Experimental Analysis of Near andTransitional Wind Turbine Wake Using Stereo Particle Image Velocimetry

For a wind turbine, the flow downstream of the turbines (wake) is influenced by the power extraction process. This causes a momentum deficit behind the rotor, resulting in lower velocities in the wake than the free-stream velocity. Wind turbines in a wind farm are likely to operate in the wakes of upstream turbines. This drops the power production of these wind turbines. Better control strategies could help in reducing these wake losses. The first step towards achieving this goal is understanding the physics of wake re-energization. The wake of an horizontal axis wind turbine (HAWT) is investigated using Stereo Particle Image Velocimetry (SPIV) in the open jet wind tunnel facility (OJF). The wind turbine used in the experiment is a 2-bladed wind turbine having the profile of Eppler E387 airfoil, with a design tip speed ratio of 6. Using SPIV, a three component velocity field is obtained for two different tip speed ratios (TSR). The wake is captured upto 5 diameters downstream with a focus on the shear layer. Both phase-locked and uncorrelated sampled data are obtained during the experiment. The results from the experiment show that the tip vortices interact and leapfrogging phenomenon occurs. After the leapfrogging the tip vortices breakdown rapidly. The downstream location at which the leapfrogging takes place is dependent on the tip speed ratio. The distribution of the stream-wise axial velocity increases immediately after the point of maximum leapfrogging, implying the point of wake re-energization. The reason for tip vortex leapfrogging may be attributed to the difference in the manufacturing of the two blades. A 6% difference in the circulation is observed between the tip vortices released by the two blades. The circulation calculated along the span of the blades shows a good match with the data obtained from blade element method simulations. Preliminary results showing the loads on two different blade sections are also calculated.Aerospace EngineeringWind Energ