Jets and vortices propel fish forward

Aquatic organisms currently outperform all artificial underwater vehicles, which typically achieve propulsion efficiencies from 40 to 60%. A fish’s soft structure and flexible mobility generate and control vorticity along its body, creating low noise, high speed, and efficient movement. Understanding these propulsion mechanisms can guide fish-like device designs.

Guo et al. performed time-resolved particle image velocimetry experiments to explore the real flow field structure generated by swimming fish as they turn. They specifically examined Misgurnus anguillicaudatus, or pond loach, an eel-like fish that propels itself forward using body fluctuations and shows good maneuverability in small, complex, and variable environments.

The team used two lasers on each side of a water tank and a camera directly underneath. Tracer particles in the fluid were illuminated by the lasers and recorded by the camera, which allowed them to obtain the movement velocity and trajectory of the particles in continuous time.

As the fish swim, their bodies undergo wave-like motion. The peaks and troughs of this motion push and pull on the surrounding fluid, creating vortices. The low order spatial modes of the data captured the main vortex flow structure, while the high order modes probed the detailed structure.

“Jets are generated between two vortices with opposite swirling directions,” said author Han Yang. “The thrust jets provide the forward power required for turning maneuvers, while the side jets provide turning torque.”

In future work, the researchers plan to focus on the variation of forces in the eel-like propulsion process. They also hope to study the swimming in three dimensions using other flow field measurement technologies.

Source: “Hydrodynamic mechanism of Misgurnus anguillicaudatus during turning maneuvers,” by Chun-yu Guo, Ze-jun Liang, Yang Han, Peng Xu, Yong-hao Wang, and Yun-fei Kuai, Physics of Fluids (2022). The article can be accessed at https://doi.org/10.1063/5.0116029 .