How Knifefish Swim: Spanning the Gap Between Eel-like and Trout-like Swimming
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1
Northwestern University, United States
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2
Brown University, United States
Knifefish undulate an elongated ventral fin, commonly referred to as the ribbon fin, to propel through the water. This form of highly agile swimming has largely been unexplored, especially compared to anguilliform (eel-like, where the entire body undulates) and carangiform (trout-like, where only the tail fin flaps) swimmers. To begin to unravel the mysteries of how knifefish swim, we focus on the flow structures that occur during forward swimming. Using 2D digital particle image velocimetry on a biomimetic knifefish robot, we find that the ribbon fin combines a mix of eel and trout flow features. The axial motion of the traveling wave transfers momentum to the fluid, similar to an eel’s full body undulations. Additionally, the oscillatory motions of the ribbon fin rays shed two alternating vortex rings ventrally per fin cycle in the pattern of a reverse von Kármán street, similar to the wake behind the flapping caudal fin of a carangiform swimmer. These flow structures combine to form a wake at a downward angle from the axis of the fin, resulting in both forward and upward forces on the robot.
While the robot has a rectangular ribbon fin, the biological ribbon fin (and most fish fins in general) always has a convex shape, where the depth of the fin is greatest in the middle and tapers towards either end. Intuition tells us that rectangular fin, for a given depth, has greater surface area, which means greater area of fluid fin interaction, hence it should be preferred over other shapes. Using fully resolved simulation we find that the mechanical cost of transport (COT) of the convex fin is lower than the rectangular fin although the rectangular fin has higher swimming velocity. In order to quantify this difference in COT between the two fin shapes, we obtain scaling for COT in terms of various parameters which affect the swimming performance of the fin. Using scaling arguments we show why a convex profile of the ribbon fin performs best.
Our robotic ribbon fin has larger but similarly scaled dimensions to that of the black ghost knifefish, yet the undulatory pattern of 2-2.5 sinusoidal waves per fin length used by the fish matches the optimal pattern of the robotic fin (Shirgaonkar et al., 2008). We decompose the kinematics into a thrust-inducing mode, where a stationary sine-wave translates backwards to impart momentum into the fluid, and a drag-inducing slithering mode, where the fin travels forward along a sinusoidal path resulting in only skin friction. The net force is the result of a dynamic force and the difference in the coefficients of drags (effective Cd) of slithering and frozen modes. As wavelength increases, the dynamic force decreases while the effective Cd increases, eventually reaching a plateau. This results in optimal thrust at 2 waves per fin length in the knifefish. We find this optimum to always occur at the same specific area (ratio of surface area to projected side area).
Acknowledgements
This material is based upon work supported by the National Science Foundation (IOB0846032 and CMMI-0941674)
References
Shirgaonkar, A. A., Curet, O. M., Patankar, N. A., & Maciver, M. A. (2008). The hydrodynamics of ribbon-fin propulsion during impulsive motion. Journal of Experimental Biology, 211(Pt 21), 3490-3503. THE COMPANY OF BIOLOGISTS LTD. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/18931321
Keywords:
flow structure,
fluid mechanics,
knifefish,
Locomotion,
morphology,
propulsion,
ribbon-fin,
Swimming
Conference:
Tenth International Congress of Neuroethology, College Park. Maryland USA, United States, 5 Aug - 10 Aug, 2012.
Presentation Type:
Poster (but consider for participant symposium and student poster award)
Topic:
Motor Systems
Citation:
Neveln
ID,
Bale
R,
Curet
OM,
Patankar
NA and
MacIver
MA
(2012). How Knifefish Swim: Spanning the Gap Between Eel-like and Trout-like Swimming.
Conference Abstract:
Tenth International Congress of Neuroethology.
doi: 10.3389/conf.fnbeh.2012.27.00415
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Received:
02 May 2012;
Published Online:
07 Jul 2012.
*
Correspondence:
Dr. Malcolm A MacIver, Northwestern University, Chicago, IL, 60613-3168, United States, maciver@northwestern.edu