Flow fields around an oscillating airfoil at the low Reynolds number region are extremely unsteady because the change direction of leading edge produces unsteady vortex motions. Studies of unsteady propulsion system of birds, insects, and fish are few and inconclusive. It has been noted that the unsteady fluid force plays an important role in biological flight. To evaluate the force correctly, it is necessary to know the unsteady properties determined from the vortex dynamics. The actual motion of a hand in swimming is obviously unsteady, and time-dependent fluid forces must be considered, because a quasi-steady-state approach towards predicting the fluid forces acting on a hand under unsteady conditions yielded errors. The three-dimensionality and the unsteady effect of a hand must be important to the estimation of the fluid forces acting on a swimmer’s hand. The purpose of this study is to investigate the relationship between unsteady fluid forces and vortex structures for a three-dimensional airfoil during the pitch-oscillating motion. Particle image velocimetry (PIV) was used to aid in understanding the flow field in the near field of the airfoil edge for the wind tunnel test. It is confirmed that the unsteady fluid forces were affected by the vortices shed from the airfoil edge during up-stroke in pitching oscillation. There are two peaks in the fluid force during one pitch-oscillating due to the vortex behavior. The vortex behavior was strongly affected by the reduced frequency, and the fluid force acting on the airfoil model increases with increasing the reduced frequency.

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