Effects of wavy roughness elements on the aerodynamic performance of NACA0012 airfoil

The effects of 2D surface roughness on the aerodynamic performance of the NACA0012 airfoil (Re = 5 ×104, AoA = 10°) are investigated with direct numerical simulations (DNS). The DNS are performed with an energy-conservative fourth-order parallel code solving the incompressible Navier-Stokes equations in generalized curvilinear coordinates. Located near the leading edge, the surface roughness elements are characterized with streamwise sinusoidal-wave geometry that is uniform in the spanwise direction but with varying wave number (from k = 0 to 12, with k = 0 corresponding to the smooth case, chosen as the baseline). The flow structures and and boundary layer separation are quantified, and it is shown that the lift and drag coefficients are strongly affected by separation bubbles that arise. The numerical results reveal that the drag coefficients increase to a peak value and then decrease with increase in wave number, while the lift coefficients decreases strongly for k ≤ 6 and then become stable. The boundary layer separation is affected by the wavenumber and with an increase in the number of separation bubbles that render the flow more complex. For k ≥ 8, massive separation occurs and almost covering the suction side of the airfoil dominating the airfoil aerodynamic performance.