Aerodynamic Measurements on a Vertical Axis Wind Turbine in a Large Scale Wind Tunnel

Abstract

This paper presents the first results of a wide experimental investigation on the aerodynamics of a vertical axis wind turbine. Vertical axis wind turbines have recently received particular attention, as interesting alternative for small and micro generation applications. However, the complex fluid dynamic mechanisms occurring in these machines make the aerodynamic optimization of the rotors still an open issue and detailed experimental analyses are now highly recommended to convert improved flow field comprehensions into novel design techniques. The experiments were performed in the large-scale wind tunnel of the Politecnico di Milano (Italy), where real-scale wind turbines for micro generation can be tested in full similarity conditions. Open and closed wind tunnel configurations are considered in such a way to quantify the influence of model blockage for several operational conditions. Integral torque and thrust measurements, as well as detailed aerodynamic measurements were applied to characterize the 3D flow field downstream of the turbine. The local unsteady flow field and the streamwise turbulent component, both resolved in phase with the rotor position, were derived by hot wire measurements. The paper critically analyses the models and the correlations usually applied to correct the wind tunnel blockage effects. Results evidence that the presently available theoretical correction models does not provide accurate estimates of the blockage effect in the case of vertical axis wind turbines. The tip aerodynamic phenomena, in particular, seem to play a key role for the prediction of the turbine performance; large-scale unsteadiness is observed in that region and a simple flow model is used to explain the different flow features with respect to horizontal axis wind turbines.

Large Eddy Simulation of Vertical Axis Wind Turbine Wakes

“…As can be observed in the figures, the wake of a VAWT is not symmetric in the spanwise direction (y-direction). This lack of symmetry, which is a typical behavior of VAWT wakes, was also reported by Battisti et al [20], Simao Ferreira et al [60] and Brochier et al [18]. We can see that in the region where the streamwise component of the blade velocity is in the opposite direction of the inflow velocity, i.e., upcoming blade region, (positive y in the figure), the wake is stronger than in the region where these two velocities are in the same direction, i.e., retreating blade region, (negative y in the figure).…”

“…Also seen in this figure is the fact that in the spanwise direction the TI profile has two maxima, which are located on the edges of the wake. This is due to the unsteady shed vorticity from the blades and is reported in the works of Fujisawa and Shibuya [21], Battisti et al [20] and Brochier et al [18]. Moreover, we see different behavior of the flow turbulence in the spanwise direction for different turbine models.…”

“…Bergeles et al [19] performed hot-wire measurements in the near wake of a Troposkein-shaped VAWT in a wind tunnel and reported the profiles of mean velocity and turbulence intensity for different tipspeed ratios and two downstream positions. More recently, Battisti et al [20] carried out wind tunnel measurements on the wake of a VAWT and provided data on time-averaged and phase-resolved velocity and turbulence intensity in a vertical plane located 1.5 rotor diameters downstream of the turbine axis. They showed that large-scale aerodynamic unsteadiness is greater in the tip region than in the central region.…”

Near‐wake flow simulation of a vertical axis turbine using an actuator line model

“…These profiles have two maxima in every studied section, which are located in the edges of the wake and are produced by the unsteady shed vorticity from the blades. () Numerical results show a good representation of the profiles in terms of the trend. However, there is a lack in the representation of the fluctuations close to the center of the wake in the first studied sections ( x / D = 0.75,1.0 and 1.25).…”

Three-Dimensional Velocity Measurements Around and Downstream of a Rotating Vertical Axis Wind Turbine

“…Dixon et al [6] investigated the detailed airfoil aerodynamics using two-dimensional PIV along a plane perpendicular to the turbine axis. Battisti et al [7] focused on the wake characteristics along a plane normal to the flow, 1.5 turbine diameters downstream of the rotor axis. Ferreira et al [8] looked at the shedding of tip vortices and their development in a VAWT wake both numerically and experimentally using PIV.…”