On critical aeroelastic modes of a tri-rotor wind turbine

This paper presents the first investigation on a high-fidelity linear time-invariant aeroelastic model of multi-rotor wind turbines with three-bladed rotors. The modeling method developed in this work integrates the rotor model from the aeroelastic single-rotor wind turbine stability software HAWCStab2 with a model of the support structure in the aeroelastic multi-body tool HAWC2. The method is used to analyze the linear aeroelastic response of the DTU 30 MW tri-rotor wind turbine to see if the system experiences any aeroelastic instabilities or has any critical low damped modes. Furthermore, a comprehensive aeroelastic analysis of the DTU 10 MW wind turbine is performed and used to compare rotor modes between the two types of wind turbines. The results from the aeroelastic analysis are visualized using a developed mode-clustering algorithm utilizing an extension of the modal assurance criterion. The results show that the tri-rotor wind turbine does not have any aeroelastic instabilities. Comparing the rotor modes of the single-rotor and the tri-rotor wind turbine shows that the dynamics of the lower rotors change significantly both in natural frequencies and damping ratios. This means that it can be sub-optimal to use the rotor from a single-rotor on a multi-rotor configuration. It is found that for multi-rotor wind turbines with arm structures the vertical arm-bending modes are low damped and therefore at risk of aeroelastic instabilities in addition to the side-side tower modes. The aeroelastic multi-rotor wind turbine modeling framework is verified by comparing frequency response func-tions with a nonlinear model in HAWC2.