The Effect of Aerodynamic Evaluators on the Multi-Objective Optimization of Flatback Airfoils

With the long lengths of today's wind turbine rotor blades, there is a need to reduce the mass, thereby requiring stiffer airfoils, while maintaining the aerodynamic efficiency of the airfoils, particularly in the inboard region of the blade where structural demands are highest. Using a genetic algorithm, the multi-objective aero-structural optimization of 30% thick flatback airfoils was systematically performed for a variety of aerodynamic evaluators such as lift-to-drag ratio (CL/CD), torque (CT), and torque-to-thrust ratio (CT/CN) to determine their influence on airfoil shape and performance. The airfoil optimized for CT possessed a 4.8% thick trailing-edge, and a rather blunt leading-edge region which creates high levels of lift and correspondingly, drag. It's ability to maintain similar levels of lift and drag under forced transition conditions proved it's insensitivity to roughness. The airfoil optimized for CL/CD displayed relatively poor insensitivity to roughness due to the rather aft-located free transition points. The CT/CN optimized airfoil was found to have a very similar shape to that of the CL/CD airfoil, with a slightly more blunt leading-edge which aided in providing higher levels of lift and moderate insensitivity to roughness. The influence of the chosen aerodynamic evaluator under the specified conditions and constraints in the optimization of wind turbine airfoils is shown to have a direct impact on the airfoil shape and performance.