An Assessment of Computational Fluid Dynamics and Semi-empirical approaches for Vertical Axis Wind Turbine Analysis

Vertical axis turbines have many attractive features and are well suited for extraction of wind and hydrokinetic energy. At low angular velocities, the rotor blades momentarily experience high angles of attack and move in and out of stall. This gives rise to unsteady aerodynamic loads that may cause vibration and structural fatigue. The high pressure drag and loss of lift during dynamic stall also leads to a lower efficiency compared to horizontal axis systems. In this work, a three-pronged approach with increasing complexity is used to understand and model this complex phenomenon, for a representative system. Empirical curve fits are first used to map the efficiency of the turbine as a function of tip speed ratio. An existing double multiple streamtube methodology (DMST) is next used, with a variety of empirical methods for modeling dynamic stall. Finally, computational fluid dynamics (CFD) results are presented and compared with test data and the DMST approach. It is concluded that DMST approaches are well suited for screening and designing vertical axis wind turbine configurations, followed by detailed modeling of the physical phenomena using well validated CFD tools.