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.
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.
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.
The findings are especially intriguing because they echo an important discovery from 1930 by Theodore von Kármán and Ludwig Prandtl, known as the "Law of the Wall." This discovery involved the study of a gas or liquid flowing along a wall, where its speed must be zero at the wall because of friction. The speed of the fluid parallel to the wall increases as the distance from the wall increases. Von Kármán and Prandtl showed more specifically that the speed increases linearly with the logarithm of this distance when the flow is fast enough so that the fluid becomes turbulent. This result is called the Law of the Wall and is of great importance in many engineering applications.
Ahlers compared the new findings about the way temperature varies in convection to the way speed varies with the Law of the Wall, noting that they are similar, although the precise relationship has yet to be understood. "They behave in the same way," said Ahlers. "But just because two things look the same doesn't mean they are the same, so we still need to build the theoretical foundation that connects them. That's what makes this a very active, very exciting field, with theorists as far apart as Beijing (China), Marburg (Germany), and Twente (the Netherlands) already trying to explain the experimental results. You make an experimental discovery, and then theorists get excited. Then they start working on it, and who knows what we will have six months down the road?"
He explained how the Law of the Wall is of importance in engineering applications. "Pumping oil from Alaska down to the United States costs billions of dollars," said Ahlers. "And if you can understand what causes the resistance that you have to overcome, then maybe you can reduce that. Even if you only reduce it by 2 or 3 percent, you've saved hundreds of millions. So it's very, very important."
Ahlers went on to say that understanding the temperature in turbulent convection is also very important because there are many applications where turbulent convection is used to cool things. In nuclear reactors, for instance, cooling is done by turbulent convection. "There are many applications of this turbulent convection system in industry, where you would also like to understand what's going on inside, what the temperature gradients are," he said. "So I can see relevance for this in applications. Although I must say that is not our motivation; our motivation is to understand the fundamental physics."
Eberhard Bodenschatz, one of the authors, was a postdoctoral fellow with Ahlers at UCSB about 20 years ago and is now director of the Max Planck Institute for Dynamics and Self-Organization in Goettingen. Co-author Xiaozhou He is a postdoctoral fellow with Ahlers and is based in Goettingen. Scientists from The Netherlands, Italy, and France are also involved.
Gail Gallessich | EurekAlert!
Study offers new theoretical approach to describing non-equilibrium phase transitions
27.04.2017 | DOE/Argonne National Laboratory
SwRI-led team discovers lull in Mars' giant impact history
26.04.2017 | Southwest Research Institute
More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.
Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...
Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.
"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...
The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.
Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...
The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...
Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.
Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...
20.04.2017 | Event News
18.04.2017 | Event News
03.04.2017 | Event News
27.04.2017 | Life Sciences
27.04.2017 | Physics and Astronomy
27.04.2017 | Earth Sciences