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An Exploration of Passive and Active Flexibility in Biolocomotion ...

An Exploration of Passive and Active Flexibility in Biolocomotion ...

An Exploration of Passive and Active Flexibility in Biolocomotion

Advances in Science and Technology Vol. 58 (2008) pp 212-219 online at http://www.scientific.net © (2008) Trans Tech Publications, Switzerland Online available since 2008/Sep/02 An Exploration of Passive and Active Flexibility in Biolocomotion through Analysis of Canonical Problems Jeff D. Eldredge 1,a , Megan Wilson 1,b and Daniel Hector 1,c 1 Mechanical & Aerospace Engineeering Department, 420 Westwood Plaza, Box 951596, University of California, Los Angeles, CA 90095, USA a eldredge@seas.ucla.edu, b meganwils@ucla.edu, c dhector@ucla.edu Keywords: Biological locomotion, fluid-structure interaction, viscous incompressible flow, viscous vortex particle method. Abstract. Most aquatic creatures achieve motility through the dynamic interaction of their flexible body with the surrounding medium. This flexibility is used to provide a spectrum of active and passive control, allowing the creature to sometimes prescribe its shape changes and at other times extract energy from the fluid. This mix is particularly important in the moderate Reynolds number regime, in which wake vortices play an important energetic role. A well-devised control strategy for a bio-inspired vehicle should – perhaps must – exploit such flexion and energy exchange; as yet, we lack sufficient understanding to develop such a strategy. In this work, we present two canonical problems that distill fundamental modes of fluid/flexible body mechanics in biological systems, which are analyzed using high-fidelity numerical simulation. The first system consists of an articulated three-link swimmer considered in free-swimming. The second system involves an articulated jellyfish, in which the active/passive flexibility mix is explored by designation of the individual hinges. Introduction Aquatic locomotion mechanics at moderate Reynolds number are fundamentally based on the reaction force supplied by the fluid against an accelerating surface. For example, the undulatory wave sent from the head to the tail of an eel causes an oscillatory exchange of energy with the adjacent fluid, resulting in a net forward thrust from the unbalanced tail motion [1]. Less well understood is the role of shed vorticity in such locomotion. Vorticity production and shedding is clearly a hallmark of the flows produced by these locomotion mechanics. Much of our current understanding of the role of vorticity is derived from experimental flow visualization and measurements [2,3,4,5]. Investigations have revealed that the manner in which vorticity is produced and processed by a flapping, undulating or contracting surface has profound effects on the thrust efficiency [5,6]. Each of these studies has supported the general conclusion that aquatic biolocomotion at moderate Reynolds numbers is strongly connected with unsteady vorticity production and transport. However, it is less well understood that most mechanics observed in this regime would also lead to self-propulsion in a perfectly inviscid medium, in which no vorticity is shed whatsoever [7]. It is important to note that most experimental and computational studies have analyzed the system in a ‘creature’ reference frame, measuring forces when the tethered undulating body is subject to a steady free stream. This reference frame certainly simplifies the analysis, allowing sustained velocity field measurements and force calculations in the case of experiments. However, a fish inevitably swims with some unsteadiness due to its undulatory mechanics, and it is not clear to what extent the conclusions drawn in the steady frame are applicable to unconstrained motion. Observations of water-borne creatures highlight the important roles that both active and passive flexibility play in aquatic locomotion [4]. They also naturally lead to the question of how these types of flexibility interact, and most importantly, how optimal performance can be achieved by All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, www.ttp.net. (ID: 164.67.190.116-18/09/08,17:09:14)

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