Experimental evidence of coupled-mode flutter in a two-meter-long non-rotating wind turbine blade

Abstract

We have conducted a series of experiments on a 30:1 scale wind turbine blade and observed coupled-mode flutter. This observation validates the theoretical studies that predict larger wind turbine blades are susceptible to coupled-mode flutter. To conduct these experiments, we built a scaled-down model of the 61-meter NREL 5 MW blade. The blade was approximately 2 meters long and was built such that the ratios between its first torsional natural frequency and the flapwise natural frequencies were similar to those of the full-scale blade. This was important, because the theoretical predictions suggest that coupled-mode flutter occurs when one of the flapwise modes and the first torsional mode of the blade coalesce. In our experiments, as the wind speed was increased, turbulence induced vibrations were observed initially in the form of oscillations of the blade’s first flapwise mode, and then its second flapwise mode, with a region of multi-modal oscillations in between. It was observed that the first torsional natural frequency of the blade decreased with increasing wind speed and the second flapwise natural frequency increased slightly. Then at a critical wind speed, the second flapwise mode and the first torsional mode merged into a flutter mode, indicating coupled-mode flutter has occurred at this wind speed.