Interaction of a Laminar Boundary Layer with a Cylindrical Roughness Element near an Airfoil Leading Edge

摘要

Surface roughness elements in the leading edge region of airfoils can cause laminar to turbulent boundary layer transition resulting in a loss in lift and a significant increase in aerodynamic drag. Especially wind turbine blades are affected since there is no maintenance. The present investigation is devoted to the interaction of a laminar boundary layer with a single, three-dimensional roughness element of cylindrical shape. Experiments were conducted in a very low turbulence wind tunnel. A two-dimensional airfoil section similar to the NACA 64_3-418 was employed at a typical Reynolds number (Re=6,000,000) for wind turbine applications. The roughness element is placed in the leading edge region. Downstream of the roughness element hot-wire boundary layer measurements were performed in a close to zero pressure gradient boundary layer. Discrete roughness heights in a range from 0 to 150 percent of the boundary layer displacement thickness were investigated. For roughness heights less than 20 percent of the displacement thickness no significant interaction with the laminar boundary layer is evident. In a range from 30 to 50 percent, clearly, an increasing mean flow distortion and the growth of boundary layer disturbances can be observed, however, not leading to boundary layer transition within the measurement region. For roughness heights higher than 75 percent of the boundary layer displacement thickness by pass transition occurs.