Modelling large deflections through reduced-order multibody structures in the floating frame of reference formulation

The floating frame of reference formulation (FFRF) is widely used to model flexible multibody systems, such as wind turbines. With the development of longer and more flexible wind turbine blades, it is essential that geometrical nonlinear effects are considered to accurately capture the large deflections these structures experience. A method is presented that uses the FFRF to this end by partitioning a structure into sub-bodies, each with its own floating frame. The elastic deformation of the sub-bodies is represented by Hurty/Craig-Bampton component modes, calculated from an existing solid finite element (FE) model; the use of a high-fidelity FE model allows for capturing complex structural phenomena (e.g. three-dimensional effects) that can arise in wind turbine blades. Interface modes are used to reduce the number of interface degrees of freedom, which require a special treatment of the constraint equations needed to connect the sub-bodies. Conventional rigid multipoint constraints are also analysed. It is demonstrated that this FFRF-based multibody approach can compute accurate nonlinear static responses when compared to standard solid finite element models, with a good compromise between computational cost and accuracy.