Journal of Fluid Mechanics



Three-dimensional flow structures and vorticity control in fish-like swimming


Q. ZHU a1, M. J. WOLFGANG a1p1, D. K. P. YUE a1 and M. S. TRIANTAFYLLOU a1c1
a1 Department of Ocean Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Abstract

We employ a three-dimensional, nonlinear inviscid numerical method, in conjunction with experimental data from live fish and from a fish-like robotic mechanism, to establish the three-dimensional features of the flow around a fish-like body swimming in a straight line, and to identify the principal mechanisms of vorticity control employed in fish-like swimming. The computations contain no structural model for the fish and hence no recoil correction. First, we show the near-body flow structure produced by the travelling-wave undulations of the bodies of a tuna and a giant danio. As revealed in cross-sectional planes, for tuna the flow contains dominant features resembling the flow around a two-dimensional oscillating plate over most of the length of the fish body. For the giant danio, on the other hand, a mixed longitudinal–transverse structure appears along the hind part of the body. We also investigate the interaction of the body-generated vortices with the oscillating caudal fin and with tail-generated vorticity. Two distinct vorticity interaction modes are identified: the first mode results in high thrust and is generated by constructive pairing of body-generated vorticity with same-sign tail-generated vorticity, resulting in the formation of a strong thrust wake; the second corresponds to high propulsive efficiency and is generated by destructive pairing of body-generated vorticity with opposite-sign tail-generated vorticity, resulting in the formation of a weak thrust wake.

(Received March 17 2000)
(Revised April 8 2002)


Correspondence:
c1 Author to whom all correspondence should be addressed.
p1 Currently with The Boston Consulting Group, Inc., Boston, MA, USA.


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