Development of a biologically derived robotic model of the bluegill sunfish caudal fin

Fish inspired robots that have historically modeled the caudal fin as a rigid or semi-rigid plate with a single undulatory actuation from the peduncle caudal area. However, a two dimensional representation of a three dimensional motion ignores aspects that are crucial in producing variations in force vector direction and the magnitude. The primary objective of this research was to develop a biologically derived robotic model of a bluegill sunfish's caudal fin, and to use this model to investigate how changes in fin kinematics and stiffness affect hydrodynamic forces and flows. A six fin-rayed robotic caudal fin was developed using information obtained from biological studies of the bluegill sunfish caudal fin. The robot model tested five distinct motions that were deduced from caudal fin steady swimming motions of the bluegill sunfish (Lepomis macrochirus). These motions investigate both slight and large changes through phase, amplitude and frequency alterations. There were five fin stiffnesses tested, which retained similar curvature and tips deflection to those of the fish. The experimentation involved the use of high speed video, microtomography scans, force collection and particle image velocimetry. The results have shown that large changes in the motions caused large changes in the forces, while slight changes have led to small but constant changes in the forces. It was determined that the lift and thrust forces generated by the robotic caudal fin peaked at particular fin stiffnesses, indicating there are optimal stiffnesses for different scenarios (flapping frequencies, flow speeds). The results of this research have shown that alterations in the kinematics and mechanical properties of the fin rays can significantly affect the forces produced. This work has provided a better understanding of three dimensional caudal fin flapping and laid groundwork for future studies.