Load application in wind turbine blades modelled as reduced-order multibody structures in the floating frame of reference formulation

Wind turbine aeroelastic simulation tools usually rely on the Blade Element Momentum (BEM) theory to calculate aerodynamic loads and a beam finite element model for the structural response. A method to transfer distributed aerodynamic loads computed by these aeroelastic codes to a multibody reduced-order model based on the Floating Frame of Reference Formulation (FFRF) is presented. The model is based on solid finite elements, thus constituting a higher-fidelity alternative to beam elements, and the number of degrees of freedom (DOFs) is reduced using the Hurty/Craig-Bampton method and interface reduction based on interface modes. The proposed method consists of calculating equivalent concentrated loads and applying them to the model using interpolation multipoint constraints (RBE3). Two approaches are introduced to avoid applying loads to the internal DOFs of the reduced-order model, by including internal interfaces at the load application cross-sections, either described by interface modes or using a minimum strain energy formulation. Results show that adding load interfaces can improve the static response to torsional moments, but the overall increase in accuracy is not substantial; additionally, it is found that minimum strain energy interfaces increase the stiffness of the model. The methodology is also applied to a 12.6 m wind turbine blade, showcasing the better torsional response of the model when compared with standard beam models.