Finite element analysis of wind turbine blades subjected to torsional loads: Shell vs solid elements

In this work the modelling of wind turbine blades subjected to torsional loads is explored. Usually, wind turbine blades are modelled using Outer Mold Layer (OML) shell models, whose mid-surface does not coincide with the nodal plane, and have been shown not to capture their behaviour correctly in torsional loadings, highly underestimating the torsional stiffness of the blade. To better understand the differences between using OML shell models and solid element models, a cylinder tube first is analysed and the conclusions from this study are applied for the wind turbine models. It was observed that artificial deformation modes were responsible for the erroneous results when using offset shell elements. These results can be drastically improved by controlling the stiffness associated with the drilling Degree Of Freedom (DOF). Furthermore, a high fidelity definition of the geometry of the blade is crucial to accurately capture its behaviour in torsion. By carefully controlling these parameters, the stiffness difference between the offset shell and solid models was reduced from 35% to 5%.