Effect of inflow conditions on tip vortex breakdown in a high Reynolds number wind turbine wake

Understanding the re-energization of wind turbine wakes is crucial for the design and control of wind farms. Close to the rotor, this process is determined by the dynamics of the tip vortices. Here, we experimentally investigate the downstream evolution of the tip vortices for different inflow conditions. The experiments were performed in the Variable Density Turbulence Tunnel at the Max Planck Institute for Dynamics and Self-Organization, which uses pressurized <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:msub> <a:mi>SF</a:mi> <a:mn>6</a:mn> </a:msub> </a:math> as the working fluid to achieve a turbine diameter-based Reynolds number of <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:msub> <b:mi>Re</b:mi> <b:mi>D</b:mi> </b:msub> <b:mo>=</b:mo> <b:mn>2.9</b:mn> <b:mo>×</b:mo> <b:msup> <b:mn>10</b:mn> <b:mn>6</b:mn> </b:msup> </b:mrow> </b:math> . An active turbulence grid was used to generate atmospheric inflow conditions with varying levels of mean shear and turbulence intensity. Hot wire measurements of the streamwise velocity component were conducted in the inflow and the wake of a model wind turbine MoWiTO 0.6 for various tip speed ratios and are used to investigate the scaling of tip vortex breakdown in the near wake. Three different scaling regimes can be identified, which we link to an initial advection phase, the vortex breakdown, and a decaying turbulence regime. While the scaling in the vortex breakdown regime is only weakly affected by variations in mean velocity shear and turbulence intensity, higher tip speed ratios lead to faster breakdown.