ABSTRACT

Wind turbines are exposed to a wide range of external conditions and resulting loads. This work investigates the influence of these loads on the deformation of different cross-sections of a rotor blade. A 3D finite element model is created and several simulations with different load distributions are carried out. Following classical beam theories, a cross-section that is planar in the undeformed configuration is assumed to remain planar. Two procedures to find this plane are proposed. The cross-sectional deformations are discussed in the context of magnitude and qualitative distribution of internal loads.

INTRODUCTION

When designing a rotor blade, a compromise is made between aerodynamic and structural performance. For aerodynamics, the shape of the shell is essential. A purely aerodynamic design would result in thin airfoils to achieve a high lift-to-drag ratio. On the other hand, from a structural point of view, a thicker airfoil can be beneficial, as not only the material but also the geometry influences the stiffness of the cross-sections and the internal load distribution. The geometry determines the second moment of inertia. If the shape of the rotor blade cross-section changes due to deformations, aerodynamic and structural properties may be affected significantly.

In wind energy, the trend is towards increasingly large rotor diameters. By simple upscaling of existing rotor blade designs, this means a substantial increase in weight. Structural design optimization by means of weight reduction will lead to softer and more flexible blades. Particularly in the case of offshore megastructures, we expect that rotor blades will become increasingly more elastic. It is assumed that significant cross-sectional deformations, both in-plane and out-of-plane, will occur, especially for very large rotor blades found in offshore wind energy systems. This effect is also called cross-sectional warping or blade breathing.

So far, cross-sectional deformation has been considered in terms of damage (e.g., Eder and Bitsche, 2015), but not in terms of aeroelasticity. Cecchini and Weaver (2005) presented a method for cross-sectional deformations in symmetrical profile sections.

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