Influence of operating conditions on unsteady wind performance of vertical axis wind turbines operating within a fluctuating free-stream: A numerical study

A fluctuating free-stream in unsteady wind environment presents a more significant challenge in wind turbine performance. In this paper, a numerical method is presented to investigate the influence of operating conditions on Vertical Axis Wind Turbine (VAWT) of NACA00XX symmetric airfoils with 12% and 22% thickness in unsteady wind condition. The Computational Fluid Dynamics (CFD) numerical method was used to analyze the aerodynamic performance and physics of flow of the VAWT. The VAWT dynamic motion of blades was introduced by sinusoidally oscillating both VAWT blades. Using a validated CFD model, steady wind simulations at Umean=7.00 m/s and 11.00 m/s were conducted and the results predicted the Power Coefficient (CP) performance for the VAWT scale. The results derived in the numerical analysis show that, within fluctuating free-stream wind conditions, thicker airfoils are desirable. Overall maximum unsteady CP of VAWT with thicker blades reveals positive deviations if the tip speed ratio λ is slightly higher than λ of the steady maximum CP, while thinner blades maximum CP marginally drops from the steady maximum CP for the same λ range. Higher frequencies of fluctuation marginally improve the unsteady wind performance of both VAWT blade profiles. High fluctuation amplitudes reveal overall performance degradation on both VAWT blade profiles more than small fluctuation amplitudes. The findings lend substantially to our understanding of both the kinematic and aerodynamic behavior on VAWT scale blades operating in unsteady wind condition, and the flow physics that causes the behavior.