Research on Multi-Objective Aerodynamic/Structural Design Optimization Method for Large Thickness Flatback Airfoils

With the operation of ultra-large wind turbines exceeding 10 MW, the blade length has significantly increased. Further consideration is needed for the contribution of the root airfoil family (with a relative thickness >40%) to improve the wind energy capture capacity of wind turbines and meet the structural constraint requirements. The large-thickness flatback airfoils offer advantages such as a larger slope of the lift curve, a higher maximum lift coefficient, and better structural characteristics. These advantages contribute to blade weight reduction and performance improvement, making it a preferable choice for balancing both aerodynamic and structural requirements. However, flatback airfoils that have not been designed with good compromises may incur severe drag penalties and fail to improve both blade structural performance and aerodynamic performance. This paper presents a multi-objective aerodynamic/structural design optimization for large-thickness flatback airfoil. The method takes the minimization of airfoil drag at the design state as the aerodynamic performance optimization objective, and maximize the sum of the cross-sectional stiffnesses of the airfoil profile in the flapwise and edgewise directions as the structural performance optimization objective. The validity of the optimization framework is verified through the aerodynamic and structural optimization of a flatback airfoil with a relative thickness of 60%, resulting in a set of 25 optimal shapes from the Pareto solution. Considering that the accuracy limitation of the rapid aerodynamic performance evaluation method used in the optimization design in predicting the aerodynamic characteristics of airfoils with large blunt trailing edge separation flow, the aerodynamic characteristic comparison of the optimized airfoil and the baseline is carried out by using a highfidelity IDDES method. For the optimized airfoil with the most balanced aerodynamic and structural performances, the drag coefficient at the design state is reduced by 33.79%, the cross-section flapwise stiffness is increased by 16.0%, and the cross-section edgewise stiffness is increased by 9.4%, verifying the effectiveness of the multi-objective design optimization method developed in this paper. Moreover, the numerical simulation results obtained by the high-fidelity IDDES method are further analyzed to reveal the physical mechanisms of the aerodynamic performance improvements of the optimized airfoil presented in this paper.