Abstract: Tuna, known for high endurance cruising, have already inspired several underwater robots and swimming studies. This study uses a biomimetic robotic tuna to investigate how different caudal fin planform geometries affect the thrust production and flow structures during Body and/or Caudal Fin (BCF) swimming. The robot was tethered to a circulating water tunnel, and swimming was simulated by moving water at a constant speed relative to the stationary robot. Three differently shaped caudal fins were tested, one rectangular, one elliptical, and one swept. Area, aspect ratio, and rigidity were kept constant between the three fins to ensure that the effect of caudal fin shape could be isolated. The fins were tested at three freestream velocities and four Strouhal numbers (St) so that comparisons between the fins could be made for a variety of swimming scenarios. The swept fin, which is the tested caudal fin most similar to one found on a fusiform swimmer, had the greatest thrust potential at high St, followed by the elliptical fin. The rectangular fin generally produced the least thrust. It was shown that in addition to producing the most thrust, the swept fin also best stabilized the leading edge vortex that developed during the second half of the stroke.

Key words: thunniform locomotion, vortex formation, caudal fin, BCF swimming