Modal test on a wind turbine blade

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In a sunny september day Vicoter completed experimental modal analysis tests to characterize a wind turbine blade on behalf of SEVA s.r.l.

The test had the objective to furnish reliable information concerning the dynamic behaviour of the structure in order to correlate a multi-body elastic model developing by the Politecnico di Milano.

The blade, designed for a variable speed wind turbine, has a total length of 19.1 m with a maximum chord of about 2.0 m. The warping is very marked and varies between a minimum of -16° near the root and a maximum of + 0.7° at the tip. A lot of care had been so used in the identification of the right orientations of the sensors positioned along the blade span in order to avoid bias in the identification phase.

Modal analysis tests were performed at the customer’s site, installing 44 uni-axial accelerometers in order to recover flap, lag and torsion modes. A verification of the stiffness of the support has been realized too in order to guarantee the compliance with the clamping constraint used in the model.

Due to the low frequencies of the first modes and the large displacement of the excitation point during the test, a special device (electrodynamic long-stroke shaker) has been employed.

The chosen installation allowed to determine the first 11 modes of the structure, in terms of frequency, modal shape, damping and generalized mass. The data thus obtained will be directly exploitable to improve the dynamic analytical model of the blade.