Segmented Gurney Flaps for Enhanced Wind Turbine Wake Recovery

Abstract. The wind turbine wake is a downstream region of velocity deficit, featuring higher turbulence and a complex helical vortex structure. In low ambient turbulence and low tip speed ratio conditions, the wind turbine wake is extremely stable. The impact of the velocity deficit is a power loss for the downstream wind turbine, which scales with the velocity cube. This study uses field tests and simulations to evaluate segmented Gurney flaps for enhanced wind turbine wake recovery and power output in a wind farm setting; where the power and loads of the retrofitted wind turbine were assessed. Four Gurney flaps were attached to each blade tip of a 3.8 MW research wind turbine. This configuration is hypothesised (in line with an ECN (now TNO Wind Energy) patent) to cause a spatial variation to the stable tip vortex and induce turbulence in the wake for faster wake mixing. Field tests using a scanning LiDAR were conducted to quantify the wind turbine wake recovery between the baseline and the retrofitted configuration in various atmospheric conditions. The results show a consistent increase in wake recovery for the Gurney flap configuration, generally at all downstream distances (span wise averaged deficits reduced by roughly 10 % at hub height, at a downstream distance of 5D), pronounced at low tip speed ratio conditions. Using crude assumptions, this implies a 4 % relative increase in wind farm efficiency for a typical wind farm with outer rows of wind turbines with segmented Gurney flaps. The impact of retrofitting on turbine power and loads remained within the measurement uncertainty band, and this limited effect is confirmed by design load simulations. In this work, a very successful field test campaign was executed which demonstrated the use of segmented Gurney Flaps as a promising add-on to promote enhanced wind turbine wake recovery for improved overall wind farm farm performance.