Time-resolved 3D Lagrangian measurements of a wind turbine wake

Wind turbines operate at high diameter-based Reynolds numbers which leads to wakes dominated by highly turbulent flows. These wakes are also characterized by the presence of coherent structures like the hub vortex and the tip vortices. Shed from the nacelle in the center of the rotor, the hub vortex dissipates quickly, yet its breakdown generates turbulence within the core of the wake, thus affecting wake re-energization. Similarly, the helical tip vortices, shed from the blade tips, are a major source of turbulence as they break down. Thus, studying the formation and dissipation mechanisms of these coherent structures is crucial for understanding turbulence evolution within the wind turbine wake and enhancing wind turbine performance.

To study the evolution of the wake in three dimension, we use a Lagrangian Particle Tracking (LPT) setup. LPT allows for volumetric measurements of highly turbulent flows by following tracer particles within a pre-defined volume over an extended period of time. Until recently, mostly 1D and 2D Eulerian results had been investigated; however, advancements in high-resolution, high-speed cameras have now made 3D measurements feasible. Illumination is provided by 3 LEDs, while high temporal resolution is achieved using four Phantom high-speed cameras. These cameras have a frame rate of up to 10 kHz and a resolution of 2560 × 1664 pixels. The wind turbine model has a rotor diameter of 16 cm. The experiment is realized in the Variable Density Turbulence Tunnel (VDTT) at the Max Planck Institute for Dynamics and Self-Organization (MPI-DS). The wind tunnel is filled with sulfur hexafluoride (SF₆) and pressurized up to 15 bars. These conditions allow us to create inflow with a diameter-based Reynolds number of Re_D = 10⁷.

We record the particles’ movement at different positions allowing to resolve the entire near-wake region up to 2 rotor diameter downstream. The time-resolved three-dimensional positions of the particles are obtained using the Low Light Lagrangian Particle Tracking (L3PT) code, which was developed at the MPI-DS. From the time-resolved particle positions, we can deduce their velocities and accelerations. Studying the wake dynamic from a Lagrangian perspective gives insight into the transport and mixing characteristics directly related to the wake recovery.