Velocity Deficit and Swirl in the Turbulent Wake of a Wind Turbine

Energy production data from several of the existing large offshore wind farms indicate that turbine arrays can suffer from a significant overall energy production shortfall, due to wakes generated by turbines upstream interacting with turbines downstream. An experimental investigation of the axial and azimuthal (swirl) velocity field in the wake of a single three-bladed wind turbine with rotor diameter of 0.91 m was conducted. The turbine was positioned in the free stream, near the entrance of the 6 m x 2.7 m cross section of the UNH Flow Physics Facility, a 72-m-long boundary layer wind tunnel. The turbine model was tested at various rotor loading conditions with blade tip-speed ratios up to 2.8.A Pitot-static tube and constant temprature hot-wire anemometry with a multiwire sensor were used to obtain velocity field measurements in the wake of the model turbine up to 20 diameters downstream. The results of an equilibrium similarity theory for the axismmetric wake with rotation are presented. The measurements obtained were used to examine the validity of the derived scaling functions for streamwise and azimuthal velocity, wake growth, and turbulence.