Load Pulses on Wind Turbine Structures Caused by Tower Interference

One of the factors which causes cyclic loading on wind turbines is the influence of the tower on the aerodynamic performance of the rotor. Although a flickering in the power output of the turbine can be observed, the load pulses acting on the blades, the main shaft, the support bearings, the power transmission system, and the tower are potentially more troublesome. Aerodynamic modelling of this interaction by means of symmetric tower flow models is not capable of capturing all the effects present such as aerodynamic coupling of the tower and blades, tower wake meandering, lateral tower loads, and break-up of the rotor wake. These effects become more important for highly loaded rotors or for very flexible blades for which the gap between the rotor and the tower is small. In this paper, the results of 2D CFD simulations are used in order to correct the predictions of traditional BEM methods. Two turbulence models are used and their advantages and drawbacks are discussed. It is shown that the rotor also induces a periodic lateral loading on the tower and a shift in frequency of the vortex street behind the tower. It is also shown that the tower causes a break up of the turbulent wake of the rotor. The BEM formulation including this correction is validated against experimental data available in the literature. The method is applied to calculate the effects on a test blade geometry.