Wind tunnal experiment study of wind turbine airfoil flapwise vibration in the traling edge windward load reduction condition

Shutdown and individual pitch for load reduction are important measures to ensure the structural safety of large wind turbines in extreme typhoon weather. When the blade trailing edge faces the wind (180° angle of attack), the aerodynamic coefficient of the airfoil is relatively small. This indicates that the trailing edge windward condition is favorable for load reduction. However, unfavorable vibrations exist in the flapwise direction under this condition. Based on wind tunnel experiments, this paper investigates the aeroelastic characteristics of three different typical wind turbine airfoils (DU00-W2-401, DU91-W2-250, and NACA64618) under the condition of the trailing edge windward load reduction. The study measures the flapwise displacement of the different airfoils at various wind speeds in the trailing edge windward condition. It is discovered that airfoils with different cross-sections of the blade exhibit flapwise vibrations at a specific flow angle of attack and reduced structural frequency. Specifically, the overall range of unstable angle of attack is from 155° to 167°. The instability region forms an inverted "V" shape, where the unstable angle of attack range increases as the reduced frequency decreases. This type of flapwise vibration exhibits frequency lock-in characteristics, meaning that the vibration frequency becomes locked to the first flapwise mode natural frequency of the structure. Additionally, the vibration amplitude increases as the structural reduced frequency decreases, and the maximum peak-to-peak vibration exceeds the chord length of the airfoils. For modern wind turbine blades with a pre-torsion design, it is advisable to minimize the occurrence of the main blade section entering the sensitive angles of flapwise instability during the trailing edge windward load reduction condition to prevent structural damage.