Effects of bend-twist coupling on flutter limits of composite wind turbine blades

The effects of bend-twist coupling on the flutter limits of large composite wind turbine blades are investigated in this work. A mixed, geometrically exact, and nonlinear finite element for anisotropic beams is developed based on Hellinger-Reissner variational principle. The nonlinear beam model is combined with Blade Element Momentum method and an unsteady dynamic stall model to perform flutter analysis of composite blades. A 5-MW composite wind turbine blade with shallow-angled skins is studied, where unbalanced laminates (i.e. ply angle and ply thickness unbalances) are implemented to introduce the bend-twist coupling in the blade through material anisotropy. The results show that with bend-twist coupling due to geometrical nonlinearities, the predicted flutter limits are around 23% lower than the same predicted in linear flutter analysis. The unbalanced skin laminates show no significant influence on the flutter limits of the blade in nonlinear flutter analysis.