NablaWave Srl - Vertical-axis wind turbine (VAWT)

CHALLENGE

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 gives knowledge of the flow in all its detail — velocities, pressure, temperature — but the analysis of forces and stresses on vertical-axis blades is not straightforward, because it needs analytical tools adequate to the complex fluid-structure interaction.

The most common analyses do not consider the interaction between the aerodynamic and structural behaviour of the blades at all, even though it is an important aspect for a correct choice of the design parameters — geometrical, structural and aerodynamical.

SOLUTION

To evaluate the aerodynamic and inertial contributions to a VAWT blade deformation, a specific coupling code was designed to link three tools together:

a solid modelling software, capable of generating the desired blade geometry from the design geometric parameters

a finite volume Computational Fluid Dynamic (CFD) code for the calculation of rotor performance

a Finite Element Method (FEM) code for the structural design analysis of the rotor blades

The methodology makes it possible to investigate flow field characteristics, determining the torque coefficient generated by the blades as a function of rotor azimuthal coordinate, as well as pressure, tangential and centrifugal forces — and so to assess the influence of aerodynamic and inertial contributions to blade stresses and deformations.

RESULTS

A full unsteady CFD calculation of a three-bladed rotor architecture with 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 came out higher than the aerodynamic one for every shell thickness analysed. Inertial displacements were proportional to shell thickness, being tied to rotor blade mass; aerodynamic displacements, being proportional to blade deformability, were higher for reduced shell thickness.

Both were higher at the blade trailing edge than at the leading edge, because of the chosen position of the blade-spoke connection — 0.25 blade chord, at the airfoil centre of pressure — which produces a higher bending moment, and a consequently higher deformation, in the rear part of the blade.

The experimental tests carried out on the blades confirmed the predictions of the coupling code, which is now used at Nablawave as a valuable and accurate design tool for vertical-axis wind turbines.