Wind Turbine Tip Vortices under the influence of Wind Tunnel Blockage Effects

The current paper describes the characteristics of the tip vortex in the near wake of a three-bladed upwind horizontal axis wind turbine with a rotor diameter of 3 m. Phase-locked stereo particle image velocimetry measurements were carried out under the influence of the wind tunnel walls that create a high blockage ratio. The location of the vortex, convection velocity, core radius, and strength were investigated and compared with similar investigations, including different blockages cases. Additionally, the same performance of the wind turbine model was simulated in the open source wind turbine tool QBlade, using the lifting line free vortex wake module in the absence of the walls. The results showed that the location of the tip vortices was more inboard the tip and more downstream the tunnel compared to the simulations and similar experiments. The convection velocity remained similar in the axial direction and changed in the lateral direction, contributing to the delay of the movement of the tip vortex outboard the tip. The strength, based on the circulation, was found with a difference of 4% between simulation and experiment