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 stabilising effect should come from the fish’s 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 therefore acts as both a stabiliser 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.

File description:

STL_binary_Ocub_units_mm_Boute_et_al.stl : Triangulated surface mesh used as (1) the geometry input file for body and part of the caudal peduncle in 3D printing of the model used in the flow tank, and (2) as input file for IGES conversion for CFD input.  The STL file has a binary format, and is created with mm as the unit for sizes.  The origin is located in the centre of volume of the surface mesh.

IGES files : Geometry input files for the CFD models

CFD result files (.cas and .dat): ANSYS version 14.5.7 'case' and 'data' files.  File name legend:Ocub = species Ostraction cubicus
tail_closed = closed caudal fin
tail_open = open caudal fin
bxx = body angle alfa
txx = tail angle omega
m (in front of number) = minus
3000its = solved with 3000 iterations