Actuator methods for wind turbine wake predictions on coarse meshes

Abstract The defining characteristic of actuator line method large eddy simulations (ALM-LES) of wind turbines is their ability to simulate tip vortices and near wake structures, which are important for wake breakdown and far wake dynamics. However, the method is increasingly being applied for the study of wind farm aerodynamics in simulation domains characterised by coarse meshes and high background turbulence, which impacts the resolution quality of the simulated tip vortices. In this work, we discuss the interplay between the mesh size and turbulence intensity levels in ALM-LES simulations. We find that spurious tip vortex merging takes place for coarser meshes, affecting the wake breakdown mechanism, mean flow, and dynamics. It is shown that the spurious features correlate with low values of the turbulent kinetic energy ratio: the ratio of the resolved to total TKE in an LES simulation. When background turbulence is added, mesh sensitivity is relaxed as turbulent structures overtake the mesh dependent small scale structures of the near wake. Both coarser meshes and higher turbulence have an effect on the resolution quality of tip vortices in ALM-LES simulations. This feature raises the question of whether employing the actuator line over the actuator disc method is worthwhile. A comparison between the two actuator methods suggests that the two methods perform similarly for coarse meshes with high background turbulence.