On the sensitivity and validation of bending, twisting, and bend-twist coupling behaviour of wind turbine blade cross-sections

Abstract

Scaling up of rotor technology is a key challenge faced by industry for future turbines. Larger rotors provide greater total energy capture, but also suffer increased loads on the blades and, subsequently, accelerated wear of all components of the turbine. To reduce the wear, load alleviation techniques have received significant interest from both academia and industry. Various authors have proposed both active and passive solutions, and one prominent technique is the implementation of Bend-Twist Coupling (BTC). When used to induce twist towards feather under flap-wise bending, BTC has been shown to reduce the maximum loads on the turbine drastically. While the advantages of BTC are well documented, the underlying modelling approaches are still mostly unvalidated. Code-to-code comparisons and validation exercises between techniques have shown significant discrepancies. It has also been demonstrated that at least some of the difference can be traced to small discrepancies in inputs and cross-sectional representation.

              This thesis contributes to the body of research on cross-sectional modelling of wind turbine blades through both numerical and experimental work. Using two cross-sectional modellers (i.e. BECAS and VABS) and 3D Finite Element Models in ABAQUS, a numerical sensitivity study has been carried out. Comparison of the compliance terms for three typical turbine blade cross-sections highlights the innate performance variability caused by manufacturing tolerances and modelling simplifications. The results show the importance of the Trailing Edge (TE) bondline, previously unstudied, to the accurate prediction of the stiffness of compliant and coupled beams. The TE bondline is also shown to contribute meaningfully to the disagreement observed between 3D shell and solid model predictions.

              The numerical insights are supported by experimental work through testing of several demonstrators, designed to contain BTC, under cantilever loading.  Each of demonstrator has a different cross-sectional design, capturing the influence of various common features of wind-turbine blade cross sections. Together, these demonstrators help validate the modelling techniques investigated and provide important practical insights for cross-sectional properties of compliant and coupled beams. As such, this work helps establish best practices for modelling and testing of bend-twist coupled beams. In doing so, it also provides reliable experimental data for validation and benchmarking purposes of current and future numerical and/or analytical modelling techniques.