Sensitivity of the aerodynamic force unsteady characteristics of wind turbine airfoil to turbulence

Atmospheric turbulence is the main inducer of the unsteady characteristics in wind turbines. This study aims to explore the sensitivity to turbulence in the aerodynamic unsteady characteristics of the wind turbine airfoil using the large eddy simulation (LES). The turbulent inlet boundary was also generated using the Consistent Discretizing Random Flow Generation (CDRFG). The calculation domain and grid were then established, according to surveying and mapping of the outfield true blade. A series of wind tunnel experiments were carried out to verify the numerical simulation. Specifically, the airfoil was numerically calculated at 2°, 8°, and 14° angles of attack, where the turbulence intensity of the incoming flow was set as 9.6%, 6.5%, and 4.8%, respectively. Among them, the turbulence scale was of the same order of magnitude as the chord length of the airfoil. The results show that the turbulence increased the unsteady characteristics of the lift coefficient of the wind turbine airfoil. Specifically, the higher the turbulence intensity was, the stronger the unsteady characteristics were. The leading edge of the airfoil was much more sensitive to the incoming flow of turbulence, whereas, there was no influence of the turbulence on the middle and trailing edge of the airfoil. Once the turbulence intensities of incoming flow were 9.3%, 6.5%, and 4.8%, the standard deviations of the lift coefficient of airfoil were 6.36, 5.42, and 4.90 times that of the uniform incoming flow at 2° angle of attack, respectively, while the standard deviations of the lift coefficient of airfoil were 3.95, 3.33, and 3.02 times of uniform flow at 8° angle of attack, respectively, and the standard deviations of lift coefficient of airfoil were at 1.78, 1.63, and 1.40 times that of the uniform incoming flow at 14° angle of attack, respectively. It infers that there were much more significant fluctuation characteristics of the lift coefficient caused by the turbulence at the small angles of attack, compared with the large angles. When the angles of attack were 2°, 8°, and 14°, about 0.5, 0.3, and 0.1 times increases were found in the chord length for the standard deviations of the pressure on the suction surface from the leading-edge point to the position, compared with the uniform incoming flow, indicating the turbulence influence of pressure fluctuation on the suction surface in a larger area at a small angle of attack. The reason was also that the more significant characteristics were observed in the lift coefficient fluctuation caused by turbulence at the small angle of attack. Furthermore, the pressure fluctuations on the suction surface were larger than those on the pressure surface. As such, the suction surface greatly contributed to the fluctuations of the lift coefficient, indicating the relatively higher sensitivity to the incoming turbulent flow. A consistent trend was found in the power spectrum curve between the fluctuating pressure at the leading edge of the airfoil and the turbulent incoming velocity in the entire frequency domain region. Therefore, the turbulence of the incoming flow was dominated by the fluctuating characteristics of the pressure at the leading edge. Going back along the chord of the airfoil, the fluctuation pressure of the airfoil suction surface only responded to the incoming flow turbulence in the low-frequency region, while the pressure fluctuation at the high frequency depended on the own flow characteristics. Anyway, the aerodynamic sensitivity of wind airfoil to turbulence can be expected to reduce the power fluctuation for the better control strategy of the wind field.