Wind-turbine wake flows - Effects of boundary layers and periodic disturbances

The effects of a rough surface boundary layer on the outputs of a wind turbine model were investigated experimentally in a wind tunnel. The very rough surface consisted of cylindrical pins, in order to model a forest canopy. The hub height of the turbine model was varied in order to see the effect of the presence of the model forest in the power and thrust coefficients. A small effect of the hub height was observed in the averaged power coefficient, where the turbine produced less for the lowest hub height. The difference was however reduced when scaling the power output with the available power in the wind instead of using the velocity at hub height. Consistent trends were present in the standard deviation of the thrust coefficient and the rotational speed, which both increased by decreasing the hub height. This underlines the fact that not only the rotor but also the tower and the bearings of a wind turbine must withstand to increased loads when operating close to a canopy.