Effects of fluctuating velocity on dynamic stall of vertical axis wind turbine airfoil

In contrast to the classical dynamic stall with steady velocity, the dynamic stall phenomenon observed in vertical axis wind turbines encompasses not only periodic variations in the angle of attack but also periodic fluctuation in velocity. Therefore, this study performs a research to deepen understanding of the dynamic stall characteristics of an airfoil used in a vertical axis wind turbine when subjected to fluctuating velocities. The formation of the leading-edge vortex is brought forward for a larger resultant velocity and postponed for a smaller one, according to the comparing the dynamic stall characteristics under different fluctuating velocities and phase angles. Meanwhile, the convection and dissipation of the leading-edge vortex is also diminished as a result of the smaller resultant velocity. As a result, the aerodynamic loads, particularly the drag coefficient, present a nonsymmetrical form. Finally, the tangential force coefficient of an airfoil under fluctuating velocity is shown to have a larger peak in the upwind region and smaller peak in the downwind region, indicating that the blade of a vertical axis wind turbine would suffer more obviously aerodynamic imbalance under a lower tip speed ratio condition.