Dynamic mode decomposition (DMD) technology is used to analyze the control of dynamic stall on a pitching S809 airfoil using an off-surface rod. The unsteady flows around the original and the controlled airfoil are simulated by using the SST k-ω turbulence model. With the introduction of the off-surface rod, the hysteresis effect of the dynamic stall process of the original airfoil is considerably reduced, and the clockwise sub-loop of the pitching moment coefficient is eliminated. The improvement of the dynamic stall process is beneficial to the safe and high-efficiency operations of wind turbine. The coherent structure of the unsteady flow fields are decoupled by the DMD method compiled by an in-house code in MATLAB. Results reveal that the hysteresis effect is dominated by mode 2 with a pitching frequency and mode 3 with twice the pitching frequency. The global energy of the two modes is reduced by the off-surface rod, which alleviates the hysteresis effect for the original airfoil.
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