A two-dimensional numerical study is conducted to investigate the influence of the reduced velocity on two-degree-of-freedom VIVs of a circular cylinder controlled by synthetic jets ejected in horizontal and vertical directions (β = 0° and β = 90°). The Reynolds number is constant at Re = 150, and the reduced velocity varies in the range of 2.5 and 15 (U* = 2.5-15). The mass ratio of the circular cylinder is 2,0, and the in-flow to the cross-flow natural frequency ratio equals 1.0 (fnx/fny = 1.0). The oscillation characters, hydrodynamic forces and vortex shedding of circular cylinders with or without synthetic jets control are analyzed and compared. The results indicate that synthetic jets ejected in vertical direction (β = 90°) can intensify both in-flow and cross-flow oscillations of the circular cylinder in the whole reduced velocity range, the vortex shedding of the controlled case (β = 90°) becomes complicated even unstable with the reduced velocity increasing. Synthetic jets ejected in horizontal direction (β = 0°) have excellent performance on suppressing the cross-flow oscillation of the circular cylinder in the whole reduced velocity range. When U* ≥ 8.0, the in-flow oscillation of the circular cylinder can even be intensified by synthetic jets ejected in horizontal direction (β = 0°).

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