This paper studies the effects of the pivot location and the reduction frequency on the aerodynamic characteristics of the SD7037 airfoil under small amplitude forced oscillation at low Reynolds number. By comparing lift coefficient-time curve with angle of attack-time curve, it is found that the hysteresis loop direction changing of the lift coefficient varies with the angle of attack is mainly due to the change of the phase of the lift coefficient-time curve. It is proved mathematically that the phase is a function of reduction frequency. What's more, when the phase of the lift coefficient-time curve increases, the hysteresis loop of the corresponding lift coefficient-angle of attack curve will change from counterclockwise to clockwise. In this process, the lift coefficient-angle of attack curve inevitably shows a straight line shape, as if the hysteresis phenomenon "disappears". In addition, by comparing the flow field structure, it is found that the pivot location and reduction frequency have different impact on this phase. While the pivot location mainly changes the effective angle of attack of the airfoil, the increase of reduction frequency affects the structure of the surrounding flow field, so that the reaction force brought by the added mass becomes stronger, and thus the lift is improved. Both the forward movement of the pivot location and the increase of reduction frequency will increase the phase of the lift coefficient curve.
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