Underwater vehicles have not changed much since the submarines of World War II. They are rigid, quite boxy and use propellers to move. And whether they are large manned ships or small robots, most underwater vehicles have a cruising speed where they are most energy efficient.
Fish take a very different approach to moving through water: their bodies and fins are very flexible, and this flexibility allows them to interact with water more efficiently than rigid machines. Researchers have been designing and building flexible fish-like robots for years, but they still lag far behind real fish in terms of efficiency.
What is lacking?
I am an engineer and study fluid dynamics. My labmates and I wondered if there was something in particular about the flexibility of a fish tail that allows fish to be so fast and efficient in the water. Therefore, we built a model and built a robot to study the effect of stiffness on swimming efficiency. We found that fish swim so efficiently over a wide range of motion because they can change how rigid or flexible their tails are in real time.
Why are people still using propellers?
Fluid dynamics applies to both liquids and gases. Humans have been using rigid objects to propel vehicles for hundreds of years – Leonardo da Vinci incorporated this concept into his helicopter designs, and the first propeller-driven boats were built in the 1830s. The propellers are easy to make, and they work just fine at their designed cruise speed.
It is only in the last few decades that advances in soft robotics have made actively controlled flexible components a reality. Now, marine roboticists are turning to flexible fish and their amazing swimming abilities for inspiration.
When engineers like me talk about flexibility in swimming robots, we usually refer to how tight the tail of a fish is. The tail is the entire rear part of the fish’s body that moves back and forth while swimming.
Consider tuna, which can swim up to 50 mph and is highly energy efficient at a wide range of speeds.
The tricky thing about mimicking the biomechanics of fish is that biologists don’t know how flexible they are in the real world. If you want to know how flexible the rubber band is, you just pull it. If you pull a fish’s tail, the stiffness depends on how much the fish is straining its various muscles.
The best researchers can do to estimate flexibility is take a swimming fish and measure how its body shape changes.
looking for answers in maths
Researchers have built dozens of robots in an effort to mimic the flexibility and swimming patterns of tuna and other fish, but none match the performance of the real thing.
In my lab at the University of Virginia, my colleagues and I ran into the same questions as others: How flexible should our robot be? And if there is no best flexibility, then how should our robot change its stiffness as it swims?
We looked for answers in an old NASA paper about vibrating airplane wings. The report suggests that when an aircraft’s wings vibrate, the vibrations change the amount of lift the wings have. Since fish fins and airplane fins have similar shapes, the same math works well to model how loud a fish’s tail is produced when flapping back and forth.
Using old wing theory, postdoctoral researcher Qiang Zhong and I built a mathematical model of a swimming fish and added a spring and pulley to the tail to represent the effects of a tense muscle. We discovered a surprisingly simple hypothesis hidden in the equations. To maximize efficiency, muscle tension needs to increase as the square of the swimming speed. Therefore, if the swimming speed is doubled, the stiffness must be increased by four times. To swim three times as fast while maintaining high efficiency, a robot like a fish or a fish needs to pull its tendon about nine times as hard.
To confirm our theory, we simply added an artificial tendon to one of our TuneLike robots and then programmed the robot to change the stiffness of its tail based on motion. We then placed our new robot in our test tank and drove it through various “missions” – like a 200m sprint where it had to dodge simulated obstacles. With the ability to change the flexibility of its tail, the robot used on average about half the energy across a wide range of motion compared to a robot with the same stiffness.
why it matters
While it’s great to build an excellent robot, what my colleagues and I are most excited about is that our model is adaptable. We can change this based on body shape, swimming style or even fluid type. It can be applied to animals and machines whether they are large or small, swimmers or fliers.
For example, our model suggests that dolphins gain much from the ability to change the stiffness of their tails, whereas goldfish do not gain much because of their body size, body shape, and swimming style. .
The model also has applications for robotic design. Higher energy efficiency when swimming or flying – which also means quieter robots – will enable fundamentally new missions for vehicles and robots that currently only have an efficient cruising speed. In the short term, it could help biologists study river beds and coral reefs more easily, enable researchers to track wind and ocean currents on unprecedented scales or enable search and rescue teams to study further and longer. allows to operate.
In the long term, I hope our research can inspire new designs for submarines and airplanes. Humans have only been working on swimming and flying machines for a couple of centuries, while animals have been improving their skills for millions of years. There is no doubt that there is much to be learned from them.
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