Meeting Abstract
It has been hypothesized, using robotic models and some biological studies, that the fish actively stiffen their tail as the swimming speed increases. However, no such modulations have been confirmed by directly investigating the stiffness of the fin of a fish swimming freely at different speeds. In this study, the caudal fin of bluegill sunfish (N = 2) was perturbed with vortex rings, as the fish swam naturally at three different swimming speeds: slow speed at which the fin was stationary, medium speed at which the fin was flapping slowly (< 1 Hz), and high speed at which the fin was flapping at 1.5 Hz (body-caudal fin swimming). The stiffness modulations were studied by comparing the fin’s deviations from natural swimming that were caused by a constant strength perturbation during different swimming speeds. The deflection in the caudal fin decreased when the swimming speed of the fish increased from the low to the medium or high speed, but did not change much when the swimming speed increased from the medium to high speed. In other words, the deflection did not drastically change with the flapping frequency of the fin. These results suggested that the fin stiffened when the caudal fin transitioned from being stationary to being flapped during swimming, but there was no consistent or strong evidence that the fin stiffened with an increase in speed, once the fin had begun flapping. This perturbation technique will be validated by perturbing flexible foils and ensuring that deviations caused by a perturbation are a reliable quantification to estimate stiffness changes in the fish fin. Additional experiments will be conducted with multiple fish swimming at the same range of swimming speeds.
