested shielding as a mitigation strategy for roughness induced boundary layer transition (RIT) by examining its various stages. Recent DNS (Suryanarayanan et al., Int. J. Heat Fluid Flow 86, 108688, 2020) explored the effect of pressure gradients (PGs) on specific RIT mechanisms by applying PGs over specified streamwise extents. To apply this fundamental understanding of RIT shielding, matched DNS and experiments of RIT over a 63(3)-418 airfoil are performed. The PG and boundary layer profiles over the leading edge of the airfoil are determined using wind tunnel measurements and XFOIL simulations which serve as an input to the DNS code. The PG is achieved in DNS through a contouring of an upper wall that is updated until the PG matches XFOIL measurements in 2D laminar DNS. Additionally, various boundary layer metrics are compared between XFOIL and DNS for consistency in boundary layer evolution. A geometrically identical 3D discrete roughness element is then introduced in experiments and DNS, and RIT processes are studied for a variety of shielding configurations at a matched chord Reynolds number. DNS and experimental results are compared to demonstrate shielding over airfoils.