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6th Annual T. Francis Ogilvie Young Investigator Lecture Eliot G. Drucker, Ph.D.Postdoctoral Associate, Department of Ecology and Evolutionary Biology, University of California, Irvine. PDF Flyer
ABSTRACT: The enormous evolutionary success of fishes, the most speciose group of vertebrate animals, is associated with pronounced diversification of swimming behavior and performance. Despite centuries of active investigation by both biologists and engineers into the mechanisms by which fishes swim, one critical level of analysis remains largely unexplored: direct investigation of the physical interaction between swimming animals’ propulsors and the aquatic medium. Presently, experimental data on the effect of motion of fish fins on the surrounding water are scarce. As a result, our understanding of how momentum is transferred from the animal to its fluid environment, and how hydrodynamic forces arise, is still in its infancy. An important task, is to characterize experimentally for freely swimming fishes the dynamics of the fin-water interface, the locus of fluid production. This talk reviews current work that has adopted from the field of fluid mechanics a modern flow visualization technique called Digital Particle Image Velocimetry (DPIV). This technique allows, for the first time, empirical and quantitative measurements of the three-dimensional water velocity flows in the wake of swimming animals. These data enable direct investigation of the hallmark of fluid force production (vorticity) and estimation of locomotor force from rates of momentum transfer. Three specific questions are addressed using DPIV. (1) What is the structure of the wake and magnitude of the forces developed by fish fins? This work focuses on the wake vortices shed by the paired pectoral fins of fishes, structures analogous to the forelimbs of terrestrial vertebrates. During constant-speed, straight-ahead swimming, fishes produce toroidal vortex rings with each pectoral fin. Mean thrust and lift forces arising from this momentum flow match closely the empirically determined counter-forces of body drag and weight. The observed force balance indicates that DPIV can be used to measure accurately large-scale vorticity in the wake of swimming fishes and therefore validates the technique as a valuable means of studying biological fluid forces. (2) Why are some fishes able to swim faster than others? This question is addressed from a hydrodynamic perspective through a comparative analysis of the wakes of slow and fast pectoral-fin swimmers. DPIV analysis reveals that faster fishes progressively reorient their vortex wake downstream with increasing speed to augment thrust, while slower fishes maintain large sideways-oriented forces at all speeds. Interspecific variation in the orientation of wake jet flow has important implications for both body stability and maneuverability. (3) How do fish maneuver? In nature, unsteady swimming involving time-dependent variation in heading, speed and acceleration accounts for the majority of the locomotor budget and is of considerable ecological importance. DPIV is used to study the wake of fishes performing unsteady turning maneuvers with the pectoral fins. During turning, fishes radically reorient vortex wake flow and generate fluid forces that exceed those of steady swimming by up to an order of magnitude, a result that underscores the impressive functional versatility of the locomotor apparatus of fishes. The application of engineering techniques to the study of organismal function holds considerable promise for future work. Ongoing experimental studies of fluid flow around moving biological surfaces are expected to illuminate further the mechanisms of aquatic animal locomotion and help clarify the hydrodynamic significance of evolutionary variation in fin design. Eliot G. Drucker is a postdoctoral researcher at the University of California, Irvine in the Department of Ecology and Evolutionary Biology. He received his baccalaureate and doctoral degrees in Biology from Harvard University. His dissertation examined the biomechanics of paired fin locomotion in fishes and the effects of body size and ecological association on swimming performance. He has taught in the School of Fisheries of the University of Washington and has served as an instructor of gross human anatomy in the Division of Biology and Medicine at Brown University. From 1998-1999 he was a National Science Foundation Postdoctoral Fellow in Biosciences Related to the Environment studying the fluid forces generated by freely swimming fishes during both steady locomotion and unsteady maneuvering behavior. His present research into how fish swim combines traditional techniques of comparative anatomy and physiology with modern approaches from the field of experimental fluid mechanics. This investigation into the mechanisms of aquatic animal locomotion, conducted in collaboration with Dr. George Lauder of Harvard University, seeks to assign hydrodynamic significance to observed evolutionary variation in the design of biological propulsors. Working at the intersection of animal behavior and hydrodynamics, he has also pursued studies of the mechanism of prey capture by fishes. Further details of his research interests may be found at http://darwin.bio.uci.edu/~edrucker/home. Dr. Drucker is the recipient of a Distinction in Teaching Award from Harvard University. |
