Aquatic manoeuvering with counter-propagating waves: A novel locomotive strategy

Department of Mechanical Engineering, Northwestern University, Evanston, IL 60208, USA.
Journal of The Royal Society Interface (Impact Factor: 3.92). 12/2010; 8(60):1041-50. DOI: 10.1098/rsif.2010.0493
Source: PubMed


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 generate 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. Animals 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 directions, the phenomenon of counter-propagating waves provides novel motion capabilities for systems using robotic undulators, an emerging technology for propelling underwater vehicles.

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    • "For example, kinematic analyses of the anal fin have been conducted (Blake, 1983; Ruiz-Torres et al., 2013), the elongate anal fin has been studied as a locomotor system for analyzing how opposing forces are used to control body position (Sefati et al., 2013), fluid dynamic function of the anal fin has been studied both experimentally (Neveln et al., 2014) and computationally (Lighthill, 1990; Lighthill and Blake, 1990; Shirgaonkar et al., 2008), the role of locomotion in prey capture has been described (MacIver et al., 2001), and a novel heave (vertical) maneuvering behavior has been identified in which counter-propagating waves on the elongate anal fin generate downwardly directed forces allowing knifefish to move vertically (Curet et al., 2011a,b). And, the elongate ribbon fin of knifefish has also been used as inspiration for robotic models of wave-like locomotion (e.g., MacIver et al., 2004; Low and Willy, 2006; Hu et al., 2009; Curet et al., 2011a; Neveln et al., 2013). "
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    • "Several approaches were employed to investigate the fish locomotion , such as stochastic analysis [6], [7], theoretical modeling [8], and computational simulations [19], [21], [24], [32]. On the other hand, biorobotic models [14]–[16] have been used to greatly broaden the study and scope of fish locomotion available to researchers. "
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