This paper presents the analysis of the modal frequencies and mode shapes of the Vestas four-rotor wind turbine using two operational modal analysis methods. The Frequency Domain Decomposition method is used for standstill analysis, and the Ibrahim Time Domain method is used for the operational analysis. In the standstill analysis, 8 modes are estimated from the singular-value plot. In the operational analysis, 8 modes are identified among these three from the standstill analysis. The operational analysis reveals that the modes related to in-plane rotor motions dominate due to the high aerodynamic damping of out-of-plane motions.


When designing a mechanical system a numerical or analytical model is often established and used to validate if the system can withstand external loads in its lifespan, to tune controllers, and to optimize the structural design. Once the dynamic model is established, then the next step is to validate the model. This is often done by comparing modal parameters such as modal frequencies, damping, and mode shapes. The reason is that the modal parameters provide important insights into the stability and characteristic dynamics of the system. If the modal parameters are not predicted correctly, it can lead to severe vibrations, and eventually cause failure. The modal parameters from the physical model can be used to gain important knowledge of the system dynamics characteristics and can be put into simulation-model to increase its fidelity.

The purpose of this paper is to extract modal parameters of the Vestas four-rotor concept demonstrator wind turbine for a range of rotor speeds. The multi-rotor wind turbine (MRWT) is shown in Fig. 1, and consists of four 225 kW rotors of which is 29 m in diameter. The height for the lower and upper platform is 29 m and 59.5 m respectively. The terrain surrounding the MRWT is seen in Fig. 2. It shows that the MRWT experiences stable wind conditions when the wind direction is onshore from the fjord. In other directions, the wind becomes more turbulence due to buildings, and complex landscape.

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