Area of turbulence
As a member of the EuHIT (European High-Performance Infrastructures in Turbulence - see here) consortium, CERN is participating in fundamental research on turbulence phenomena. To this end, the Laboratory provides European researchers with a cryogenic research infrastructure (see here), where the first tests have just been performed.
The last day of data collection, tired but satisfied after seven intense days of measurements. Around the cryostat, from left to right: Philippe-E. Roche, Éléonore Rusaouen (CNRS),
Olivier Pirotte, Jean-Marc Quetsch (CERN), Nicolas Friedlin (CERN),
Vladislav Benda (CERN). Not in the photo: Laurent Le Mao (CERN), Jean-Marc Debernard (CERN),
Jean-Paul Lamboy (CERN), Nicolas Guillotin (CERN), Benoit Chabaud (Grenoble Uni), and Gregory Garde (CNRS).
CERN has a unique cryogenic facility in hall SM18, consisting of 21 liquid-helium-cooled test stations. While this equipment was, of course, designed for testing parts of CERN's accelerators, it can also be used for other laboratory experiments, notably for studying “very intense turbulence” in fluids.
Very intense turbulence is a natural phenomenon observed in numerous situations – in the atmosphere and in the ocean, in the slipstream of planes and trains, and even in stars – but it is very difficult to study. “The appearance of this phenomenon depends mainly on three factors,” explains Philippe Roche, a researcher at CNRS. “The velocity of the fluid, its viscosity and the system's size (several kilometres in the case of atmospheric or oceanic turbulence, for example).” The greater the velocity and size and the lower the viscosity, the more intense the turbulence. In fluid mechanics, the level of turbulence is described by the Reynolds number (Re)*: a high Reynolds number indicates high turbulence.
Diagram of the cryostat in which the experiment takes place.
The cryogenic facility in SM18 is the ideal place to generate a very intense turbulence phenomenon in perfectly controlled laboratory conditions: its cooling system distributes cryogenic helium, whose viscosity is extremely low (200 times lower than that of air in the experimental conditions), and that, combined with the extremely large size of the cryostat in which the experiment takes place (1.1 metre in diameter and 4.6 metres high - see diagram), leads to Reynolds numbers of the order of 107. “This infrastructure at CERN is capable of producing turbulence of an intensity comparable to that observed in certain atmospheric phenomena,” says Olivier Pirotte, head of the Mechanical and Engineering Support section of the Cryogenics group and coordinator of the EuHIT project at CERN. “But to achieve this result, we have to create conditions as close to the theoretical model as possible, which means working with a jet of helium that’s 100% gaseous.” The Cryogenics group has therefore developed a heating device that, when installed in the liquid-helium supply line, ensures that the fluid passes into its gaseous state before being injected into the cryostat.
In order to study the area of turbulence and to “see” what happens at its core, the EuHIT researchers are designing sensors specially adapted for the cryostat provided by CERN. The data collected (velocity, temperature and pressure) will allow them to learn a lot about this phenomenon. For the moment though, the experiment is being fine-tuned, in collaboration with CNRS. The first physics measurements are expected to be taken when the experiment receives a share of the helium flow in October (see box); the second run, this time at full flow, is expected to take place in January.
*Reynolds number (Re) = (velocity x size) / kinematic viscosity.
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CERN, a member of EuHIT (European High-Performance Infrastructures in Turbulence)
CERN’s involvement in research into turbulence phenomena dates back to the year 2000. A new cryogenic infrastructure had just been installed in SM18 to test the String for the future LHC. A change in the test schedule in 2000 freed up the new cryogenic system, and so CERN Management agreed to make it available to researchers specialising in the study of turbulence phenomena, allowing them to conduct experiments in early 2001.
From 2002 to 2007, the cryogenic facility was used exclusively for the LHC. In 2007, once the tests for the LHC had been completed, the "turbulence" community met at CERN and decided to apply for funding from Europe. CERN thus became a partner in the EuHIT consortium (within the EU's Framework Programme 7).
Thanks to the European funding, CERN is able to provide, at no extra cost to the Laboratory, an exceptional research infrastructure for European researchers for two weeks per year from 2015: one week of shared flow (which means that the liquid helium is shared between various tests conducted in parallel) and one week at full flow (where 100% of the liquid helium goes to EuHIT).
by Anaïs Schaeffer