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

Kurzfassung

The accurate damping estimation is of great importance for the design and validation of wind turbine rotor blades. We present experimental and operational modal analysis results conducted on a large wind turbine blade using a novel procedure that can reduce testing time and equipment. Reference modal parameters were obtained by a hammer test. Afterwards, the eigenfrequency and damping ratio were estimated by a free-decay test. This was performed by harmonically exciting the rotor blade by hand until large amplitudes were achieved, after which the oscillations were left to decay. We show that the estimated modal parameters are a function of oscillation amplitude. The experiment was conducted for the first three out-of-plane and for the first two in-plane rotor blade bending modes. The damping ratio increases with oscillation amplitude, while the eigenfrequency is only minimally affected. The test requires only limited equipment and simple algorithms.