Philippe-E. Roche, Frédéric Gauthier, Robert Kaiser, Julien Salort
Rayleigh-B\'enard cells are one of the simplest systems to explore the laws
of natural convection in the highly turbulent limit. However, at very high
Rayleigh numbers (Ra > 1E12) and for Prandtl numbers of order one, experiments
fall into two categories: some evidence a steep enhancement of the heat
transfer while others do not. The origin of this apparent disagreement is
presently unexplained. This puzzling situation motivated a systematic study of
the triggering of the regime with an enhanced heat transfer, originally named
the "Ultimate Regime" of convection. High accuracy heat transfer measurements
have been conducted in convection cells with various aspect ratios and
different specificities, such as altered boundary conditions or obstacles
inserted in the flow. The two control parameters, the Rayleigh and Prandtl
numbers have been varied independently to disentangle their relative influence.
Among other results, it is found that i) most experiments reaching very high
$Ra$ are not in disagreement if small differences in Prandtl numbers are taken
into account, ii) the transition is not directly triggered by the large scale
circulation present in the cell, iii) the sidewall of the cell have a
significant influence on the transition. The characteristics of this Ultimate
regime are summarized and compared with R. Kraichnan prediction for the
asymptotic regime of convection.
View original:
http://arxiv.org/abs/1202.0661
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