"Squid Swimming: Characterization and Categorization of Aquatic Locomotion with Lessons for Mechanical Design"

Paul S. Krueger, Department of Mechanical Engineering, Southern Methodist University

Many common modes of aquatic locomotion are fundamentally more complex than traditional mechanical propulsion methods. First, aquatic locomotion is unsteady, involving oscillations of appendages or cyclic (i.e., pulsed) jetting. Second, swimming animals can produce a wide array of behaviors by deforming their body and/or utilizing multiple propulsors. This complexity challenges realistic analysis of many aquatic propulsion systems, hampering understanding of animal ecology and development of versatile bio-inspired designs which can leverage both the robustness and efficiency of their biological counterparts.

Results from squid swimming, including biological (brief squid Lolliguncula brevis) and mechanical systems, will be presented to illustrate the progress in and challenges to understanding complex aquatic propulsion. Squid utilize a dual mode propulsion system comprising a pulsed jet and muscular fins with no bony structures. The performance (thrust and efficiency) of the jet is strongly dependent on the volume of ejected fluid, with short jets being dominated by compact vortical structures (vortex rings). Shorter jets provide increased propulsive efficiency, and tests with a bio-morphic mechanical system (‘Robosquid’) have demonstrated that the propulsive efficiency for this operating condition can exceed that of equivalent steady jets. Despite this efficiency advantage, there appears to be no preference for jetting mode (long vs. short) across swimming speed in juvenile and adult brief squid, which may be related to the interplay of the two propulsion systems. The fins, on the other hand, provide maneuvering and stabilizing forces in addition to thrust using flows characterized by a wide variety of 3D vortical structures and vortex arrangements.

The complexity and variety of observed flow features complicates analysis of squid swimming and efforts to understand how the two propulsion systems work together. As a first step toward addressing this challenge, a general approach for characterizing and comparing flows based on properties of flow field critical points is developed and presented. Generic results from clustering theory can then be applied to group flows with similar features, and the resulting groups can be subsequently analyzed for distinctive characteristics. Application of the approach to 3D flow field measurements from swimming squid illustrates the effectiveness of the approach, which is general enough to be applied to any swimmer or flyer.

Paul Krueger received his B.S. in Mechanical Engineering in 1997 from the University of California at Berkeley. He received his M.S. in Aeronautics in 1998 and his Ph.D. in Aeronautics in 2001, both from the California Institute of Technology (Caltech). In 2002 he joined the Mechanical Engineering Department at Southern Methodist University. He is a recipient of the Rolf D. Buhler Memorial Award in Aeronautics and the Richard Bruce Chapman Memorial Award for distinguished research in Hydrodynamics. In 2004 he received the Faculty Early Career Development (CAREER) Award from the National Science Foundation and he was elected the ASME North Texas Section Young Engineer of the Year in 2009. In 2012 he received the Ford Senior Research Fellowship from SMU. His research interests include unsteady hydrodynamics and aerodynamics, vortex dynamics, vortex-boundary interactions, bio-fluid mechanics, and pulsed-jet propulsion.

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