Secrets of fish tails will help design underwater drones

By uncovering the secrets of how fish swim so efficiently at various speeds, scientists are getting ideas for the next generation of underwater drones. 

Typically, underwater vehicles are designed for one cruising speed – and fall short at other speeds. The technology is very limited compared to the impressive skills of fish, which swim well at various speeds by adjusting their tail stiffness. It would be very useful for underwater drones to move both quickly and slowly. 

Professor Dan Quinn at the University of Virginia School of Engineering and Applied Science, in collaboration with postdoctoral researcher Qiang Zhong, developed a key strategy for enabling multi speed missions in underwater vehicles. The experts have demonstrated a simple way to implement their strategy in robots.

When scientists design swimming robots, it is challenging to determine the ideal stiffness of the piece that propels the robots through the water. This difficult because different levels of stiffness are needed for various conditions. 

“Having one tail stiffness is like having one gear ratio on a bike,” said Professor Quinn. “You’d only be efficient at one speed. It would be like biking through San Francisco with a fixed-gear bike; you’d be exhausted after just a few blocks.”

Most likely, fish solve this problem by adjusting their tail stiffness in real-time, tuning into different levels of stiffness depending on the situation.

While there is no known way to measure the stiffness of a swimming fish, Quinn have combined fluid dynamics and biomechanics to create a model for how and why tail stiffness should be tuned.

“Surprisingly, a simple result came out of all the math: Stiffness should increase with swimming speed squared,” said Professor Quinn.

“To test our theory, we built a fishlike robot that uses a programmable artificial tendon to tune its own tail stiffness while swimming in a water channel. What happened is that suddenly our robot could swim over a wider range of speeds while using almost half as much energy as the same robot with a fixed-stiffness tail. The improvement was really quite remarkable.”

“Our work is the first that combines biomechanics, fluid dynamics, and robotics to comprehensively study tail stiffness, which helps to uncover the long-existing mystery about how tail stiffness affects swimming performance,” said Zhong. 

“What is even more fantastic is that we are not just focused on theory analysis, but also on proposing a practical guide for tunable stiffness. Our proposed tunable stiffness strategy has proved effective in realistic swimming missions, where a robot fish achieved high speed and high efficiency swimming simultaneously.”

The study is published in the journal Science Robotics.

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By Chrissy Sexton, Earth.com Staff Writer