Thanos Kanatsoulis: FEA (Finite Element Analysis) for wind turbine blades and tower

The Finite Element Method (FEM) is a numerical method for solving problems of engineering and mathematical physics. In the FEM, the structural system is modeled by a set of appropriate finite elements, interconnected at points called nodes. Elements may have similar physical properties with materials such as thickness, coefficient of thermal expansion, density, Young’s modulus, shear modulus and Poisson’s ratio. The FEM is used for problems with complicated geometries, loadings, and material properties where analytical solutions cannot be obtained. The method can predict the performance and behavior of the design and identify the weakness of the design accurately, in order to find the optimal design. The first part of this presentation is a design of wind turbine blades and tower on Solidworks 3D CAD platform. On this part we will discover the design methods of the mesh-like elements, as well as focus on the analysis method that is going to be used in order to find out the mechanical resistibility of the structures. Through the finite element analysis of the structure we can find out, not only the resistibility, but also the stress concentration of each part of the design.

The second part includes the finite element simulation. The simulation in Solidworks uses the displacement formulation of the finite element method to calculate component displacements, strains and stresses under external loads that are calculated on the first part, based on real data collected by the NTUA lab. The data is based on a HP turbine at the small wind test site of NTUA. The exact geometry during the meshing process is known by the program and the more accurately the mesh matches the product geometry, the more accurate the analysis results will be. The analysis of the parts is a static stress analysis that relies on the external load calculated on the first part. The point is to ensure the geometry remains in the linear elastic range and does not enter the plastic range. Consequently, using the appropriate data and design, we can find out the exact effect in our final result.

As far as the tower is concerned, a dynamic test called the ‘frequency test’ is also held. Every design has its preferred frequencies of vibration, called resonant frequencies, and these frequencies are characterized by a specific shape (or mode) of vibration. The frequency analysis in our simulation uses an Eigen value approach to determine the natural modes of vibration for any geometry. If a design’s natural mode and expected service vibration environment are closely matched, a harmonic resonance may occur, leading to excessive loads which will result in failure. The tower should be designed in such a way that its natural frequency (fn) does not coincide with the excitation frequencies (such as the rotor’s rotation speed (frotor), and the number of blade times frotor (fbp)).