Boxfish swimming paradox resolved: forces by the flow of water around the body promote manoeuvrability

Abstract

The shape of the carapace protecting the body of boxfishes has been attributed an important hydrodynamic role in drag reduction and in providing automatic, flow-direction realignment and is therefore used in bioinspired design of cars. However, tight swimming-course stabilization is paradoxical given the frequent, high-performance manoeuvring that boxfishes display in their spatially complex, coral reef territories. Here, by performing flow-tank measurements of hydrodynamic drag and yaw moments together with computational fluid dynamics simulations, we reverse several assumptions about the hydrodynamic role of the boxfish carapace. Firstly, despite serving as a model system in aerodynamic design, drag-reduction performance was relatively low compared with more generalized fish morphologies. Secondly, the current theory of course stabilization owing to flow over the boxfish carapace was rejected, as destabilizing moments were found consistently. This solves the boxfish swimming paradox: destabilizing moments enhance manoeuvrability, which is in accordance with the ecological demands for efficient turning and tilting.

The armored carapace of the boxfish

“…The boxy shape and rigid carapace considerably restrict the movement of boxfish. Of note, this structure has led to extensive fluid dynamics studies as the unusual shape of the boxfish body and the placement of its fins create a number of vortices around the body and result in a sophisticated self-correcting swimming motion [24][25][26][27][28][29]. While a model for underwater locomotion, the boxfish is only capable of relatively slow swimming speeds of just above five body lengths per second [30].…”

On the diverse roles of fluid dynamic drag in animal swimming and flying

“…S ¼ pa 2 for a sphere of radius a). In animal locomotion, one usually expects streamlined shapes to be favoured, but other animal shapes indeed exist and considerations of ecological relevance such as enhanced manoeuvrability have been suggested to be the evolutionary reason for bluff-body type shapes such as the archetypal example of boxfishes (Ostraciidae: Tetrodontiformes) [57], where pressure drag can be expected to be larger than skin friction. Pressure drag is also the main type of drag in transient manoeuvres with impulsive rapid motions and massive separations behind the moving body, such as the rowing-type motions of median and paired fin propulsion in fish swimming, the fast c-starts of body and caudal fin fish swimming [58] or the impulsive accelerations of the strike manoeuvre in aquatic predators [59,60].…”

The comparative hydrodynamics of rapid rotation by predatory appendages

“…The walls of the cylinder were defined with a velocity equal to freestream flow (20 m s −1 ) and the multi-frustum surface was defined with a no-slip condition. We used Menter's shear stress transport model in ANSYS Fluent (Menter, 1994) to resolve the flow patterns in the wake, which accurately calculates the steady-state drag of bluff bodies at similar Re (Goyens et al, 2015;Van Wassenbergh et al, 2015b). A mesh convergence analysis showed that the drag did not vary substantially when refining from 11 million to 16 million cells (+0.1%) (Fig.…”

“…The drag coefficient is typically determined from drag measured by a force transducer for a body exposed to the uniform flow generated by a flume (e.g. Van Wassenbergh et al, 2015b). In contrast, a rotating body is exposed to flow that varies linearly along its length.…”