High- fidelity computational assessment of the performance of a vertical axis wind turbine

Abstract

Even though Vertical Axis Wind Turbines (VAWT) are known to have lower efficiencies than Horizontal Axis Wind Turbines (HAWT), their simple construction and inherent design features qualify them as viable and cheap options for low cost wind energy systems. Hence, the VAWT serves as a viable application in rural sectors where it can be extensively used for electricity generation, pumping water and so on. Computational modeling of the flow fields in the vicinity of a VAWT for four different tip speed ratios of the turbine blade is the focus of this work. Here the flow is numerically computed using two different turbulence modeling approaches namely the Reynolds Averaged Navier-Stokes (RANS) with the Spalart-Allmaras turbulence model, which is popularly used for external aerodynamic analysis and the Large Eddy Simulation (LES) with the Wall-Adapting Local-Eddy Viscosity (WALE) model. The prime focus of this study is on LES approach of turbulence modeling, where large scale motion is computed explicitly and small scale motion is modeled. This approach can accurately capture turbulent flow structures and the effect of large scale motions caused by wind forces and their fluctuations. The key differences between the capabilities of both the modeling approaches is reported here. The effect of the close proximity of the turbines on their power coefficients is numerically studied using dual counter rotating VAWT rotors placed closed enough to each other. Improvement in the performance of the dual rotor is found due to the mutual aerodynamic interference. This study will lay the foundation for further studies such as design optimization of VAWTs, optimization of inter-turbine spacing in order to maximise the wind power density of the wind farm, etc.