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The unsteady flow fields above NACA 0015 airfoil pitching with/without upstream turbulence generator are investigated in a water tunnel by mean of particle image velocimetry (PIV). The turbulence was generated by a square bar mesh situated at the inlet of the test section. The airfoil pitching waveform is performed under the condition calculated from the angle of attack histogram of a vertical axis wind turbine (VAWT). By using PIV, the instantaneous vortex structures above the pitching airfoil can be revealed. It allows us to study the free stream turbulence effects on dynamic stall over an airfoil at pitching waveform the same as VAWT. It is found that the free stream turbulence intensity has significant impacts on the dynamic stall process. The dynamic stall process is delayed to higher incidence angles on increasing the turbulence intensity.

The concept of pitch control has been implemented in the design of a small vertical-axis wind turbine. Benefits gained can be shown by the experimental and numerical results presented in this paper. As found, the method of variable pitch control outperforms the one of fixed pitch control. The present results show that the former can make remarkable improvement on the starting torque as well as the aerodynamic characteristics at low tip speed ratios.