Thermal convection experiments

Thermal convection  at the Fête de la Science, Orsay, October 14th 2016

As every year, the last October many laboratories of the campus of the University of Paris-Sud (Orsay) participated with different activities at the 25th edition of the “Fête de la science”. This event, organized by the French Ministry of National Education, Higher Education and Research, is a big science festival which involves all the scientific areas from anthropology to nanotechnology, political sciences to astrophysics. With thousands of exhibitions, debates and scientific initiatives,  the “Fête de la science” represents a great opportunity for people of all ages and all backgrounds to get closer to the scientific world and all the questions and answers which animate it.

Concerning the domain of Earth Sciences, the GEOPS (Geosciences Paris Sud)  lab of the campus of Orsay opened his doors at the public on the 14th and 16th October. On these days different experiments have been arranged in order to present the various topics of research in which the lab is interested as, for example, the study of the water cycle for the preservation of water resources or the reconstruction of planets evolution. In this frame, Gianluca Gerardi (PhD student of the FAST laboratory within the CREEP network) and Arnaud Salvador (PhD student of both the FAST and GEOPS labs) took part to the manifestation on Friday 14th, showing an experiment on thermal convection to children from primary and secondary school.

The experiment set up consisted in (Fig.1):

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Fig.1: Set up of the experiment. Before starting it, Gianluca and Arnaud explained the experience to the children.

  • 3 different tanks filled with water (central) , pure glucose syrup (right) or a mixture between the two fluids (left), at which a thermo liquid crystal (TLC) slurry has been added at a later time;
  • 3 different laser placed on the top of each tank which allowed for the 2D plane sheet (vertical) illumination of the TLC slurries;
  • Local heat source at the bottom of the tanks represented by a Peltier element covered by 4mm-thick, 25 mm-diameter Cu disk.

 The goal of the experience was to let the schoolchildren understand the concept of viscosity of a fluid and the role it plays in the development and evolution of the Rayleigh-Bénard (R-B) convection.

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Fig. 2 (a): Running the experiment: in the first stage, only the central tank, the one filled with water, shows hot fluid rising up. We injected a drop of orange dye to better visualize the formation and evolution of the turbulent plume of water.

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Fig. 2 (b) Running the experiment: in the second stage, beside the turbulent plume rising to the top of the central tank, a laminar plume, with the classical mushroom structure, can be seen in the right tank filled with the glucose syrup.

Once the heater is turned on, in fact, a thermal boundary layer (TBL) grows over it by heat conduction and it then evolves depending on the fluid properties. In case of water (i.e. low viscosity), the viscous dissipation which opposes the buoyancy force induced by heating is low and a plume quickly forms (Fig.2 a), then rising to the top of the tank in a turbulent fashion (Fig.2 b). By contrast, in case of glucose syrup (i.e. high viscosity), the viscous dissipation is strong and it takes several minutes before a laminar plume develops and rises to the top of the tank with the classic mushroom shape (Fig.2 b). This last phenomenon, in particular, well resembles the dynamics of a mantle plume rising from the Earth’s core-mantle boundary to the lithosphere. Therefore, as final part of the experience, we made a parallel between the experiment and what is actually observed in Earth’s mantle, underlying how this type of experiments are useful for a better understanding of  the Earth’s dynamics.

The experience has been repeated for different classes composed of schoolchildren of 8-9 or 13-14 years old. We were impressed by the curiosity that the children showed us! They took actively part to the experience, first trying to guess what was expected from it and then asking lots of questions on the different aspects of the experiment that we highlighted. Nevertheless, we were enthusiastic for the teaching experience we went through, since we had to test new ways of explaining what we usually expose to a public mostly composed of experts of the sector. In this light, we realized how events as the “Fête de la science” are a unique occasion, for both researchers and citizens of all backgrounds, to meet and improve their communication.

Written by Gianluca Gerardi

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