Surface Roughness Heterogeneity Effects on Five Turbine Wind Farm Evaluated Using Large Eddy Simulation

The effect of an abrupt transition in the surface roughness from relatively rough conditions to smoother conditions on a five-turbine wind farm is examined using a large eddy simulation (LES). The LES are carried out using a high-order numerical framework with a state-of-the-art subgrid scale model and with the wind turbines modelled as rotating actuator disks. The surface roughness transition in the absence of turbines leads to an increase in the mean wind speed close to the bottom surface and a decrease in the turbulence intensity. The same effects are observed in the presence of wind turbines, and as a result, the evolution of the wakes of the individual turbines and the combined wake of all turbines is affected. The velocity deficit in the wake of each wind turbine grows approximately linearly from one to four diameters downstream of the turbine. This wake growth rate is different for a five-turbine wind farm sited on homogeneously rough surface and that sited on a heterogeneously rough surface. The wind turbine wakes also display an asymmetry between the profiles above and below the rotor hub heights. The presence of a surface roughness heterogeneity is found to reduce this asymmetry. These results have implications for the development of analytical models and ultimately for optimal layout of wind farms on heterogeneously rough surfaces.