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First published online October 17, 2008
Journal of Experimental Biology 211, 3490-3503 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.019224

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The hydrodynamics of ribbon-fin propulsion during impulsive motion

Anup A. Shirgaonkar¹ and Oscar M. Curet¹

Neelesh A. Patankar^1,*

Malcolm A. MacIver^1,2,*

¹ Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208, USA
² Department of Biomedical Engineering, R. R. McCormick School of Engineering and Applied Science and Department of Neurobiology and Physiology, Northwestern University, Evanston, IL 60208, USA

^* Author for correspondence (e-mail: n-patankar{at}northwestern.edu)

^* Author for correspondence (e-mail: maciver{at}northwestern.edu)

Accepted 14 August 2008

Weakly electric fish are extraordinarily maneuverable swimmers, able to swim as easily forward as backward and rapidly switch swim direction, among other maneuvers. The primary propulsor of gymnotid electric fish is an elongated ribbon-like anal fin. To understand the mechanical basis of their maneuverability, we examine the hydrodynamics of a non-translating ribbon fin in stationary water using computational fluid dynamics and digital particle image velocimetry (DPIV) of the flow fields around a robotic ribbon fin. Computed forces are compared with drag measurements from towing a cast of the fish and with thrust estimates for measured swim-direction reversals. We idealize the movement of the fin as a traveling sinusoidal wave, and derive scaling relationships for how thrust varies with the wavelength, frequency, amplitude of the traveling wave and fin height. We compare these scaling relationships with prior theoretical work. The primary mechanism of thrust production is the generation of a streamwise central jet and the associated attached vortex rings. Under certain traveling wave regimes, the ribbon fin also generates a heave force, which pushes the body up in the body-fixed frame. In one such regime, we show that as the number of waves along the fin decreases to approximately two-thirds, the heave force surpasses the surge force. This switch from undulatory parallel thrust to oscillatory normal thrust may be important in understanding how the orientation of median fins may vary with fin length and number of waves along them. Our results will be useful for understanding the neural basis of control in the weakly electric knifefish as well as for engineering bio-inspired vehicles with undulatory thrusters.

Key words: aquatic locomotion, vortex shedding, propulsion, vortex rings

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				C. M Postlethwaite, T. M Psemeneki, J. Selimkhanov, M. Silber, and M. A MacIver Optimal movement in the prey strikes of weakly electric fish: a case study of the interplay of body plan and movement capability J R Soc Interface, May 6, 2009; 6(34): 417 - 433. [Abstract] [Full Text] [PDF]