Influence of low-level jet on structural loads of near-surface wind turbines

Wind turbines have been widely applied to convert the kinetic energy of wind into electrical energy. Among them, the yaw system of wind turbine is the one of the most important components to rotate the rotor optimally into the wind direction. However, the load fluctuation has posed a great challenge on the safety and reliability of yaw system. This study aims to clarify the influence of the low-level jet (LLJ) on the structural loads of the wind turbine. The harmonic superposition was firstly used to generate the atmospheric turbulence using the LLJ wind speed spectrum and a jet model. Then, the power and thrust were calculated for a 15 MW wind turbine using the blade element momentum (BEM) and geometrically exact beam theory (GEBT). The numerical accuracy was also validated against the theoretical values. It was found that the overall numerical error was less than 10%, where the numerical error below the rated wind speed was less than 5%. A systematic analysis was made to determine the power performance, rotor yaw moment, rotor pitch moment, and tower foundation moment in the 15 MW wind turbine under different LLJ conditions. The results showed that two coupling effects of the jet and strong shear were identified in the height range of the wind turbine. Specifically, the dominant type was changed with the jet height. Once the jet height was lower than the hub height, the dominant type was the jet; when the jet height was higher than the hub height, the dominant type was the strong shear. Correspondingly, the rotor power, rotor pitching moment, and rotor yaw moment of the LLJ increased by about 40%, 50%, and 1.5 times than before, respectively. The fluctuation intensity of the yaw moment for the tower base also increased by 60% of the loads in the condition of the LLJ spectrum. Therefore, the regions with the frequent LLJ streams needed to be considered for the design of the yaw bearings, tower foundations, and components of wind turbine. At the same time, the power spectrum characteristics of the rotor yaw and pitch moment were distributed in a double-staircase type, which was mainly divided into a decreasing and a secondary increasing stage. Among them, the secondary increasing stage was caused by the coupled response of multi-scale frequency in the atmospheric turbulence and the rotational frequency of wind turbine. When the jet height was lower than the hub height of wind turbine, the pitching moment of the rotor caused the wind turbine to tilt downward. There was the risk of the wind turbine blades hitting the tower. A full consideration was also made to determine the impact of the LLJs on the structural load characteristics, when installing the high-rise structures, such as the wind turbines in the areas with the frequent LLJs. Finally, there was the anisotropic Reynolds stress in the incoming flow. A strong response of the tower base moment was then observed at different frequencies in the corresponding time period. The larger moment fluctuation was posed a threat to the yaw direction of the tower base. Specifically, the maximum moment fluctuation was about 60% higher than that in the Kaimal spectrum. Therefore, controlling the fluctuation characteristics of tower load in the yaw direction can greatly contribute to the less impact on the fatigue characteristics of wind turbines in the regions with the frequent LLJs. © 2022 Chinese Society of Agricultural Engineering. All rights reserved.