Experimental technique to investigate compliance of fish fin during natural swimming

The compliance of a fish fin and how it contributes to swimming has gained extensive attention in the biology and engineering research and design communities. Many studies have hypothesized that fish can actively control the stiffness of their fin rays and fins during swimming to optimize propulsion, but such active controls have not been confirmed through experiments with live fish during natural swimming. This is partly because there are no experimental devices or methodologies that can facilitate controlled experiments during natural swimming to investigate the compliance of the fish fins. It is proposed, through this research, that the compliance of a fish fin can be investigated by applying an external perturbation, like a vortex ring, to the fin while it is being used in locomotion and by measuring the displacement of the fin from its natural swimming motion. To that end, the goal of this thesis was to develop and validate a perturbation device (vortex generator) and a technique to investigate compliance of fins during natural swimming. Experiments were conducted to understand how the vortex formation, size and speed could be altered by tuning the design features on the vortex generator. The evaluation of the vortex generator, the development of the experimental protocol, and the investigation of compliance changes in a fish fin at different swimming speeds were done by conducting experiments with live bluegill sunfish at different steady-swimming speeds and with flexible foils of known flexural rigidities. The evaluation of the technique was conducted by building an engineered system with tunable compliance and estimating the compliance using the perturbation technique proposed herein. The results from the experiments showed that (a) the vortex generator was able to produce an adequate range of vortex rings that could be used to perturb the fins during natural swimming, (b) the fish at slower swimming speed responded similarly to a compliant foil, while the fish at higher swimming speeds responded similarly to a stiffer foil, suggesting a stiffer fin at higher swimming speeds, (c) we can successfully predict changes in the compliance of a system by comparing the maximum deflection and rate of deflection from the unperturbed state to the maximum deflection of a system. Perturbation of fins is a key technique to uncover not only compliance but also behavioral responses that cannot be understood through the study of normal locomotion alone. The outcomes of this research will continue to advance the understanding of fish swimming as fish continue to be one of the greatest inspirations for advanced underwater vehicles.