Unsteady wake analysis of horizontal wind turbine using proper orthogonal decomposition

Wind turbine wakes have posed a great challenge to the aerodynamic performance and fatigue loads of the downstream wind turbines. This study aims to explore the impacts of blade tip speed ratio on the unsteady wake characteristics of horizontal wind turbines under neutral atmospheric stable conditions. Large Eddy Simulation (LES) with the actuator line model was adopted to investigate the flow field around the wind turbine under the different ratios of tip speed. A snapshot of proper orthogonal decomposition was selected to distinguish the coherent structures in the wind turbine wakes wind turbines. Results show that the tip speed ratio presented a remarkable influence on the velocity profile of the wind turbine wakes within the downstream distance range of 12 times of wind turbine diameter. More importantly, the influence was relatively weakened with the increase of the distance downstream. Furthermore, the velocity deficits reached the maximum at the downstream distance of around two times of wind turbine diameter, which recovered completely around the downstream distance of 28.5 times of wind turbine diameter for all tip speed ratios. As such, the tip speed ratio posed notable impacts on the turbulence intensity of wind turbine wakes. There was the weakest turbulence intensity, when the tip speed ratio was 2, while the maximum, when the tip speed ratio was 3, indicating the strongest momentum and energy exchange. Correspondingly, the tip speed ratio of 3 was greatly beneficial to the wake recovery during the operation of the wind turbine. Meanwhile, there was also a significant influence of tip speed ratio on the scale and energy contained in coherent structures in the wind turbine wakes. The wind turbine model demonstrated that the largest turbulence structures and the highest energy were achieved in the wind turbine at the tip speed ratio of 3. The coherent structures were composed of high- and low-speed alternately counter-rotating vortices in the wind turbine wake, the scale of which in the vertical direction reached the boundary layer thickness. The wind energy absorption and utilization of wind turbines can be mainly reflected in the scale of turbulence structures in the spanwise of the wind turbine wakes. Therefore, there was a significant influence of tip speed ratio on the velocity deficit, the turbulence intensity, flow structures, and the scales in the wind turbine wakes. This finding can provide a strong reference to clarify the influence of blade tip ratio on the statistical characteristics and structural evolutions of wind turbine wakes, particularly for the optimal design and operation strategy of wind turbines.