Vortex dynamics of clapping plates

Abstract

AbstractVortex 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 three-dimensional 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.