Surface roughness effects on the wake characteristics of wind turbines

Abstract. We use the large-eddy simulation (LES) technique to investigate surface roughness effects on the atmospheric boundary layer flows over large wind farms and their power production efficiency. We consider four representative turbine-array layouts including perfectly-aligned, perfectly-staggered, laterally-staggered, and vertically-staggered configurations. The arrangement of each configuration has 120 turbines installed in 30 rows with aligned or staggered configurations along the wake-wise direction. We perform the LESs of neutrally-stratified atmospheric boundary layer over homogeneous flat surfaces with three different aerodynamic roughness lengths (i.e., z₀ = 0.5, 0.1, 0.01 and 0.0001 m). Emphasis is placed on the overall farm power production efficiency, as well as the structure and characteristics of the cumulated wakes originated from the clustered turbines in the wind farm cases where the incident flows to the upstream turbines of the farms have the same mean velocity at the hub height but different mean wind shears and turbulence intensity levels. The simulation results show that the different turbulence intensity levels of the incoming flow lead to considerable influence on the overall wind farm power production performance. The inflow turbulence intensity is stronger, the overall wind farm power production is higher. The incoming flows also cause a significant effect on the spatial distribution of the mean velocity deficit, turbulence intensity, and turbulent shear stress in the wake region. In particular, when the turbulence intensity of the incoming flow is stronger, the turbine-induced wake (velocity deficit) recovers faster, and the locations of the maximum turbulence intensity and turbulent stress are closer to the turbine.