Study On The Effects Of Tidal Turbine And Array On The Flow Field

As a clean and renewable energy, tidal current energy has received widespread attention to meet the growing needs for the sustainable development of our economy. However, natural tidal flowsare disturbed by the operation of tidal stream turbines, which change the potential distribution of the available energy and have an influence on the hydrodynamic environment. It is important to predict the changes of the tidal flow field in order to optimizethe turbine array configuration, and it is also beneficial to further strengthen the potential of extractable energy and minimize the impacts on the hydrodynamic environment.In this study, a series of experimentshave been carried out to investigate the potential impacts of horizontal axis turbine on the flow field. It is found that the influence of turbine rotor on the upstream flow pattern is very small.Secondary flows, almost within the area of velocity deficit, appear in the near field of wakes. The hanging pole, used to fix the turbine rotor, has a blockage effect on the flow movement above the turbine rotor. Due to the velocity decrease around support structure, the distribution of velocity deficit in the vertical direction moves to the free surface with the distance increasing downstream.In order to better understand the development of wakes downstream, this study further analyzes the influence factors ofwake attenuation, with acomparison with other experiment studies. The results show that the distribution of flow characteristics in the near wake area is mainly determined by the tip speed ratio of turbine rotor, and the wake recovery rate is influenced by both the turbulence intensity of ambient flow and turbine blockage ratio.Whilethe shape of flow characteristics distribution usually changes during wake recovery process,which is mainly caused by the nonuniformdistribution of velocity on the cross section.A three dimensional numerical model is employed and validated using the experiment results.The validation results indicate that the 3D hydrodynamic model can be used to investigate the natural tidal flow, and it has a good accuracy to predict the changes of flow field around turbine arrays. Compared with a single turbine, thesmall-scale turbine array, arranged in a single row, causeslarger velocity changes, and retards the progress of wake recovery with the wakes from individual turbines merging to form a single wake. While the large-scale turbine array, arranged cross the flow section, heightens both the high water level and low water level, with the greater impacts being on the low water level during the ebb tide. Due to the large blockage effect, the velocities above and below rotor-tip increase significantly while the velocity decreases in the rotor swept area, which accelerates the wake recovery.The turbine array configuration has effects on the distribution of extractable energy. The distribution pattern of tidal power extracted by a single row array is mainly depended on the natural flow features, and the extractable energy potential could be improved with the decrease of lateral spacing. The multi-row array in a staggered manner changes the natural tide flows, it results in the significantly nonuniform distribution of extractable power, with lower energy in the array. The wake influence of the upstream turbines is attenuated with the increased lateral spacing or longitudinal spacing. Correspondingly, the turbine performance in large arrays is generally improved. However, the lateral spacing decreases the blockage ratio of array andaccelerates the wake attenuation, and thus it is more effective to improve the energy extraction and reduce the changes of hydrodynamics around turbine array.