Coupled aerostructural shape and topology optimization of horizontal-axis wind turbine rotor blades

Traditionally, Aerodynamic Shape Optimization (ASO) and Structural Topology Optimization (STO) are considered as two separate, consecutive stages in the aerostructural design process of wind turbine rotor blades. Since such a modeling strategy does not adequately account for the coupling effects between the blade aerodynamics and its structural performance, in this paper a Coupled Multi-objective Shape and Topology Optimization (CMSTO) approach is presented, which simultaneously optimizes the outer shape and the interior structural layout of the turbine blade from aerodynamic and structural requirements. The framework uses the weighted sum method, whereby the sum of the aerodynamic and structural objectives multiplied by specific weighting factors is minimized employing an incremental-iterative update procedure. The coupled optimization process is performed by sequentially carrying out shape and topology optimization steps for beam-type structures, in accordance with a staggered scheme. For the aerodynamic and structural optimizations the rotor power coefficient and the blade structural compliance are considered as the objectives, respectively. The aerodynamic response of the blade is evaluated by the Blade Element Momentum (BEM) method, which, together with a reduced beam-type Finite Element Method (FEM) model that simulates the structural response, facilitates the application of a gradient-based algorithm with analytical sensitivities. Accordingly, the computational efficiency of the CMSTO framework is warranted. The shape design variables are characterized by the locations of Non-Uniform Rational B-Splines (NURBS) control points that parameterize the blade outer shape. The topology design variables are represented by the relative densities assigned to the finite elements modeling the blade cross-sections, in correspondence with the Simplified Isotropic Material with Penalization (SIMP) method. The CMSTO framework is used to optimize the NREL 5 MW reference rotor blade, whereby the results are compared to those obtained from a separate ASO-STO approach and a pure STO approach. The rotor power coefficients calculated by the CMSTO and ASO-STO approaches are about 4% larger than that of the reference rotor blade. In addition, the blade structural compliance computed by the CMSTO approach is reduced by an extra 16% and 41% compared to the compliances found by ASO-STO and pure STO approaches, respectively. Such significant improvements clearly demonstrate the benefits of the CMSTO approach in the quest of wind turbines with higher power output and better structural performance