Computer Science
Scientific paper
Jan 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002iaf..confe.538a&link_type=abstract
IAF abstracts, 34th COSPAR Scientific Assembly, The Second World Space Congress, held 10-19 October, 2002 in Houston, TX, USA.,
Computer Science
Scientific paper
temperature oscillations at the Rayleigh-Benard convection threshold in supercritical He 3 contained in a shallow cavity heated by a constant heat flux at the bottom wall, the top wall being maintained at its initial temperature. These oscillations have also been reproduced by numerical simulations by several independent groups in Japan (A. Onuki) and in France by our group making use of different numerical methods. We bring here an interpretation of the origin of these temperature oscillations. In fact Meyer and co-workers covered the (height of the cavity- Temperature gradient) Rayleigh-Benard stability diagram with regions where the temperature response of the bottom of the cavity exhibits similar time evolution. Whereas in ideal gases the temperature increase at the bottom wall of the cavity is monotonous after the threshold, it exhibits quite different behaviors in very compressible fluids. In particular it can oscillate or show a simple overshoot before reaching its steady state value. We demonstrate with the aid of numerical simulations that the piston effect, discovered thanks to microgravity experiments, is at the origin of these oscillations. To be short, the mechanism is the following. One should remember that the bulk temperature in a very expandable fluid is the result of a competition between heat transferred by convection and that transferred by the piston effect both being adiabatic and proportional to the heat flux at the boundary. The later which is quite convection independent in the bulk, is the result of the combined and opposite effects of the constant heating piston effect at the bottom wall which heats up the bulk fluid homogeneously and the cold piston effect generated at the top thermostated boundary. When convection starts at the threshold, the thermal plume rises up in the cell and brings hot fluid to the cold upper wall. At this time the thermal boundary layer is squeezed and the amplitude of the cooling piston effect expansion waves it generates is increased and provokes a general fast cooling of the bulk. But as the bulk temperature decreases it provokes a drop in the cooling piston effect. At that time the heating piston effect becomes again dominant and the bulk temperature increases again to generate the observed damped oscillations.
Amiroudine Sakir
Zappoli Bernard
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