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

Abstract

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.