The effect of the boundary layer on the wake of a horizontal axis wind turbine

This paper analyses the effect of a mean shear similar to an atmospheric boundary layer on the wake of a wind turbine by means of Large Eddy Simulation. More specifically, a comparison is made between the wake in the presence of a smooth boundary layer and that in the absence of a boundary layer (i.e., an unconfined uniform incoming flow). The numerical simulations show that the presence of a smooth boundary layer lowers the power output, however, the rings of tip vortices in the presence of a power law incoming flow are more stable than for a uniform incoming flow. More importantly, the length of the wake region for the case with the smooth boundary layer is about 12D, which is much shorter than for a uniform incoming flow (namely 20D). Strong downwash, observed at this distance in the presence of a smooth boundary layer, results in a higher velocity magnitude and lower turbulence in the far wake of the wind turbine when compared with a uniform flow. A mechanism explaining these observations is also proposed. This new knowledge may result in denser wind farms, compared with wind farms established on smoother surfaces. (C) 2019 Elsevier Ltd. All rights reserved.