Traditionally, underwater vehicles have been using propellers for locomotion but they are not only inefficient but generate large acoustic signature. Researchers have taken inspiration from efficient swimmers like fish to address the issue with alternate propulsion mechanism. Mostly, research on fish locomotion involved studying a foil tethered to a fixed point inside uniform flow. A major drawback of such study is that neither it resembles a freely swimming fish nor it takes into consideration the dynamics of moving fish on propulsive forces. Hence, in our current study, we focus on comparing the performance of a free swimming fin over tethered fin both experimentally and numerically.
Experimentally, we focus on the oscillatory form of locomotion where the caudal fin pitches to generate necessary thrust as seen in boxfish. We intend to investigate the Caudal fin kinematics and its physical properties on locomotion performance. To better understand, we build an automated robo-physical model that swims in a circular path so as to carry extensive experiments. We focus on understanding the effect of flexibility, shape and thickness of caudal fin on performance. Currently, we have studied three different flexibility and for each flexibility, we studied three different shape. We found there must be an optimal flexibility for minimising the Cost of Transport (COT). We also propose that the steady forward speed linearly varies with tail tip velocity. Furthermore, we investigated the effect of thickness of fin and considered uniform and tapered fin with equal area moment of inertia.
Numerically, we investigated the effect of phase offset between heave and pitch motion on the performance of a freely swimming fin and compared that to a tethered fin. A freely-swimming fin self propels and moves with steady speed while a tethered fin remains stationary and actuates under uniform flow. We model the fin as a rigid body undergoing prescribed motion in an inviscid fluid and solved for coupled interaction using panel method. We show the effect of phase offset for optimum performance and found a significant difference between tethered and freely swimming fin. / M.S. / Underwater vehicles use propeller based mechanism but they are inefficient and generate noise. Researchers have taken inspiration from nature to replace propellers with efficient propulsion mechanism. In the current study, we design a robotic model to understand the effect of various kinematic and physical properties of tail fin on performance. Our research is unique from past study in the aspect that most research involved studying performance using a robotic model fixed at its position which does not resemble a freely-swimming fish. Hence, in our current study, we focus on comparing the performance of our freely swimming model with tethered fin.
The robot has one degree of freedom and can pitch its tail to generate thrust. We intend to investigate the tail fin kinematics and its physical properties on locomotion performance. We focus on understanding the effect of flexibility, shape and thickness of fin on performance. Currently, we have studied three different flexibility and for each flexibility, we studied three different shape. We showed there exists an optimal flexibility for maximising efficiency.
For any fin undergoing combined pitch and heave motion, there exists a phase offset between them which will maximise the performance. Researchers have tried to understand its impact using both experiment and numerical simulation. In the current study, we study the impact of phase offset between pitch and heave for a freely-swimming fin and compare that to a fixed fin. A freely-swimming fin self propels and moves with steady speed while a tethered fin remains stationary and actuates under uniform flow. We show the effect of phase offset for optimum performance and found a significant difference between tethered and freely swimming fin.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/101811 |
| Date | 23 December 2020 |
| Creators | Nayak, Anshul |
| Contributors | Mechanical Engineering, Pendar, Hodjat, Qiao, Rui, Tafti, Danesh |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf, application/pdf, application/octet-stream |
| Rights | Attribution-ShareAlike 4.0 International, http://creativecommons.org/licenses/by-sa/4.0/ |
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