Body Undulations Of Fish And Eels
June 24, 2013

How Do Fish Swim?: Researchers Explore Complex Body Undulations Of Fish And Eels

Brett Smith for - Your Universe Online

Looking to biology for inspiration seems to be all the rage in engineering circles these days, and a team of Northwestern University and NYU scientists has discovered new insights into how undulations help a fish to swim.

The team’s findings, which were published in the journal PLOS Computational Biology, could have implications for the development of new underwater vehicles.

“If we could play God and create an undulatory swimmer, how stiff should its body be? At what wave frequency should its body undulate so it moves at its top speed? How does its brain control those movements?” said Neelesh Patankar, professor of mechanical engineering at Northwestern. “Millennia ago, undulatory swimmers like eels that had the right mechanical properties are the ones that would have survived.”

Instead of creating mechanical models to mimic all the details of an undulatory swimmer, the team used computer methods to test their theories about the evolution of fish. They were able to determine the optimal values for a low muscle activation frequency and high body stiffness, two desirable characteristics.

“The stiffness that we predict for good swimming characteristics is, in fact, the same as the experimentally determined stiffness of undulatory swimmers with a backbone,” said co-author Amneet Bhalla, graduate student in mechanical engineering at Northwestern. “Thus, our results suggest that precursors of a backbone would have given rise to animals with the appropriate body stiffness,” added Patankar. “We hypothesize that this would have been mechanically beneficial to the evolutionary emergence of swimming vertebrates.”

The researchers also found that species must be resilient to small evolutionary physical changes. As long as the body stiffness is above a certain value, minor generational changes do not affect the ability to swim quickly, the researchers found.

The team also looked into neural control of swimming motions by analyzing the arc of modeled undulations to see if the motion was the result of a single bending torque or of several bending torques at specific points along its body. They found that a simple movement pattern can result in a complicated-looking swimming action.

“This suggests that the animal does not need precise control of its movements,” Patankar said.

To reach their findings, the researchers applied a common physics concept known as “spring mass damper” to the fish body. The concept, found in everything from car suspension to Slinkies, determines movement in systems that are losing energy.

The study’s unique approach unified the concepts of active and passive swimming by calculating the conditions necessary for the fish to swim effectively. Active swimmers move by a force that comes from within the fish, while passive swimmers move as a result of the surrounding water.

Other engineers have been inspired by birds in their research. The US military recently unveiled a robotic bird so realistic, it even reportedly fooled actual birds. Some birds were seen flying in formation with the “Robo-Raven” while hawks and other birds of prey were seen attacking it. Some observers suspect that this bird-like robot could be the latest evolution of the American military drone.