QUANTIFICATION OF AEROELASTIC RESPONSE OF BLADETOWER INTERACTION IN MULTIMEGAWATT WIND TURBINES

The trend in the current development of modern horizontal axis wind turbines (HAWTs) is towards light-weight, bigger rotor diameter and longer towers. The bigger, lighter and more flexible structure is more dynamically active and sensitive to smaller excitations. Furthermore, with increasing tower length the tower base becomes bigger and transportation problems appear. Nowadays, tower dimensions have almost reached roads limits. On the other hand, the influence of external factors such as wind shear and rotor-tower interaction become more important. To ensure that the dynamic behavior of the wind turbine structure will not influence the stability of the system and to further optimize the structure dimensions, a fully detailed analysis of the entire wind turbine structure is essential. Hence, the aim of this work is to investigate the bidirectional blade-tower interaction of a multi-megawatt upwind HAWT. A high-fidelity aeroelastic model based on fluid-structure interaction is presented. The blade aerodynamics was predicted from solving the unsteady incompressible Navier- Stokes equations by means of computational fluid dynamics (CFD), while structure deformation was calculated using finite element (FE). The dynamic response of the wind turbine structure is accomplished in a strongly coupled manner. Both elastic blade and tower are considered, where the blades are modeled as an equivalent cantilever beam while the tower is discretized into finite elements. The numerical model provides insight into wind turbine aerodynamic performance and structure behavior. The study showed that passage of the blade in front of the tower causes a noticeable dip in the rotor aerodynamic torque. Furthermore, the rotor has a strong influence on the tower shedding frequency causing different wake structures between the upper and the lower parts. The dynamic response of the tower is synchronized with azimuthal angle of the rotor and the blades suffer oscillatory deformation particularly in the flapwise direction. This creates cyclic fatigue loads and structure deformation three times per rotation which are considered to be important for the tower design and fatigue-life analyse.