Aerodynamic Performance of Wings with Spanwise-Coherent Surface Roughness

Abstract

Abstract: Spanwise-coherent roughness textures feature a series of extended roughness elements in the spanwise direction, corresponding to an intermediate geometry between classical two-dimensional (2-D) strips and fully three-dimensional roughness. However, these configurations have not been extensively studied, and their aerodynamic impact is not well captured. To address this knowledge gap, spanwise-coherent roughness textures are applied to a flat plate boundary layer under zero pressure gradient and to a NACA 0012 airfoil at a Reynolds number of 300,000. On the flat plate, spanwise-coherent textures result in an earlier laminar-turbulent transition, produce boundary layers thicker than over a smooth wall, and are systematically 23 % times thicker than the equivalent 2-D roughness; and tend to feature a less full boundary layer with shape factor 20 % times greater than for 2D roughness. There is no obvious trend in boundary-layer characteristics with the spanwise occupancy, defined as the proportion of a spanwise strip occupied by raised roughness elements. Meanwhile, the boundary-layer thickness and shape factor both increase with the aspect ratio (spanwise extent versus streamwise length) of each roughness element. It is hypothesized that the boundary-layer behavior is dominated by streamwise vortices generated at the spanwise edges of each roughness element, which enhance turbulent mixing and cause the boundary layer to thicken. On the airfoil, spanwise-coherent roughness reduces the lift coefficient before stall by 5-10 % compared to 2D roughness and 10 % compared to a smooth surface, consistent with a decambering effect from the thicker boundary layer. Spanwisecoherent roughness also causes the stall angle to be increased to 16 from 14 (2D roughness) and by 10 (smooth wall). The dependence of the aerodynamic characteristics on the roughness-element aspect ratio is relatively mild, and further work is needed to establish the precise influence of each geometric parameter.