Numerical and experimental investigations of bend-twist coupling effects for a small wind turbine blade

Abstract

Material bend-twist coupling has been widely studied in the research community as a passive control mechanism for a wind turbine. However, there is a lack of research in incorporating it in the rotor design process, with little research being conducted for its application to small wind turbines. The present study focuses on this issue by including bend-twist coupling in the design of a 500W wind turbine by using a combination of parametric studies and a multidisciplinary constrained optimisation approach. By doing so, it aims to establish the effectiveness of bend-twist coupling as a tool for passive load alleviation in small wind turbines. The effectiveness is tested through obtaining a significant decrease in the flapwise blade root bending moment accompanied by only a marginal decrease in the AEP, when compared with the baseline uncoupled turbine. The reference blade is designed under limitations imposed by the rules of the Small Wind Turbine Design Contest. Bend-twist coupling is introduced in the blades with a fixed aerodynamic design. The rotor performance is analysed inHAWCStab2. The internal structure of the blade is created with the intention of producing flexible blades with a single composite material used throughout the blade. Carbon-epoxy and glass-epoxy FRPs are considered as the material to be chosen in the unidirectional laminae. The cross-sectional stiffness analysis is conducted using BECAS for a range of fibre layup angles in both carbon-fibre and glass-fibre blades. Carbon outperformed glass for all fibre angles with regard to the amount of coupling seen in the crosssections. Apart from flapwise bend-twist coupling, other secondary torsion couplings are also present. A load response study is carried out for varying positive fibre layup angles in both carbon-fibre and glass-fibre blades, under steady wind conditions for the operational wind speed range. An increase in the flapwise blade tip displacement and a reduction in the flapwise bending loads with increasing fibrelayup angles is observed for both glass-fibre and carbon-fibre blades. The HAWTOpt2 aero-structural design tool using OpenMDAO as its core is utilised to implement the optimisation. The spanwise fibre layup angle and laminate thickness distribution are the only design variables that were allowed to be manipulated by the optimiser. The objective function is comprised of two different individual objective functions weighed according to the situation. The optimisation cases failed due to inaccurate gradients of the objective function and constraints. Due to time constraints the manufacturing of the blade and, static load and wind tunnel testing were not carried out.