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First published online January 5, 2005
Journal of Experimental Biology 208, 327-344 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01356
Body-induced vortical flows: a common mechanism for self-corrective trimming control in boxfishes
1 Department of Biological Sciences, Old Dominion University, Nor folk, VA
23529-0266, USA
2 Options of Bioengineering and Aeronautics, California Institute of
Technology, Pasadena, CA 91125, USA
3 School of Natural Resources and Department of Biology, University of
Michigan, Ann Arbor, MI 48109, USA
4 Department of Aerospace Engineering, Technion, Haifa, 3200,
Israel
5 Department of Ecology and Evolutionary Biology, University of California,
Los Angeles, CA 90095-1606, USA
* Author for correspondence (e-mail: ibartol{at}odu.edu)
Accepted 22 October 2004
Boxfishes (Teleostei: Ostraciidae) are marine fishes having rigid carapaces that vary significantly among taxa in their shapes and structural ornamentation. We showed previously that the keels of the carapace of one species of tropical boxfish, the smooth trunkfish, produce leading edge vortices (LEVs) capable of generating self-correcting trimming forces during swimming. In this paper we show that other tropical boxfishes with different carapace shapes have similar capabilities. We conducted a quantitative study of flows around the carapaces of three morphologically distinct boxfishes (spotted boxfish, scrawled cowfish and buffalo trunkfish) using stereolithographic models and three separate but interrelated analytical approaches: digital particle image velocimetry (DPIV), pressure distribution measurements, and force balance measurements. The ventral keels of all three forms produced LEVs that grew in circulation along the bodies, resembling the LEVs produced around delta-winged aircraft. These spiral vortices formed above the keels and increased in circulation as pitch angle became more positive, and formed below the keels and increased in circulation as pitch angle became more negative. Vortices also formed along the eye ridges of all boxfishes. In the spotted boxfish, which is largely trapezoidal in cross section, consistent dorsal vortex growth posterior to the eye ridge was also present. When all three boxfishes were positioned at various yaw angles, regions of strongest concentrated vorticity formed in far-field locations of the carapace compared with near-field areas, and vortex circulation was greatest posterior to the center of mass. In general, regions of localized low pressure correlated well with regions of attached, concentrated vorticity, especially around the ventral keels. Although other features of the carapace also affect flow patterns and pressure distributions in different ways, the integrated effects of the flows were consistent for all forms: they produce trimming self-correcting forces, which we measured directly using the force balance. These data together with previous work on smooth trunkfish indicate that body-induced vortical flows are a common mechanism that is probably significant for trim control in all species of tropical boxfishes.
Key words: vortex, boxfish, leading edge vortices (LEV), digital particle image velocimetry (DPIV), stability, pressure, pitch, yaw
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