Gas behaves differently from fluid at thermally driven turbulence
Researchers from Twente (The Netherlands) and America have discovered that gas behaves differently from fluid in case of turbulent convection. Turbulent convection is a current that is driven by a temperature difference. This conclusion is surprising because first-year students in physics learn that turbulence is universal and that only dimensionless indicators matter. The researchers have now discovered that at a very high temperature difference by using gas the temperature in the centre of the vessel where convection takes place, decreases and that the thermal conduction increases. This is exactly the other way round by using fluids. Obviously, the behaviour of turbulence is not universal at high temperature differences. This is of importance, for instance, in the heating mantle of earth. Researchers Detlef Lohse (Twente University) and FOM Ph.D. student Francisco Fontenele Araujo will have their conclusions published on 2 February 2007 in Physical Review letters.
A (hot air) balloon rises through heating. Also water in a closed vessel rises like a bubble or plume after local heating. Cooling down at the top causes the plume to descend. Then it becomes warm again, rises and thus a so-called convection cylinder arises. This is called a turbulent Rayleigh-Bénard convection, which is an important modelsystem in hydrodynamics.
By comparing theoretical calculations and experimental measurements Fontenele Araujo and Lohse recently discovered that also the so-called Non-Oberbeck-Boussinesq effects (NOB effects) significantly influences the behaviour of the convection. This phrase indicates the way in which turbulent convection is generally analysed according to the approach of the researchers Oberbeck and Boussinesq (OB). In this approach is assumed that all characteristics of fluids in a vessel are constant, except the density, for an increasing of the temperature causes a decreasing of the density of water (over four degrees Celsius) or gas. At NOB effects the characteristics of water and gas are not constant. They will change under the influence of temperature.
A remarkable difference between fluid and gas
Measuring NOB effects in turbulent Rayleigh-Bénard convection is not new, but only recently Lohse and his fellow-workers have been carrying out a quantitative comparison between OB and NOB convection for the first time. The research was restricted to NOB effects in fluids such as water and glycerol. Lohse and colleagues ascertained an increase of the mean temperature and a decrease of the thermal conduction.
The scientists found it even more exciting to investigate what takes place with gas because also the density is dependent on temperature, as a result of which a boundary layer of density arises. Besides, various characteristics, such as viscosity, expansion and diffusion are dependent on temperature as well as density.
They examined the NOB effects on turbulent Rayleigh-Bénard convection of the ethane gas under pressure. In this investigation too, they combined new theoretical calculations with experimental measurements executed in a vessel. And now the researchers discovered a decrease of the mean temperature compared to the mean top and bottom temperatures and an increase of the actual thermal conduction. Not only is this effect reverse to that of the NOB convection in fluid, but it is also larger. This means that the NOB effects have to be taken into account in the calculations at large temperature differences. Situations for this occur, for instance, in the heating mantle of the earth.
Researchers are now able to model turbulences in the heating mantle in a better way.
Rewards of brainstorming
The interaction between theory (UT) and practice (University of California, Santa Barbara, United States of America) was of importance in this research. The seed was laid during one of the workshops - also financed by the FOM-Foundation - that the Lorentz Centre in Leiden is frequently organizing for physical researchers from all over the world. They will get ample opportunity for discussion and spouting creative ideas, from which often nice research follows and which in this case has led to a publication in Physical Review Letters.
For more information, please contact Professor Detlef Lohse, Phone +31 (0)53 489 24 70
or
Drs. Francisco Fontenele Araujo, Phone: +31 (0)53 489 46 82.
The article is entitled: 'Non-Oberbeck-Boussinesq effects in gaseous Rayleigh-Bénard convection'; it will be pubished in Physical Review Letters on 2 February 2007. The authors are Guenter Ahlers, Franscisco Fontenele Araujo, Denis Funfschilling, Siegfried Grossmann and Detlef Lohse.
Films of model calculations and observations of turbulent convection are shown on the website of ex-FOM researcher Federico Toschi: http://www.iac.rm.cnr.it/~toschi/
and on: http://www.ipgp.jussieu.fr/~labrosse/movies.html
(particularly the first film)