The gymnotiform mode of aquatic locomotion is characterized by the use of an elongated anal fin, generically referred to as a ribbon fin. The extraordinarily maneuverable weakly electric black ghost knifefish (Apteronotus albifrons) uses this mode of locomotion. These animals also have an unusual sensory adaptation: the use of a self-generated electric field to detect surrounding objects. This allows them to hunt at night in the murky waters of Amazon Basin rivers, where they are indigenous. Electric fish are a leading model system within neurobiology for the study of the neural control of sensing and movement. These animals also offer a unique opportunity to develop novel sensing and propulsion technologies for use in systems such as underwater vehicles.

The knifefish sends continuous sinusoidal traveling waves along the fin. By altering the direction of the traveling wave, it swims backward as gracefully as it swims forward, and frequently reverses the direction of swimming during natural behaviors such as prey capture. In order to better understand how the fish controls its movement, we are studying the hydrodynamics of ribbon fin propulsion. We investigated the mechanism of propulsive force generation by a non-translating, non-rotating ribbon fin - the situation relevant to the low-speed, rapid forward-backward maneuvers of the knifefish.

Using flow visualizations of numerical simulation data [1], we found that the fin generates both surge force (equivalently, thrust, i.e. the propulsive force in the swimming direction), and heave force (force in the perpendicular direction). The thrust generation mechanism involves the generation of a longitudinal, central jet running along the lower end of the fin, and an associated series of vortex rings attached to the lower edge of the fin.

Smaller secondary vortex rings were observed to be emitted at an angle to the swimming direction on both sides of the fin surfaces. This indicates that the peculiar combination of the morphology and the actuation pattern of the fin (traveling waves) of the knifefish may be utilized by the animal to generate significant sideways forces for rapid maneuvers. The mechanism of heave generation consists of longitudinally oriented vortex rolls shedding at the lower edge of the ribbon-fin [1]. The central jet becomes further evident from the velocity blobs that are advected down the length of the fin. This ``bucket effect?? gives rise to fluid carried by successive crests and troughs of the fin-wave and expelled into the surrounding fluid at the trailing edge of the fin. This results in an undulatory propulsive force.

[1.] A. A. Shirgaonkar, O. M. Curet, N. A. Patankar, M. A. MacIver, The hydronamics of ribbon-fin propulsion during impulsive motion, Journal of Experimental Biology