Modulating yaw with an unstable rigid body and a course-stabilizing or steering caudal fin in the yellow boxfish (Ostracion cubicus)

Abstract

Despite that boxfishes have a rigid carapace that restricts body undulation, they are highly manoeuvrable and manage to swim with remarkably dynamic stability. Recent research has indicated that the rigid body shape of boxfishes shows an inherently unstable response in its rotations caused by course-disturbing flows. Hence, any net stabilizing effect should come from the fishes' fins. The aim of the current study was to determine the effect of the surface area and orientation of the caudal fin on the yaw torque exerted on the yellow boxfish, Ostracion cubicus , a square cross-sectional shaped species of boxfish. Yaw torques quantified in a flow tank using a physical model with an attachable closed or open caudal fin at different body and tail angles and at different water flow speeds showed that the caudal fin is crucial for controlling yaw. These flow tank results were confirmed by computational fluid dynamics simulations. The caudal fin acts as both a course-stabilizer and rudder for the naturally unstable rigid body with regard to yaw. Boxfishes seem to use the interaction of the unstable body and active changes in the shape and orientation of the caudal fin to modulate manoeuvrability and stability.

Keels of boxfish carapaces strongly improve stabilization against roll

“…As mentioned in the introduction, the absence of stabilizing effects of the keels on yaw and pitch could be expected, as this would be in line with the ecological demands of the boxfish to maintain sufficient manoeuvrability and agility [10,12]. To manage yaw and pitch, boxfish seem to rely on active control by the fins to tune their level of stability or manoeuvrability according to the circumstances [13].…”

“…The fluid in the model had a water density ( ρ ) of 998.2 kg m −3 and a dynamic viscosity of 1.001 Pa s. To account for the effects of turbulence at relatively low Reynolds numbers, Menter's shear stress transport (Transition SST) model was used, which is a robust four-equation eddy-viscosity turbulence model widely used in CFD [37] . This model was previously validated against force and torque measurements in a flow tank [13,24]. Convergence was safely reached before the end of the imposed 2000 iterations.…”

“…Carapace geometry varies greatly between species [6][7][8] and serves as a bony armour that provides protection against bites of coral-reef-dwelling predators [9]. Boxfish have accurate control over manoeuvres, which is essential for foraging in spatially complex habitats such as coral reefs [10][11][12][13]. In fish, propulsion is usually driven largely by undulations of the body.…”

“…The hypothesis that the keels contribute to stabilizing rectilinear swimming by generating self-stabilizing yaw and pitch torques (H2) was inferred from vortical flows and pressures measured experimentally near the boxfish keels [22,23,25]. However, recent studies on two boxfish species, Lactophrys triqueter with a triangular body shape and Ostracion cubicus with a rectangular body shape, demonstrated that the overall torque by flow past the carapace under yaw and pitch angles of attack is not self-stabilizing but rather destabilizing [13,24]. Nevertheless, a role in reducing destabilization seems unlikely, as the destabilizing hydrodynamic properties of the carapace improve the manoeuvrability of the boxfish, and the role of the keels to reduce this effect appears to be in conflict with the ecological demands of the boxfish in terms of manoeuvrability and agility [10,12].…”

Thrust Improvement of a Biomimetic Robotic Fish by Using a Deformable Caudal Fin

“…However, the stiffness and shape of the caudal fin usually cannot be changed when the robotic fish is freely swimming. In contrast, live fish is capable of modulating the stiffness or shape during swimming in real-time to adapt to surrounding aquatic environment [ 16 , 17 , 18 , 19 ]. Accordingly, novel mechanisms to adjust stiffness are developed for biomimetic robotic fishes in these years.…”