TIMEFREQUENCY CHARACTERISTICS OF UNSTEADY AERODYNAMIC FORCES FOR FEATHERED WIND TURBINE AIRFOIL UNDER TOWER BLADE INTERACTION

Abstract
Due to the interference between the wind turbine blades and the flow field of the tower, the actual value of the aerodynamic force is quite different from the theoretical value. The difference in aerodynamic force caused by this interference has a nonnegligible impact on the reliability of the blade and tower structure. Taking the airfoil DU91W2250 as the research object, based on the transient numerical analysis and proper orthogonal decomposition method, considering the interaction between the blade and the tower flow field, the timefrequency characteristics and the influence law of unsteady aerodynamic forces for the feathering airfoil are analyzed, the influence degree of the relative position of the tower blade and the geometric parameters on the mean value of aerodynamic force, the fluctuation range and frequency at different Reynolds numbers are quantified, and the influence mechanism of flow field interference on aerodynamic force are revealed through the analysis of flow field modal energy distribution. Results show that, the vertical and horizontal distances from the aerodynamic center of the airfoil to the geometric center of the tower as well the tower diameter relative to the chord length of the airfoil, which are defined as the dimensionless distance parameters y*, x* and D*, have varying degrees of influence on the aerodynamic force. Among which, y* has the greatest influence on the mean value of lift and drag coefficients, but has no obvious influence on the frequency. The greater the absolute value of y* is, the closer the mean value of C_{l} is to the C_{l} value of a single airfoil. The smaller the absolute value of y* is, the greater the fluctuation amplitude of lift and drag coefficients is, and y* increases from −12 to 12, the minimum value of average lift coefficient is −0.48, and the maximum value is 1.16. When x* decreases and D* increases, the mean reverse drag force increases, the fluctuation amplitude increases, and the fluctuation frequency decreases slightly. When x* is less than the critical value 5, the average drag force of the airfoil with tower is reversed. Within the calculation range, compared to the single airfoil, the maximum deviation of the mean value of lift coefficient for the airfoil with tower is −221.94% and its maximum fluctuation is 28.0% of the lift coefficient of single airfoil. While the maximum deviation of the mean drag coefficient of for the airfoil with tower is −1189.3% and its maximum fluctuation is 121.1%. Due to the influence of the high pressure area in front of the tower, the airfoil flow field exhibits obvious symmetrical pulsation excitation, resulting in the deviation and fluctuation of the aerodynamic forces.

