Wake Alleviating Devices for Offshore Wind Turbines

The wake behind an offshore wind turbine can persist for several turbine diameters, so decreasing the space between wind turbines in an array leads to strong wake-turbine interactions and a decrease in efficiency of the wind turbines downstream. The dominant structures in the wake of a horizontal axis wind turbine are large helical tip vortices. Implementing devices on the blade tips of a wind turbine can induce mixing into the tip vortex core, encouraging breakup of the tip vortices and wake dissipation. A wake that dissipated more quickly can maximize the farm-level efficiency by allowing more turbines to be installed in a fixed area. This study focuses on quantifying the effectiveness of three different blade-mounted devices in speeding up the dissipation of the wake of an offshore horizontal axis wind turbine. Experiments were conducted in a low speed, low turbulence wind tunnel. A small scale wind turbine model was designed using optimum rotor theory to match the tip speed ratio of an offshore wind turbine. The baseline case consisted of a wind turbine rotor without blade-mounted devices. It was tested in the wind tunnel under a range of free stream conditions, and the rotational speed was measured to determine the operational tip speed ratios. A second test case was the same rotor, but with winglets at the blade tips designed to weaken the tip vortices. A third test case was the baseline case rotor with serrated blade tips, designed to introduce turbulence into the core of the tip vortex. Smoke flow visualization and particle image velocimetry (PIV) were used to observe the dissipation of the turbines’ wake. The effectiveness of the blade-mounted devices on wake dissipation was evaluated with a special interest in optimizing the overall energy harvested by an offshore wind farm of a fixed area. It was shown that both tip treatments tested have the capacity to reenergize the flow and decrease the momentum deficit in the wake of a wind turbine.