Modal dynamics of a three-bladed tri-rotor wind turbine

In this paper, the structural modal frequencies of DTU three-bladed tri-rotor reference wind turbine are analyzed using a high-fidelity linear time-invariant model. In the analysis, the three rotors rotate at equal speed and are aligned in the same plane. The high-fidelity model is established by combining a model of the support structure with three identical time-invariant rotor models. This makes it is possible to use a direct eigenvalue analyze to get the model frequencies as a function of rotor speed. The proposed method is validated by modeling a single-rotor wind turbine and compare modal frequencies with the aeroelastic stability tool HAWCSTAB2. After the validation, the tri-rotor wind turbine is analyzed. The modal frequencies showed that the torsional tower mode is at the same frequency level as the first tower fore-aft and side-side tower mode and that some tower modes change significantly during rotation.