Aeroelastic analysis of a rotating wind turbine blade using a geometrically exact formulation

Abstract

In this paper, an aeroelastic analysis of a rotating wind turbine blade is performed by considering the effects of geometrical nonlinearities associated with large deflection of the blade produced during wind turbine operation. This source of nonlinearity has become more important in the dynamic analysis of flexible blades used in more recent multi-megawatt wind turbines. The structural modeling, involving the coupled edgewise, flapwise and torsional DOFs, has been performed by using a nonlinear geometrically exact beam formulation. The aerodynamic model is presented based on the strip theory, by applying the principles of quasi-steady and unsteady airfoil aerodynamics. Compared to the conventional steady aerodynamic model, the presented model offers a more realistic consideration of fluid–structure interactions. The resulting governing equation, expanded up to the third-order terms, is analyzed by using the reduced-order model (ROM). The ROM is developed by employing the coupled mode shapes of a cantilever blade under free loading condition. The specifications of the 5MW-NREL wind turbine are used in the simulation study. After verifying the ROM results by comparing them with those of the full FEM model, the model is used in additional static, modal and transient dynamics analyses. The results indicate the important effect of geometrical nonlinearity, especially for larger structural deformations. Moreover, nonlinear analyses reveal the important effects of torsion induced by lateral deformations. It is also found that the governing equation is more efficient, and sufficiently accurate, when it is developed by using the second-order kinetic terms, third-order potential terms and the second-order aerodynamic terms together with third-order damping. Finally, the effects of nonlinearities on the flutter characteristics of wind turbine blades are evaluated through frequency and dynamic analyses.