First identification and quantification of detached-tip vortices behind a wind energy converter using fixed-wing unmanned aircraft system

In the present study, blade-tip vortices have been experimentally identified in the wake of a commercial wind turbine using the Multi-purpose Airborne Sensor Carrier Mark 3 (MASC Mk 3) unmanned aircraft system (UAS) of the University of Tübingen. By evaluation of the wind components, detached blade-tip vortices were identified in the time series. From these measurements, the circulation and core radius of a pair of detached blade-tip vortices is calculated using the Burnham–Hallock (BH) wake vortex model. The presented data were captured under a dominating marine stratification about 2km from the North Sea coastline with northern wind direction. The measured vortices are also compared to the analytical solution of the BH model for two vortices spinning in opposite directions. The model has its origin in aviation, where it describes two aircraft wake vortices spinning in opposite directions. An evaluation method is presented to measure detached-tip vortices with a fixed-wing UAS. The BH model will be used to describe wake vortex properties behind a wind energy converter (WEC). The circulation and core radius of detached blade-tip vortices will be calculated. Also a proposition of the model for WEC wake evaluations will be made to describe two independent co-rotating vortices. Quantifying blade-tip vortices helps to understand the process of vortices detaching from a rotor blade of a wind turbine, their development in the wake until finally dissipating in the far wake and contributing to overall atmospheric turbulence. This is especially interesting for set-ups of numerical simulations when setting the spatial resolution of the simulation grid.