Experimental modal analysis of aeroelastic tailored rotor blades in different boundary conditions

Bend-twist coupled blades are intended to reduce the loads on the overall wind turbine by passively adapting to current wind conditions. The coupling results from complex-design shapes and structures using advanced finite element models utilising shell and volume elements. These models are however prone to mispredict the structural dynamic behaviour of the rotor blades. In particular, normal modes with both bending and torsion contributions, as well as local vibrations of the blade shells include computational uncertainties. Therefore, in order to update flawed model parameter assumptions, a modal characterisation of blade prototypes including mode shapes is essential. In the present study results of a modal test campaign involving four identical rotor blades of 20 m length with geometric bend-twist coupling are reported. Design, realisation, and post-processing of the experiments have been carried out under careful consideration of a pre-existing FE shell model. Modal data is obtained at two different stages of the manufacturing process and for one blade in two separate boundary conditions, i.e. free-free in elastic suspensions and clamped to a test rig. Due to the high sensor density in both configurations, the identified normal modes do not only include coupled eigenforms but also mode shapes illustrating cross-sectional vibrations; the latter attributed to the deflection of the blade shells. The acquired dataset is found to be well-suited for the validation of the numerical model and represents a reliable basis for updating.