Boxfish carapace hydrodynamics: a test of the self-stabilization and low body-drag hypotheses
Van Wassenbergh, S.; Van Manen, K.; Stamhuis, E.J. (2014). Boxfish carapace hydrodynamics: a test of the self-stabilization and low body-drag hypotheses. Integrative and Comparative Biology 54: E213-E213
In: Integrative and Comparative Biology. Oxford University Press: McLean, VA. ISSN 1540-7063; e-ISSN 1557-7023
Previous work by Bartol en co-workers indicated that the shape of the bony carapace of boxfish (Ostraciidae) plays a significant role in the hydrodynamic stability during swimming. Their flow tank measurements and flow visualizations showed specific vortical flow patterns that were argued to be responsible for course self-stabilization: when the animals pitches or yaws a certain angle with respect to the swimming direction, fluid forces exerted on the carapace would cause stabilizing moments about its centre of mass, reorienting the fish parallel to the flow. Additionally, drag coefficients reported for certain species (e.g. 0.1 for Lactophrys triqueter) are remarkably low for such poorly streamlined shapes. Both swimming-direction self stabilization and a low body drag fit best to migratory swimmers, whereas boxfish usually are slow speed manoeuverers of tropical reefs. To test this apparent contradiction, flow tank drag force and turning moment measurements of realistic body models and computational fluid dynamics simulations were performed for the species L. triqueter and Ostracion cubicus at a series of yawing and pitching angles of attack (Re = 6.5 x 104). Both the empirical and computational approach found strong de-stabilizing moments instead of stabilizing moments, thus rejecting the self-stabilization hypothesis. The minimal drag coefficient for L. triqueter was considerably higher than reported earlier (Cd = 0.27), the minimal Cd for O. cubicus being of similar magnitude (Cd = 0.29). These results indicate that the shape of the boxfish carapace promotes manoeuvrability, and any stabilization must be controlled by action of the fins.
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