Wake redirection via active yaw control is a promising strategy to improve the whole wind farm performance. To successfully apply such an operational control in the real-world engineering, it is necessary to have a precise knowledge of the yawed turbines and their wakes. In the current work, a series of numerical simulations are performed by the high-fidelity SOWFA tool, to study the occurring asymmetry in oppositely signed yaw angles, as well as its effects on wake-steering performance. The simulation results demonstrate that the wake curl is asymmetric between positive and negative yaw angles, arising from the interaction of the counter-rotating vortices with the vertical wind veer. Some key wake properties are also observed to be asymmetric with respect to the yaw angle direction, especially the lateral wake deflection, which can affect the efficiency of active yaw control. What's more, a load study of wake steering is conducted, showing that the positive yaw offsets are less harmful to the tested NREL 5MW wind turbine. In conclude, this work emphasizes the necessity and importance of considering the yaw direction in implementing yaw-based wind farm control.
Wind energy is a kind of renewable energy with great development prospects (Chehouri et al, 2015). In order to maximize wind energy extraction within the limited available land, multiple turbines are commonly installed in an organized array in the wind farm. However, a consequent drawback is the strong wake interference among turbines, which has a significant impact on the wind farm performance, not only increasing turbine loads, but also decreasing energy capture. To mitigate the negative effects of wake interaction, many efforts have been made before. For instance, some scholars paid attention on altering wind farm layout, example studies include (Park et al, 2015; Kirchner-Bossi et al, 2018), and increasing streamwise distance between the consecutive turbines was suggested. What's more, some active wake control strategies were proposed, which can be divided into two classes, one about reducing the axial induction of the upstream turbine (Annoni et al, 2016), and the other is the wake redirection techniques, including pitch angle control, tilt angle adjustment and active yaw control (Bastankhah et al, 2019; Miao et al, 2017). In Fleming et al. (2014), they tested several wake redirection strategies and found that active yaw control is the most promising method to improve wind plant performance. It is implemented by intentionally misaligning the upstream turbine rotor to the incoming wind direction, thereby deflecting the wake and avoiding the downstream wind turbines. Although the power output of the yawed turbine itself is reduced, the total power of entire wind farm can potentially increase, as the downwind turbine is less affected by the upstream wakes.