Modal identification of a light and flexible wind turbine blade under wind excitation

System identification is the main goal when performing modal testing of mechanical structures. In cases where only the structural responses are measured, the identification technique is addressed in the literature as operational modal analysis (OMA). Applications of OMA are found in structures where the excitation from the ambient can not be removed or it is the only possible one. Since the ambient forces can not be measured, the identification of the modal parameters is possible if some hypotheses about its random nature are made. In this paper, the modal identification of a small and flexible wind turbine blade is performed under the OMA framework. Although the tested structure was in laboratory condition, traditional experimental modal analysis was not possible due to the small size and high flexibility of the blade. Therefore, this paper demonstrates the importance of OMA in the modal identification of this type of structure (simultaneously light and flexible). An artificially generated wind was used instead as source of excitation, which presented good properties necessary for OMA (stationarity and broadband frequency). The stochastic subspace identification method was used to identify the modal parameters, leading to good identification in a prescribed frequency band of interest. A procedure to analyze and validate the experimental results is also given at the end.