Nonlinear aeroelastic modelling of large wind turbine composite blades

Abstract

The increasing size and flexibility of large wind turbine blades introduces significant aeroelastic effects, which are caused by fluid-structure interaction. These effects might result in aeroelastic instability problems, such as edgewise instability and flutter, which can be devastating to the blades and the wind turbine. Therefore, developing a reliable and efficient aeroelastic model to investigate the aeroelasticity characterisation of large wind turbine blades is crucial in the development of large wind turbines.

There are several aeroelastic models available today for wind turbine blades. Almost all of them are linear models based on assumption of small blade deflections, and do not take account of large deflection effects on modelling responses and loads. However, with the increasing size and flexibility of large wind turbine blades, this assumption is not valid anymore because the blades often experience large deflections, which introduce significant geometric nonlinearities. Additionally, existing cross-sectional analysis models, which are used to extract cross-sectional properties of wind turbine composite blades for aeroelastic modelling, are either time-consuming or inaccurate.

This thesis aims to provide a reliable and efficient aeroelastic modelling of large wind turbine blades through developing 1) a cross-sectional model, which can extract cross-sectional properties of wind turbine composite blades in a reliable and efficient way; and 2) a nonlinear aeroelastic model, which is capable of handling large blade deflections.

In this thesis, a cross-sectional analysis model for calculating the cross-sectional properties of composite blades has been developed by incorporating classical lamination theory (CLT) with extended Bredt-Batho shear flow theory (EBSFT). The model considers the shear web effects and warping effects of composite blades and thus greatly improves the accuracy of torsional stiffness calculation. It also avoids complicated post-processing of force-displacement data from computationally expensive 3D finite-element analysis (FEA) and thus considerably improves the computational efficiency. A MATLAB program was developed to verify the accuracy and efficiency of the cross-sectional analysis model, and a series of benchmark calculation tests were undertaken. The results show that good agreement is achieved comparing with the data from experiment and FEA, and improved accuracy of torsional stiffness calculation due to consideration of the shear web effects is observed comparing with an existing cross-sectional analysis code PreComp.

Additionally, a nonlinear aeroelastic model for large wind turbine blades has been developed by combining 1) a blade structural model, which is based on a mixed-form formulation of geometrically exact beam theory (GEBT), taking account of geometric nonlinearities; and 2) a blade load model, which takes account of gravity loads, centrifugal loads and aerodynamic loads. The aerodynamic loads are calculated based on combining the blade element momentum (BEM) model and the Beddoes-Leishman (BL) dynamic stall model. The nonlinear aeroelastic model takes account of large blade deflections and thus greatly improves the accuracy of aeroelastic analysis of wind turbine blades. The nonlinear aeroelastic model was implemented in COMSOL Multiphysics, and a series of benchmark calculation tests were undertaken. The results show that good agreement is achieved when compared with experimental data, and its capability of handling large deflections is demonstrated. After the validation, the nonlinear aeroelastic model was applied to the aeroelastic simulation of the parked WindPACT 1.5MW wind turbine blade and to the stability analysis of the blade. Reduced flapwise deflection from the nonlinear aeroelastic model is observed compared to the linear aeroelastic code FAST. The calculated damping ratio of the edgewise mode is much lower than the calculated damping ratio of the flapwise mode, indicating that edgewise instability is more likely to occur than flapwise instability. It is also demonstrated that improper rotor rotational speeds can result in edgewise instability.