The effects of chordwise flexibility gradient on the hydrodynamic performance of an oscillating fin

Abstract

In fish locomotion, to adapt to different swimming requirements, a fish modulates its fin stiffness and shape utilizing its complex muscular system to maximize performance. The optimal chordwise stiffness profiles may have potential to maximize the propulsive performance. However, previous studies on the effects of chordwise flexibility of fins were mainly focused on isotropic flexible plates made of a single material with a uniform fin flexibility. Therefore, there is a need to study chordwise varying stiffness fins that can mimic the caudal fin and fish locomotion, utilizing using a heterogeneous mixture of different materials. In this thesis, chordwise varying stiffness fins were fabricated utilizing an accurate multi-material 3D printing technique. Three isotropic flexible and three gradient flexible rectangular plates were made from two different types of digital materials to construct the flexibility gradient along the chordwise direction. Experiments were conducted to document and compare the hydrodynamic performance of different oscillating fins. The thrust performance and kinematics of the oscillating fins were measured using direct force measurement. It was observed that the gradient chordwise flexible plate produced a larger thrust than the rigid plate due to the passive morphology of the plate under the interaction with the surrounding fluid and oscillatory motion. The flow fields around the fins under oscillation were studied using a time-resolved particle image velocimetry system. The flow fields show vortex formation and detachment delay due to the flexible compliance of the fin. The vortex of the rigid reference case detached from the fin and began to dissipate at the moment when the stroke reversal started. In case of the flexible fin, the detachment was delayed in time and the integrity of the shed vortex was preserved for a longer period. The delay of the thrust generation was also observed accordingly. We compared the thrust performance between the gradient and isotropic flexibility group and studied the phase difference between the driving amplitude and bending angle to investigate the effect of effective stiffness on thrust generation among different fins. It was found that the maximum thrust for each sample occurred when the phase difference between bending and driving angles for all flexible fins was 0.40π±0.01π and the optimal bending angles for the highest instant thrust among gradient and isotropic flexible group were 12.3° and 17.0° respectively. Finally, the driving amplitude had a larger influence on the isotropic group than the gradient flexibility group. It was found that an increase in driving amplitude catalyzed the reversal of the inverse relationship between thrust generation and effective stiffness in the isotropic flexibility group but did not significantly affect the gradient flexibility group.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology