THESIS
2017
xv, 97 pages : illustrations (some color) ; 30 cm
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...[
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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.
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