Interaction between Free-Stream Turbulence and Tip-Vortices of Wind Turbine Blades with and without Winglets

Experimental investigations of the free-stream turbulence impact on the tip-vortices generated in wind turbine blades have been performed. The investigation is done by exposing an efficient laboratory scale wind turbine to different turbulence levels generated by two static grids installed in the cross section of a wind tunnel. Different winglet configurations to figure out the optimum design that can prevent the tip induced flow are studied. The power gained when adding different winglets is measured when exposing the turbine to turbulence. It is found that the strength of the vortices is reduced depending on the turbulence levels. Furthermore, higher power extraction associated with more expansion of wake and wake-border is shown. This is an indication for increasing of the mixing between the free-stream and the wake. Tip-vortex analysis of high turbulence levels has shown additional interaction of large-eddy scales contained in the free-stream turbulence. The turbulence helps to suppress the tip vortices, and thus, reduces the tip losses. Further investigations of the near and far wake-surrounding intersection are performed to understand the energy exchange and the free stream entrainment that help in wake recovery.