Stereo PIV Investigation On The Wake Characteristics Of Horizontal Axis Wind Turbine In Wind Tunnel

As a kind of clean and renewable energy, wind power has gained growing attention with the rapid developing in the world scope. The large-scale wind farms and large-capacity generators have become the mainstream, which highlights the importantce of wind turbine arrangement during the construction period. The wake effect is considered as one of the critical influencing factors.The wake effect not only leads to the output decrease of downstream wind turbine generators, but also exacerbates the turbulence intensity, which brings extra loads on the wind turbine blade. So the structural fatigue damage is increased and will definitely reduce the service life of wind rotors. The aspects mentioned above are also directly related to the economic benefits of wind farm.In order to reduce the negative impact of wind turbine wake, it is very important to be ware of the wake characteristics and then describe it accurately. But at present, most methods for wake effect calculation are based on too many hypothesises, which are unable to depict the three-dimensional flow field accurately and even the evolution of the tip vortex. Therefore, in the long run, in order to form a complete wake theory, there is a great need to perform detailed experimental observation on the wake. On the other hand, the investigation on wake effect also improves the recognition of wind turbine aerodynamic characteristics. In other words, the research on the wake provides an important basis for the optimal layout of wind turbines and then improves the efficiency of wind farms.In this paper, based on the three-dimensional particle image velocimetry technique, the experimental research on the wake of a horizontal axis wind turbine model is carried out in a big wind tunnel. The specific contents and results are as follows:(1) The calibration of flow properties in wind tunnel test section is conducted during the preparation period. With the help of hot wire anemometer, the flow uniformity and stability in the test section are examined. The results provide a critical reference to comfirm the appropriate region for measuring, guaranteeing the accuracy for wake measurement.(2) On the basis of blade element momentum theory and Wilson theory, the wind turbine blade is designed through MATLAB programming tool. Aiming at obtaining optimum power, the distributions of chord and twist angle along the blade are designed. Through this procedure, the geometry of the wind turbine model is finalized.(3) Using the wind turbine model obtained in the former step, the measurement of wind turbine wake is conducted in wind tunnel with the help of three-dimensional particle image velocimetry technique. The high resolution and transient wake flow information is obtained at two different tip speed ratios. The experimental observations are valuable for revealing the complete generation and development of the tip vortex.The experimental analysis reveals that the helix trajectory of tip vortices after shedding from blade tips hinders the wake velocity recovery, which forms a deficit zone for axial velocity after wind rotor till the vortices "leapfrogging" phenomenon. After that, the dissipation of tip vortices weakens the shielding effect and the wake velocity begins to recover.(4) In view of engineering application, the experimental data is also useful for the validation of empirical/semi-empirical wake models. Through the comparisons, it is showed that, due to different assumptions and simplifications, the widely applied wake models are not consistent with the experimental data, revealing that the precision of these wake models still needs to be improved.(5) Furthermore, the tip vortex "leapfrogging" phenomenon is proved as the mark of transition from near wake to far wake region. Then a new wake-jet model is built by adding near wake properties to the jet modelling. Through the validation with measurements, it is proved that the accuracy of wind turbine wake estimation is improved when adopting the wake-jet model.In conclusion, this paper shows the significance on revealing the physical mechanism of wind turbine wake flow, specifys the standardization of near wake region and far wake region, then proceeds the improvements on the wake-jet model, making it more applicable for evaluating the wake effect in wind farm.