Vortex dynamics of clapping plates

Vortex formation and force generation of clapping plates with various aspect ratios (AR) and stroke angles were investigated. Experiments were performed with a pair of hinged rectangular plates that were rotated symmetrically in a static fluid, and defocusing digital particle image velocimetry was employed to measure the threedimensional flow field. Single-plate cases were also studied to compare with clapping plate cases. As AR decreases, both circulation of the tip vortex and area enclosed by the vortex loop increase inversely. An empirical power-law relationship with a negative exponent is found between total impulse and AR for a given stroke angle. The sensitivity of the force generated by the plates to the change of AR is larger at the smaller stroke angle because of faster acceleration and deceleration. The increase in impulse per plate from the single-plate case to the clapping double-plate case is larger for lower AR. These results reveal that low AR wings are more efficient in propulsive force generation in some specific modes of unsteady flapping flight. The evolution of the wake structures is found to depend on AR and stroke angle.