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Undulating fins of the knife fish move it swim forward and backward, left and right by generating counter-propagating waves.
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[Collapse all sections] Summary
"Gymnarchus niloticus is a native of Africa found in the Nile River. Gymnarchus has some interesting traits. It swims gracefully with equal facility either forward or backward. Instead of propelling itself by lashing its tail sideways, as most fish do, it keeps its spine straight--not only when swimming straight ahead, but even when turning. Its propulsion is accomplished by means of the undulations of the fin along its back. Gymnarchus navigates with great precision, darting after its prey and evading obstacles in its path. What is surprising is that it does so just as precisely when swimming backward." (Courtesy of the Biomimicry Guild)
Excerpt
"Many aquatic organisms swim by means of an undulating fin. These undulations often form a single wave travelling from one end of the fin to the other. However, when these aquatic animals are holding station or hovering, there is often a travelling wave from the head to the tail, and another moving from the tail to the head, meeting in the middle of the fin. Our study uses a biomimetic fish robot and computational fluid dynamics on a model of a real fish to uncover the mechanics of these inward counter-propagating waves. In addition, we compare the flow structure and upward force generated by inward counter-propagating waves to standing waves, unidirectional waves, and outward counter-propagating waves (i.e. one wave travelling from the middle of the fin to the head, and another wave travelling from the middle of the fin to the tail). Using digital particle image velocimetry to capture the flow structure around the fish robot, and computational fluid dynamics, we show that inward counter-propagating waves gen- erate a clear mushroom-cloud-like flow structure with an inverted jet. The two streams of fluid set up by the two travelling waves ‘collide’ together (forming the mushroom cap) and collect into a narrow jet away from the cap (the mushroom stem). The reaction force from this jet acts to push the body in the opposite direction to the jet, perpendicular to the direction of movement provided by a single travelling wave. This downward jet provides a substantial increase in the perpendicular force when compared with the other types of fin actuation. Ani- mals can thereby move upward if the fin is along the bottom midline of the body (or downward if on top); or left–right if the fins are along the lateral margins. In addition to illuminating how a large number of undulatory swimmers can use elongated fins to move in unexpected direc- tions, the phenomenon of counter-propagating waves provides novel motion capabilities for systems using robotic undulators, an emerging technology for propelling underwater vehicles." (Curet et al. 2010:1)
About the inspiring organism
Aba
Gymnarchus niloticus Cuvier, 1829
[Aba aba, Frankfish]
Some organism data provided by: FishBase
Organism/taxonomy data provided by:
Species 2000 & ITIS Catalogue of Life: 2008 Annual Checklist
Bioinspired products and application ideas
Application Ideas: Efficient underwater propulsion.
Industrial Sector(s) interested in this strategy: Transportation
Experts
McIver Lab
Malcolm A. McIver
Department of Mechanical Engineering, Department of Biomedical Engineering, and Department of Neurobiology and Physiology at Northwestern University
References
Curet OM: Patankar NA; Lauder GV; MacIver MA. 2010. Aquatic manoeuvering with counter-propagating waves: a novel locomotive strategy. Journal of the Royal Society Interface. Published online before print December 22, 2010, doi: 10.1098/​rsif.2010.0493:
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