"Wake effect" is a universal phenomenon in large-scale wind farms, which will adversely affect the benefit of wind farms. In this study, we adopt a tandem arrangement of two wind turbines to investigate a new wake effect restraining strategy called "Counter-Rotating Wind Turbine (CRWT) system" based on actuator line model and large eddy simulation. From the numerical results, the power output of the downstream wind turbine in counter-rotating configuration increased compared with co-rotating configuration. As the longitudinal spacing increases, the fluctuation of the power output of the downstream wind turbine increases, and the rotation direction has little effect on it. The yaw moment (Myaw) and blade-root out-of-plane bending moment (Moop) increase with increasing longitudinal spacing due to the local wake meandering caused by the amplified unstable shed vortex, but the rotation direction of the downstream wind turbine has little effects on them. Moreover, the low-speed shaft torque (Mlss) of the downstream wind turbine with the reverse rotation strategy increases when the longitudinal spacing is closer, which is consistent with the power output result.


Wind energy is widely used as one of the most mature technologies and renewable energy sources that can be developed on a large scale. With the increasing demand for wind power generation, and considering many factors such as the location of the wind turbine, clusterization of wind turbines to form a large wind farm for grid-connected power generation has become the main form of wind energy development. In large wind farms, the wake velocity loss and the increase in turbulence intensity in the wake region can not only result in a sharp reduction in the total power output but also cause an impressive increase of the fatigue load of wind turbine structure, this phenomenon is known as the "wake effect" (Frandsen S, et al., 2005). In order to reduce the influence of "wake effect" in wind farms, many scholars have proposed a number of wake effect restraining strategies based on different principles, such as "Yaw control strategy", "Vertically-staggered strategy" and "Counter-Rotating Wind Turbine(CRWT) system", etc.

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