A numerical study of Vertical Axis Wind Turbine performances in twin-rotor configurations

Abstract

Placing two counter-rotating rotors of a Vertical Axis Wind Turbine (VAWT) can lead to a significant power enhancement and a faster wake resorption. This global power output is directly related to the spacing between both rotors permitting a mutual confinement effect. In addition, the relative direction of angular velocity of both rotors can strongly impact the overall performances of the machine. A range of two-dimensional (2D) Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations has been managed in order to study the aerodynamic interactions occurring in a pair of VAWT. By comparing with a single-rotor of VAWT, it has been shown than the global power enhancement of a double-rotor VAWT is linked with an extension of the lift production range in one of the two first quartiles of the upwind path. Moreover, the region of the extra power generation seems to be dependant on the relative rotational directions of counter-rotating rotors. In all cases, the extent of lift generation can be associated with a suppression of the cross-stream velocity induced by the confinement of the neighbouring turbine. This local flow perturbation, closed to the inner region, leads to an augmentation of the incidence experienced by the blades in the upwind path, increasing the global lift and torque recovered by the turbine.