Su Wang, Jiajia Chen, Le Zhou, Xin Shen, Chun Li, Zhaohui Du
With the continuous increase in wind turbine power capacity, ultra-long flexible blades face intensified aeroelastic instability risks due to reduced structural stiffness, enhanced modal coupling, and aerodynamic nonlinearity. In addition to the analysis of basic vibration characteristics, this study focuses on energy-related mechanisms of aeroelastic instability under various working conditions. Using a numerical model integrating Dynamic Blade Element Momentum Theory (DBEMT) and Geometrically Exact Beam Theory (GEBT), over 400 time-domain simulations were conducted to characterize instability onset and development. Results reveal four distinct aeroelastic instability regions, each dominated by specific modes. In Region A, flutter dominated by the 2nd flapwise mode is observed. In Region B, flutter dominated by the 1st edgewise mode is observed. In Region C, flutter dominated by the 2nd edgewise mode is observed. While in Region D, where the medial angle of attack (AoA) of the blade has exceeded the stall angle, stall-induced vibration dominated by the 1st flapwise mode is observed. Energy analysis shows aerodynamic work concentration near the blade tip drives instability, with diverse energy exchange patterns across regions. Except for some operating conditions in region C, where instability is dominated by edgewise energy absorption, most aeroelastic instability conditions are dominated by flapwise energy absorption. Torsional degree of freedom contributes minimally to aerodynamic work, but the torsional vibration exerts a notable influence on the AoA. This, in turn, changes the comprehensive aerodynamic forces impacting the blade as well as the general aeroelastic stability. This study clarifies the relationship between operating conditions and energy-driven instability, offering some reference values for the design work and safety assurance of ultra-long flexible blades of the wind turbine.