Keels of boxfish carapaces strongly improve stabilization against roll
Van Gorp, M.J.W.; Goyens, J.; Alfaro, M.E.; Van Wassenbergh, S. (2022). Keels of boxfish carapaces strongly improve stabilization against roll. J. R. Soc. Interface 19(189): 20210942. https://dx.doi.org/10.1098/rsif.2021.0942
In: Journal of the Royal Society. Interface. The Royal Society: London. ISSN 1742-5689; e-ISSN 1742-5662, more
Boxfish (Ostraciidae) have peculiar body shapes, with conspicuous keels formed by their bony carapaces. Previous studies have proposed various hydrodynamic roles for these keels, including reducing drag during swimming, contributing to passive stabilization of the swimming course, or providing resistance against roll rotations. Here, we tested these hypotheses using computational fluid dynamics simulations of five species of Ostraciidae with a range of carapace shapes. The hydrodynamic performance of the original carapace surface models, obtained from laser scanning of museum specimens, was compared with models where the keels had been digitally reduced. The original carapaces showed no reduced drag or increased passive stability against pitch and yaw compared to the reduced-keel carapaces. However, consistently for all studied species, a strong increase in roll drag and roll-added mass was observed for the original carapaces compared to the reduced-keel carapaces, despite the relatively small differences in keel height. In particular, the damping of roll movement by resistive drag torques increased considerably by the presence of keels. Our results suggest that the shape of the boxfish carapace is important in enabling the observed roll-free forward swimming of boxfish and may facilitate the control of manoeuvres.
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