Tip vortex breakdown in a high Reynolds number wind turbine wake

Wind turbines are exposed to widely varying inflow conditions that depend on the local boundary layer meteorology. These inflow conditions are characterised by varying degrees of mean velocity shear and turbulence intensity, which affect the performance and durability of the turbine as well as the downstream evolution of the wake. These effects are challenging to study in the field due to the large scales involved, and most wind tunnel experiments are conducted at low Reynolds numbers. The near wakes of wind turbines are dominated by vortices shed from the tips of the blades. As they advect downstream, the tip vortices form a helical structure with three convoluted spirals. Here, we investigate the effect of the inflow turbulence, inflow shear, and tip speed ratio on their breakdown. We present results from high Reynolds number experiments in the Variable Density Turbulence Tunnel (VDTT) at the Max Planck Institute for Dynamics and Self-Organization. This wind tunnel uses pressurized SF₆ as the working fluid to achieve a diameter-based Reynolds number of Re_D = 3 x 10⁶. Because the VDTT achieves high Reynolds numbers at low velocities, high tip speed ratios can be achieved at reasonable rotation rates. An active grid with 111 individually-controllable paddles is used to generate inflow profiles with varying degrees of velocity shear and turbulence intensity. Streamwise hot-wire measurements at multiple downstream positions quantify the breakdown of the tip vortices behind a MoWiTo 0.6 model turbine.