THESIS
2020
xv, 117 pages : illustrations (some color) ; 30 cm
Abstract
Millions of years’ natural selection has endowed aquatic animals with the remarkable
swimming abilities such as acceleration, efficient cruising in strong currents, quick
maneuverability. These extraordinary features can be well adopted to bio-inspired autonomous
underwater robots and ship propulsion. A practical importance lies in fin propulsions low noise
emission, cavitation resistance, impact resistance and efficiency. Bioinspired fin propulsion for
manmade watercraft could be the next evolutionary step in the technology of water propulsion
systems.
Gradually flexible models undoubtedly offer a deeper insight into biologically inspired and
natural fin propulsion. Previous studies regarding gradually varied flexibility however used
simple rectangular fins that might differ f...[
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Millions of years’ natural selection has endowed aquatic animals with the remarkable
swimming abilities such as acceleration, efficient cruising in strong currents, quick
maneuverability. These extraordinary features can be well adopted to bio-inspired autonomous
underwater robots and ship propulsion. A practical importance lies in fin propulsions low noise
emission, cavitation resistance, impact resistance and efficiency. Bioinspired fin propulsion for
manmade watercraft could be the next evolutionary step in the technology of water propulsion
systems.
Gradually flexible models undoubtedly offer a deeper insight into biologically inspired and
natural fin propulsion. Previous studies regarding gradually varied flexibility however used
simple rectangular fins that might differ from fish tail form fins of gradual stiffness. Fin shape
and flexibility play a crucial role in granting fish species their swimming abilities, and it calls
for systematic and in-depth studies. In this work, we studied the effect of the trailing edge using
curved trailing edge geometries unlike the straight edged fork shape fins in previous literature.
We also studied the effect of chordwise gradual stiffness using 3D printing technology to
produce the model fins of uniform thickness, thus our results are not affected by a varied cross-sectional
thickness profile.
This study uncovers the effect on propulsive performance of two natural features of fins, the
trailing edge shape and the gradual flexibility. Using particle image velocimetry flow
measurement and force measurement in a water tank, we found that the convex trailing edge
shape always outperforms the concave shape in the studied Reynolds number range (5500-14500). Towards the high Reynolds number range of our studies the simplest geometry, the
trapezoidal fin prevailed. It is found that a gradually flexible fin possess better propulsive
performs than a uniformly flexible one, under the range of Reynolds numbers being studied.
Compared with the uniform fin, the more gradually flexible fins are better in the lower Reynolds
range, while those that are less flexible generated more thrust and are more efficient in the high
Reynolds range. Our results show that, given a cost-effective manufacturing process of fin
propulsors, natural form with gradual flexibility can provide better performance for future
watercraft.
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