By Marlene Cimons, National Science Foundation
Jellyfish create doughnut-shaped currents of rotating water when they swim. Visually, they resemble what happens when someone blows smoke rings from a cigar.
More importantly, however, this unusual method of propulsion, these so-called “vortex rings,” enable jellyfish to go further on less energy, an idea that scientists hope to translate into new engineering designs.
“We’re very interested in figuring out things that animals do better than we, as engineers, can do,” said John Dabiri, professor of aeronautics and bioengineering at California Institute of Technology, who is heading the project. “We’d like to co-opt some of those ideas.”
In particular, they want to build new underwater research vehicles that can remain beneath the ocean surface for years at a time, rather than only hours or months, and on less fuel.
“It is important to have underwater vehicles that can study the changing properties of the ocean, such as temperature and pH, so we can improve our knowledge of the ocean and how it works,” said Dabiri, who recently was among those named to receive a prestigious $500,000 MacArthur “genius’’ award, a “no-strings attached’’ fellowship. “This is especially important in trying to understand the impact of climate change on the ocean.”
Jellyfish propel themselves by contracting cells in their bell-shaped outer skin and generating jet forces in the tail end, with tentacles trailing behind. “Pretty much all underwater swimmers create these vortex structures, but theirs are a lot more complicated [than jellyfish] in flow currents,” Dabiri said. “Their rings are jumbled together in ways more difficult to measure.”
Beyond inspiring new energy-saving underwater technology, understanding the fluid dynamics of the jellyfish also ultimately could provide important information applicable to other related areas, such as blood flow in the human heart or the design of wind power generators.
The National Science Foundation is supporting the research with $170,000 as part of the American Recovery and Reinvestment Act of 2009.
“In the short term, we’re using this money to buy equipment for the labs to build the experiments we take out into the water, which is supporting the small businesses that are building these devices for us,” Dabiri said. “In the long term, the most important investment is in energy efficiency. By making underwater vehicles that are less reliant on huge amounts of fuel, the fuel that is saved either can go to other uses or stay in the ground.”
Dabiri, a biophysicist whose research encompasses several fields, including theoretical fluid dynamics, evolutionary biology and biomechanics, has shown that explaining the workings of locomotion depends on a mathematical analysis of the fluid vortex rings that jellyfish form in the surrounding water by contracting their bells. His research team, in ocean experiments, “scuba dive up close to the jellyfish,” to video them and take certain measurements.
To get a rough idea of what the animals are doing, the researchers add dye to the water. Then, in order to gather more quantitative data, they illuminate the water with a laser, allowing the scientists to see the sediment generated in the water by the jellyfish movement. “We can track the motion of those particles over time to infer the water velocity,” a process known as digital particle image velocimetry, he said.
“We’ve already demonstrated reductions in energy use by 30 percent compared to conventional propeller-driven submarines,” Dabiri said.
Today’s ocean explorer vehicles can spend only short periods of time underwater, or, in the case of gliders, must change their locations frequently. Most other sea research is conducted primarily with satellite technology. The latter “gives coverage of the ocean surface, but doesn’t tell you what’s happening beneath the surface,” he said.
Ocean gliders can remain underwater for months, but must constantly change their depth in order to function. “It’s difficult to get one of them to stay at the same depth for an extended period of time,” Dabiri said. “We’d like to develop vehicles that can remain at a fixed depth for longer periods of time.”