EUGENE, Ore.–From the time of Leonardo da Vinci’s famous sketches of the churning forces at work in roiling water, scientists have been trying to understand one of the universe’s most elusive mysteries–turbulence. A report published in Thursday’s (April 20) issue of Nature magazine describes research that pushes forward science’s ability to produce and measure turbulent flows in the laboratory by a factor of 1,000.

"We’ve attained levels of thermal turbulence three orders of magnitude beyond what has been reached before," says University of Oregon physicist Russell Donnelly who heads the research group. "This is by far the most intense thermal turbulence ever achieved on Earth in a controlled experiment."

Donnelly sees his work as an important step forward for science.

"Fluid turbulence is undoubtedly the most outstanding problem of classical physics that has remained unsolved to this day," he says. "Despite more than 200 years of focused research, there is no comprehensive theory of turbulence. This is quite remarkable."

While the new findings do not provide the elusive comprehensive theory, they are expected to shed light on some important and practical questions.

For example, scientists have previously been unable to model accurately the intense levels of turbulence that develop around moving bodies–from the wind roaring over the wing of a 777 jet aircraft in flight to the water swirling around the hull of a ship plowing through the sea. On the environmental front, advanced understanding of turbulence will assist scientists modeling the gargantuan churning motion of oceanic and atmospheric currents. Donnelly’s work even puts scientists within reach of modeling one of the most turbulent places in the solar system, the surface of the Sun.

"The advance that this work represents is our ability to control under laboratory conditions what is arguably the most ubiquitous phenomena in the universe–turbulent thermal convection–to near astrophysical levels," says UO physicist Joe Niemela, one of the co-principal investigators of the research group.

The published findings are the result of work begun in the fall of 1996 when the National Science Foundation (NSF) awarded Donnelly and his co-researchers $5 million to develop technology that can produce and study high intensity turbulence.

The research group formed the Oregon Cryogenic Helium Turbulence Laboratory (CHTL), housed on the University of Oregon campus, and developed an advanced low-temperature testing device. Known generically as a cryostat, the device looks something like a thermos bottle for Paul Bunyan–one meter tall and filled with extremely cold helium gas. At extremely low temperatures (about 450 degrees below zero or just a few degrees above absolute zero), helium gas is the slipperiest fluid known to science.

"I believed this slipperiness would make low temperature helium the perfect substance for studying turbulence," Donnelly says. He was right.

"The cryostat worked like gangbusters, right out of the chute," he reports.

In simplified terms, physicists use a number, called the Rayleigh number, to represent thermal turbulence. Due to the limiting physical qualities of air and water, however, scientists using these test fluids have been unable to create controlled conditions that yield Rayleigh numbers much higher than one billion (10⁹)–not nearly high enough to represent the wide variety of turbulence found in nature. Scientists using more conventional cryostats have attained Rayleigh numbers of 10¹⁴. Using super-slippery low-temperature helium and sophisticated new sensors developed for the latest cryostat, Donnelly’s team was able to measure Rayleigh numbers ranging from 10⁶ to 10¹⁷.

"Nothing I know of in physics covers such a wide range of observations in one apparatus," Donnelly notes.

The research team already is at work on a number of new experiments and techniques.

But the Oregon CHTL group’s vision for pushing the limits of what is known about turbulence go far beyond what is already in the works. The NSF originally funded the cryostat as a pilot project and now, having demonstrated the success in the pilot program, the CHTL research group hopes to build a 10-meter high, 5-meter diameter cryostat that will hold 60,000 liters (about 15,000 gallons) of helium. Such a cryostat would achieve Rayleigh numbers of up to 10²⁰, Donnelly says.

Besides Donnelly and Niemela, other collaborators on the project are Michael McAshan, former chief of cryogenic research at the Superconducting Supercollider project in Texas and now at Fermilab in Illinois; senior research associate Ladislav Skrbek of the University of Oregon; and co-principal investigator Katepalli Sreenivasan, a professor of physics and mechanical engineering at Yale University.

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