ENERGY, vol.176, pp.320-334, 2019 (SCI-Expanded)
In the current study, the aerodynamic performance of NACA 4412 airfoil with sandpaper which was used as an alternative control device instead of vortex generators was investigated. It was especially considered how the sandpaper affected the growth of the separation bubble. Regarding the flow visualization, the smoke-wire experiment was done at different angles of attack and Reynolds numbers. Moreover, the hot-wire experiment was employed to measure velocity as well as the values of the turbulence statistics over and near wake of the airfoil. Besides the mentioned experiments, the force measurement was performed to obtain aerodynamic forces, and quasi-wall shear stress was measured by means of a hot-film sensor. With the roughness, the instability in the long transition region at low Reynolds number was bypassed. Flow momentum was increased in the boundary layer on the airfoil surface, and the flow over the roughened airfoil exhibited more resistance against adverse pressure gradients. The flow triggered by the roughness element suppressed the bubble, which meant pre-stall flow control carried out with resulting C-L increment and C-D reduction. Finally, using the sandpaper over the airfoil provided prominent benefits in terms of postponing of the stall and enhancement of aerodynamic performance for wind turbine blades. (C) 2019 Elsevier Ltd. All rights reserved.