Effect of free-stream turbulence on the flow around a S809 wind turbine blade

Two-dimensional Particle Image Velocimetry (2-D PIV) measurements were performed to study the effect of free-stream turbulence (FST) on the flow around a smooth and rough surface airfoil, at zero angle of attack and under stall conditions. A 0.25-m chord model with an S809 profile, common for horizontal-axis wind turbine applications, was tested at a wind tunnel speed of 10 m/s, resulting in Reynolds numbers based on the chord of Re_c ≈182,000 and turbulence intensity levels of up to 6.14%. Results indicate that when the flow is fully attached, turbulence significantly decreases aerodynamic efficiency (from L/D ≈ 4.894 to L/D ≈ 0.908). This yields to higher loads and fatigue on the blades. On the contrary, when the flow is mostly stalled, the effect is reversed and aerodynamic performance is slightly improved (i.e., a 5\% increase, from L/D ≈ 1.696 to L/D ≈ 1.787) due to increased free-stream turbulence. At α= 16°, 12% and 7% increases are observed for the lift and drag coefficients, respectively. Moreover, analysis of the mean flow over the suction surface shows that, while free-stream turbulence is delaying stall, surface roughness acts to advance separation to a location closer to the leading edge. The combination of free-stream turbulence and surface roughness results in the further advancement of separation. This might be a result of the dynamics between the boundary layer scales and the free-stream turbulence length scales when relatively high levels of active-grid generated turbulence are present.