1 Risø National Laboratory for Sustainable Energy, Technical University of Denmark2 Department of Wind Energy, Technical University of Denmark3 unknown
The modal analysis technique has been used to identify essential dynamic properties of wind turbine blades like natural frequencies, damping characteristics and mode shapes. Different experimental procedures have been considered, and the most appropriateof these has been selected. Although the comparison is based on measurements on a LM 19 m blade, the recommendations given are believed to be valid for other wind turbine blades as well. The reliability of the selected experimental analysis has beenquantified by estimating the unsystematic variations in the experimental findings. Satisfactory results have been obtained for natural frequencies, damping characteristics and for the dominating deflection direction of the investigated mode shapes. Forthe "secondary" deflection directions, the observed experimental uncertainty may be considerable - especially for the torsional deflection. The experimental analysis of the LM 19 m blade has been compared with results from a state-of-the-art FE-modeling ofthe same blade. For some of the higher modes substantial discrepancies between the natural frequencies originating from the FE-modeling and the modal analysis, respectively, are observed. In general the qualitative features of measured and computed modesshapes are in good agreement. However, for the "secondary" deflection directions, substantial deviations in the absolute values may occur (when normalizing with respect to the primary deflection direction). Finally, suggestions of potential futureimprovements of the experimental procedure are discussed.