1. Introduction and background
The vertical axis wind turbine (VAWT) has an inherently nonstationary
aerodynamic behavior, 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 e
although to a lesser extent e of the corresponding Reynolds
number [1]. 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.
Until now, wind tunnel tests, involving considerable time and
financial resources, have been the only way to fully characterize the
behavior of a rotor, in order to obtain the operating torque curves
for the implementation of the turbine control system. Nevertheless,
Computational Fluid Dynamics (CFD) can nowadays be considered
as a powerful design tool, whose integration into industrial
development and production life-cycles is continuously rising. As
observed by Caridi [2], this was made possible because of two main
factors: