On the aeroelasticity of an extreme scale wind turbine

Abstract

Recent decades, an ongoing trend has emerged in the upscaling of wind turbines in order to compete with traditional energy resources. For conventional wind turbines, aside from improving safety, the driving factors for new designs have always been increased efficiency and performance. This has led to novel wind turbine designs comprising thin-walled structures and light-weight composite materials that produce more power per unit. The current cost of wind energy is strongly dominated by operational and maintenance costs throughout the full lifetime of a wind turbine. Most wind turbines do not reach their design lifetime due to various reasons, from which fatigue failure is the most prominent one. The events responsible for gearbox or blade failure are caused by complex interactions between aerodynamics and structural responses that are inherently of unsteady nature. Aeroelasticity has become increasingly important for a safe and cost-effective design. Additionally, longer and lighter wind turbine blades undergo extremely large, low-strain deformations. A more accurate understanding of the aeroelastic behavior demands nonlinear analysis methods.

Most aeroelastic solvers in aerospace industry rely on linear structural models. This thesis work has modified the existing semi-nonlinear aeroelastic analysis method of the in-house developed CFD code at NLR by using Nastran's nonlinear structural module. Both analysis methods were utilized to obtain a converged static aeroelastic solution for two different operational conditions of the design curve. Subsequently, mode shapes of the deformed state of the structure were determined, to be used in the flutter analysis.

A 108-meters theoretically designed blade, provided by the Dutch blade design company We4Ce, was analysed using both methods. The results of static aeroelastic analysis and flutter analysis were compared to assess the effect of structural nonlinearities on the aeroelastic behavior. It is concluded that linear analysis overestimates the structural deformations and that pre-stressing due to nonlinear deformations and follower-forces alter the dynamic properties of the wind turbine blade. For the test case considered, the inclusion of geometrical nonlinearities resulted in a change of behavior of various modes. Naturally-low damped modes were affected negatively, eventually leading to dynamically diverging behavior. This thesis has successfully provided the first steps in the implementation of a nonlinear aeroelastic analysis for extreme scale wind turbines, including the capability of performing stability analysis.