Effect of boundary conditions in turbulent thermal convection ^(a)

Abstract

We report an experimental study aiming to clarify the role of boundary conditions (BC) in high Rayleigh number <?CDATA $10^8 < {\textrm{Ra}} turbulent thermal convection of cryogenic helium gas. We switch between BC closer to constant heat flux (CF) and constant temperature (CT) applied to the highly conducting bottom plate of the aspect-ratio-one cylindrical cell 30 cm in size, leading to dramatic changes in the temperature probability density function and in power spectral density of the temperature fluctuations measured at the bottom plate, while the dynamic thermal behaviour of the top plate and bulk convective flow remain unaffected. Within our experimental accuracy, we find no appreciable changes in Reynolds number Re(Ra) scaling, in the dimensionless heat transfer efficiency expressed via Nusselt number Nu(Ra) scaling, nor in the rate of direction reversals of large scale circulation.

Modulated turbulent convection: a benchmark model for large scale natural flows driven by diurnal heating

“…Under cryogenic conditions, the cell is “thermally fast” (see the detailed discussion in 25 ): it faithfully and uniformly follows temperature modulations imposed to its plates. Contrary to the requirement of high heat capacity of the plates for studies of conventional steady-state RBC flow, for studies of harmonically modulated convective flows their heat capacity must be low, as it limits the modulation frequency due to the finite cooling rate on the negative slope of temperature modulation.…”

“…1 b, using a unique working fluid: cryogenic He gas with favorable, well-known 24 and in situ tunable properties. Cryogenic conditions ( K) are essential, as they provide highly conductive but low thermal capacity plates assuring uniform boundary conditions and little thermal load on the heat flow under study 25 ; see " Methods " for details.…”