The coordination between body and fin motion is a key feature of underwater propulsion for fish. However, the interactions between median fins – the dorsal, anal and caudal fins – are not well understood. Using a model of the bluegill sunfish, Han et al. conducted simulations of fin-fin interactions to study their effects on propulsion.
With high-speed cameras, the authors captured the deformation and motion of the body and fins of a bluegill sunfish swimming in a tank. This kinematic data was modeled and fed into 3D flow simulations to observe the relationships between fin size, flapping phase and kinematic performance.
They found major impacts of fin sizes and relative phases on the fish’s thrust, drag and efficiency. In particular, the caudal fin is the primary source of thrust enhancement, while the dorsal and anal fins offer the majority of drag reduction on the fish’s body. Increasing the dorsal and anal fin height can increase the thrust and decrease body drag and power consumption, and using phase-leading dorsal and anal fins further reduces the body drag and improves the propulsive efficiency of the caudal fin.
“The fish dorsal and anal fins debilitate the interactions between the posterior body vortices by working as a fence to prevent the vortices generated by the posterior body part from transferring to the opposite side of the body and thus reduce the drag on the fish trunk,” said author Pan Han.
These results have impacts in the evolutionary understanding of fish fins as well as potential biomimetic applications. “The findings of this work offer new insights into the design of bio-inspired, fish-like underwater robots aiming at high-speed, high-efficient swimming,” said Haibo Dong, another author on the paper.
Source: “Hydrodynamics of median-fin interactions in fish-like locomotion: Effects of fin shape and movement,” by Pan Han, George V. Lauder, and Haibo Dong, Physics of Fluids (2019). The article can be accessed at https://doi.org/10.1063/1.5129274.
