Abstract

This paper presents computational fluid dynamics (CFD) simulations of a three-straight-bladed Vertical Axis Wind Turbine (VAWT) operating at a wind speed of 9 m/s and low Tip Speed Ratio (TSR) ranging from 1.44 to 3.3. The turbine blade has NACA 0021 profile and the rotor diameter is 1.030 meters. CFD modelling starts with domain enclosure blades solely and later includes the main shaft to investigate its wake effects. Precursor studies such as spatial-temporal resolutions and rotational convergence are performed. The time-averaged simulation results are validated against available experimental measurements and numerical predictions from open literatures. Using fewer grids with improved mesh quality, simulation takes less wall-clock time to complete and produces better predictions compared to other published CFD results. The main shaft is found to have noticeable effects on the simulation accuracy, as its wake will have large impacts on downstream field by reducing the blade power generation once it passing through that region. Further quantitative results show that while the overall average power coefficient (Cp) of VAWT is about 0.37, lower than the Betz limit (0.593), some instantaneous power coefficients are over-performed (i.e. 86°-109° for blade 1, 206°-229° for blade 2 and 326°-349° for blade 3) with the maximum value of 0.63 above the Betz limit. Further flow visualizations around blade 1 have shown that the blade of VAWT cannot always generate power during one turbine revolution. The vorticity contours exhibit that large vortex structure presents and the blade experiences vortex-induced separation between 160° to 360° azimuthal angles which lead to negative torque production.