Vertical-axis wind turbine (VAWT)
A multi-objective structural optimization of a composite blade
University of Padua
Study the influence of aerodynamic and inertial loads in order to optimize the structure of the blade.
In order to limit blade deformations, the inertial contribution is predominant compared to the aerodynamic one for all composite thicknesses considered. Therefore, in order to increase the structural resistance of the blade, it is important to control the mass distribution.
The Vertical Axis Wind Turbine (VAWT) has an inherently nonstationary aerodynamic behaviour, mainly due to the continuous variation of the blade angle of attack during the rotation of the machine: this peculiarity involves the continuous variation both of the relative velocity with respect to the blade profile and – although to a lesser extent – of the corresponding Reynolds number. This phenomenon, typical of slow rotating machines, has a significant effect both on the dynamic loads acting on the rotor and on the generated power and, therefore, on performance.
Performing CFD calculations provides knowledge about the flow in all its details, (such as velocities, pressure, temperature, etc.), but the analysis of forces and stresses of vertical wind turbine blades is not straightforward, because of the need to develop adequate analytical tools to account for the complex fluid-structure interaction.
The most common analyses do not consider the interaction between aerodynamic and structural behaviour of the blades, even if this is an important aspect to take into account for a correct choice of the design parameters (geometrical, structural and aerodynamical) of the blades.
In order to evaluate the aerodynamic and inertial contributions to a VAWT blade deformation, a specific coupling code has been designed to link together the following tools:
1. a solid modelling software, capable of generating the desired blade geometry depending on the design geometric parameters
2. a finite volume Computational Fluid Dynamic (CFD) code for the calculation of rotor performance
3. a Finite Element Method (FEM) code for the structural design analysis of rotor blades
The methodology permits to investigate flow field characteristics, determining the torque coefficient generated from the blades as a function of rotor azimuthal coordinate, as well as pressure/tangential and centrifugal forces, thus assessing the influence of aerodynamic and inertial contributions to blade stresses and deformations.
A full unsteady CFD calculation of a three-bladed rotor architecture, characterized by a NACA 0012 profile, was carried out to demonstrate the potential of the VAWT design tool. Blade deformation analyses were performed for three different values of blade shell thickness.
The computed inertial contribution to blade deformation resulted higher with respect to the aerodynamic one for all the analysed blade shell thicknesses. Inertial displacements resulted proportional to shell thickness, being connected to rotor blade mass. On the contrary, aerodynamic displacements, being proportional to rotor blade deformability, resulted higher for reduced blade shell thickness.
Both inertial and aerodynamic displacements resulted higher at blade trailing edge than at leading edge, due to the selected position (0.25 blade chord, in correspondence to the airfoil centre of pressure) of the blade-spoke connection, determining a higher bending moment (and a consequent higher deformation) in the rear part of the rotor blade.
The experimental tests carried out on the blades confirmed the correct predictions of the coupling code, which is now used at Nablawave as a valuable and accurate design tool for vertical axis wind turbines.