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First published online August 8, 2008
Journal of Experimental Biology 211, 2669-2677 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.015883
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The `upstream wake' of swimming and flying animals and its correlation with propulsive efficiency

Jifeng Peng1,* and John O. Dabiri1,2

1 Bioengineering, California Institute of Technology, Pasadena, CA 91125, USA
2 Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, CA 91125, USA

* Author for correspondence (jfpeng{at}caltech.edu)

Accepted 18 May 2008

The interaction between swimming and flying animals and their fluid environments generates downstream wake structures such as vortices. In most studies, the upstream flow in front of the animal is neglected. In this study, we demonstrate the existence of upstream fluid structures even though the upstream flow is quiescent or possesses a uniform incoming velocity. Using a computational model, the flow generated by a swimmer (an oscillating flexible plate) is simulated and a new fluid mechanical analysis is applied to the flow to identify the upstream fluid structures. These upstream structures show the exact portion of fluid that is going to interact with the swimmer. A mass flow rate is then defined based on the upstream structures, and a metric for propulsive efficiency is established using the mass flow rate and the kinematics of the swimmer. We propose that the unsteady mass flow rate defined by the upstream fluid structures can be used as a metric to measure and objectively compare the efficiency of locomotion in water and air.

Key words: wake, upstream, fluid dynamics, locomotion


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© The Company of Biologists Ltd 2008