Convection at very high Rayleigh number: signature of transition from a micro-thermometer inside the flow

Julien Salort (1) , Frédéric Gauthier (1) , Benoît Chabaud (1) , Olivier Bourgeois (2) , Jean-Luc Garden (3) , Ronald Du Puits (4) , Andre Thess (4) , Philippe-Emmanuel Roche (1)

1 NEEL - HELFA - Hélium : du fondamental aux applications
2 NEEL - TPS - Thermodynamique et biophysique des petits systèmes
3 NEEL - Institut Néel
4 TU - Technische Universität Ilmenau

In 1991 and 2001, two groups reported a qualitative change in the second order statistics of local temperature fluctuations in Rayleigh-Bénard cells for Rayleigh numbers in the range Ra ~ 10^{12} - 10^{13}. In the second group, this change was accompanied by an abrupt increase of the heat transfer, and these concomitant observations were interpreted as two signatures of the transition to Kraichnan's ''Ultimate Regime'' of convection. In the first group, a second change on temperature statistics was reported at lower Ra but no heat transfer enhancement was found. The 1991 observations have been questioned putting forward a finite size cut-off of the probe . If this finite size argument was valid, it should also hold for the 2001 study, as the probes had the same size (200 microns) and the cell heights used in 1991 and 2001 were respectively 40 and 20 cm. In this case, the only signature of the transition to the Ultimate Regime in the 2001 study would be the increase in heat transfer. To test the finite size argument, we designed a third experiment in a 40 cm-high cell using a specially developed probe 10 times smaller than the ones used previously. A micro-machining process was developed to produce NbN micro-thermometers with temperature-sensitive spot of typical size 20 microns x 1 microns deposited on a poorly conducting glass fiber of diameter 17 microns (or less) and few mm in length. Following the 2001 study, the temperature fluctuations were monitored using the exponent of the increments of the second order structure function. This quantity, which is formally equivalent to the traditional spectral density, provides in practice a more accurate observable thanks to a better immunity to noise. This new experiment shows a heat transfer transition between Ra=10^{12} and 10^{13} and, concomitantly, a qualitative change in the statistics of temperature fluctuations. The nature of this change is very similar to the one reported in 2001 with larger probes. Both present observations are consistent with the 2001 observation.

Domaines: Autre [cond-mat.other]

Julien Salort, Frédéric Gauthier, Benoît Chabaud, Olivier Bourgeois, Jean-Luc Garden, et al.. Convection at very high Rayleigh number: signature of transition from a micro-thermometer inside the flow. 12th EUROMECH European Turbulence Conference, Sep 2009, Marburg, Germany. pp.159-162, ⟨10.1007/978-3-642-03085-7_40⟩. ⟨hal-00430188⟩