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  1. 2012.09.17 Physicists make discovery about temperature in convection


    This is a drawing of the container used to study convection. The 8-foot tall cylinder was heated at the bottom and cooled at the top. Credit: UCSB An international team of physicists is working to ascertain more about the fundamental physical laws that are at work in a process known as convection, which occurs in a boiling pot of water as well as in the turbulent movement of the liquid outer core of the Earth. The team's new finding specifies the way that the temperature of a gas or liquid varies with the distance from a heat source during convection. The research is expected to eventually help engineers with applications such as the design of cooling systems, for instance, in nuclear power plants. Read more at:

    Guenter Ahlers, professor of physics at UC Santa Barbara, worked with his team at the Max Planck Institute for Dynamics and Self-Organization in Goettingen, Germany, on this important discovery about turbulent convection. The results will be published in the September 7 issue of Physical Review Letters, and are available online now. The experiments took place in a cylinder that was placed under the turret of a large pressure container. The 8-foot tall cylinder was heated at the bottom and cooled at the top. There were about 100 thermometers inside it, and it was pressurized with sulfur hexafluoride, an inert gas. Convection occurred inside the cylinder because, in the presence of gravity, the warmer gas at the bottom tends to rise to the top, while the colder gas tends to sink. "We like sulfur hexafluoride because it is harmless –– not poisonous, not chemically reacting –– and because it is a heavy molecule," said Ahlers. "A heavy molecule enables us to produce more vigorous convection with the same temperature difference. The strength of the convection is measured by a parameter called the Rayleigh number. We go to Rayleigh numbers as high as 10 to the 15 –– a million billion –– which is very large by our standards." Ahlers enjoys the ability to oversee and even run the continuing experiments remotely on a computer in his office at UCSB (or anywhere else in the world), even though the laboratory is 5,000 miles away. He explained that convection occurs naturally in astrophysics and in Earth systems. For example, the outer layer of the sun is composed of convection cells. Convection occurs in the Earth's atmosphere and oceans. The liquid iron in the outer core of the Earth undergoes vigorous convection and has Rayleigh numbers well above 10 to the 20. That convection generates the magnetic field of the Earth. In their paper, the scientists present experimental and numerical data that show that, except for a very thin layer in the immediate vicinity of the plates, the temperature of this system varies linearly with the logarithm of the distance from the confining plates. They discovered this profile and measured it in detail. Read more at: