ABSTRACT
The main purpose of a structural model of a wind turbine is to be able to determine the temporal variation of the material loads in the various components in order to estimate the fatigue damage. Further, a dynamic system is used when analysing the stability of the wind turbine design, including perhaps the control system. To calculate the deflections and velocities of the various components in the wind turbine in the time domain, a structural model including the inertia terms is needed. Then the dynamic structural response of the entire construction can be calculated subject to the time dependent load found using an aerodynamic model, such as the BEM method. For offshore wind turbines, wave loads and perhaps ice loads on the bottom of the tower must also be estimated. One way of setting up the structural model, based on the principle of virtual work, is presented here in detail. However, more formal ‘finite element’ methods have also been used in different aeroelastic codes. The velocity of the vibrating wind turbine construction must be subtracted when calculating the relative velocity seen locally by the blade as shown in equation (9.32). The loads therefore depend on the deflections and velocities of the structure, which again depend on the loads. The structural and aerodynamic models are therefore highly coupled and must be solved together in what is known as an aeroelastic problem.
