Reliable damping ratio estimation from a 50+ m wind turbine rotor blade for prototype testing in the low-frequency regime

Kurzfassung

The design and validation of wind turbine rotor blades requires accurate damping estimates. Depending on scope, testing may involve complex sensor setups. We present experimental and operational modal analysis results from a large wind turbine blade using a novel procedure demanding little time and equipment. This procedure can be performed as prototype testing in the later stages of the blade manufacturing process. The novel experimental procedure consists in exciting the structure's lowest eigenfrequencies by hand and then analysing the free-decay vibration. The structure is excited at or near one resonance until large amplitudes develop. The structure's eigenmodes are well separated. During this ramp-up time the other eigenmodes decay until the vibration is predominantly mono-component. The excitation is then stopped and the vibration left to decay. The modal parameters of the one excited eigenmode can be estimated from the acquired free-decay data. In the case of the rotor blade, the eigenfrequency and damping ratio are found to be functions of the oscillation amplitude. We present the results for the first two out-of-plane and for the first in-plane rotor blade bending modes. We show that with this experimental procedure the logarithmic decrement approach provides results that are consistent with standard and more complex modal analysis techniques such as Stochastic Subspace Identification (SSI). The procedures requires only the output data of the accelerometers and simple algorithms.