A novel forecasting method of flutter critical wind speed for the 15 MW wind turbine blade based on aeroelastic wind tunnel test

Flutter is one of the primary challenge that has to be overcome for the development of super-large wind turbine blade. Although the vibration wind tunnel test based on an aeroelastic scale model is the most effective solution, similarity ratio and measuring accuracy of the model cannot be solved accurately by traditional methods. In this study, a novel aerodynamic-stiffness mapping integrated three-dimensional complete aeroelastic scale model design method of super-long flexible blades based on the principle of equivalent stiffness of main beam was proposed for the first time. Later, synchronous full-wind angle wind tunnel tests of vibration and force measurement were carried out using the high-speed photography technology and a high-frequency six-component balance. The nonlinear dynamic response spectral characteristics of NREL-15MW super-long flexible blades were discussed systematically. A comparative analysis of flutter performances and critical instability state of wind turbine blades based on blade tip deflection and blade root reaction force was carried out, which proved the feasibility of forecasting flutter performances according to blade root reaction force. Finally, the blade root reaction force method for flutter instability forecasting of super-long flexible blades was put forward. Results show that the proposed aeroelastic scale model design and experimental method can simulate dynamic performances and flutter behaviors of wind turbine blades accurately and effectively. In the test, it finds that the super-long flexible blades flutter in pitch angle ranges of 93°–96° and 284°–287°. In the flutter ranges, the flutter critical wind speed decreases firstly and then increases with the increase of pitch angle, reaching the valley (5.4 m/s) at 94°. The blade root reaction force and blade tip deflection have consistent divergence and strong correlation. It is suggested that the wind turbine blade enters into the flutter critical state when the flutter index of blade root reaction force is δ ≥ 2%.