Evidence of self-correcting spiral flows in swimming boxfishes
The marine boxfishes have rigid keeled exteriors (carapaces) unlike most fishes, yet exhibit high stability, high maneuverability and relatively low drag given their large cross-sectional area. These characteristics lend themselves well to bioinspired design. Based on previous stereolithographic boxfish model experiments, it was determined that vortical flows develop around the carapace keels, producing self-correcting forces that facilitate swimming in smooth trajectories. To determine if similar self-correcting flows occur in live, actively swimming boxfishes, two species of boxfishes (Ostracion meleagris and Lactophrys triqueter) were induced to swim against currents in a water tunnel, while flows around the fishes were quantified using digital particle image velocimetry. Significant pitch events were rare and short lived in the fishes examined. When these events were observed, spiral flows around the keels qualitatively similar to those observed around models were always present, with greater vortex circulation occurring as pitch angles deviated from 0°. Vortex circulation was higher in live fishes than models presumably because of pectoral fin interaction with the keel-induced flows. The ability of boxfishes to modify their underlying self-correcting system with powered fin control is important for achieving high levels of both stability and maneuverability. Although the challenges of performing stability and maneuverability research on fishes are significant, the results of this study together with future studies employing innovative new approaches promise to provide valuable inspiration for the designers of bioinspired aquatic vehicles.
Copyright © 2008 IOP Publishing Limited. Received 20 August 2007, accepted for publication 11 January 2008. Published 4 February 2008. Print publication: Issue 1 (March 2008). We especially thank M Grosenbaugh at Woods Hole Oceanographic Institution (WHOI) for his generosity in providing lab space and access to DPIV equipment during the course of these experiments. We would also like to acknowledge E Anderson for providing invaluable technical assistance in the operation of the WHOI DPIV system. We thank Jonathan Brown for assistance in data analysis and Emilio Graff for writing a Matlab routine for data processing. This work was supported by the Office of Naval Research under grants N00014-96-1-0607 (MSG) and N00014-02-1-0180 (MG).