Fish create moving vortices to power their propulsion

Humans have always admired the grace and skill exhibited by fish as they move through their viscous environment, and engineers have looked to them for inspiration when designing watercraft. Fish can be especially useful in developing underwater vehicles. Now, a new study published in the journal Physics of Fluids takes a closer look at fish propulsion.

Scientists at Harbin Engineering University in China have demonstrated that fish, using tightly controlled body fluctuations, create a pair of moveable vortices or high and low pressures, which allow them to move. The research lays the groundwork for the design of a new propeller. 

“Fish need to evade predators, catch prey, and move through complex environments, and their motion advantage is unmatched by any man-made machinery. As such, many outstanding questions about the relationship between form and function in fish are biomechanically driven,” wrote the researchers.

“The study of motion law, corresponding mechanical characteristics, and revelation of the hydrodynamic basis of the fish swimming process have significant implications for the optimal design of underwater vehicle propulsion methods.”

Using high speed cameras and particle image velocimetry, the researchers analyzed zebra fish swimming in a tank. A fish speeds up as it bends its caudal fin to the side and then returns to a neutral position by straightening the whole body. This single tail swing, the researchers discovered, creates two vortices spinning in opposite directions. 

On opposite sides of the body, one vortex is high pressure, while the other is low pressure. The low pressure pulls the fish, while the high pressure pushes the fish from the other direction, thus providing the zebrafish with propulsion. 

When the fish was in a “J shaped” position with its tail bent, the high pressure area moved to the rear of the caudal fin and the low pressure area moved to the front of the caudal fin. This process repeats itself, allowing the fish to move continuously. 

“The entire zebrafish in the swimming process is regarded as a body wave,” said study co-author Yang Han. “Whether they accelerated forward or changed directions, the fish maintained a wavelike motion at all points of the body from the start of movement.”

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By Zach Fitzner, Earth.com Staff Writer