Effect of boundary conditions in turbulent thermal convection

TL;DR

This experimental study investigates how different boundary conditions, specifically constant heat flux and constant temperature, affect turbulent thermal convection in cryogenic helium gas at high Rayleigh numbers, revealing significant impacts on temperature fluctuations but minimal effects on overall flow scaling.

Contribution

The paper provides the first detailed experimental comparison of boundary condition effects on turbulent convection at very high Rayleigh numbers, showing their influence on temperature statistics without altering flow scaling.

Findings

Boundary conditions significantly change temperature fluctuation characteristics.

Flow scaling laws such as Re(Ra) and Nu(Ra) remain unaffected.

Large-scale circulation reversal rates are unchanged by boundary condition variations.

Abstract

We report an experimental study aiming to clarify the role of boundary conditions (BC) in high Rayleigh number $10^8 < {\rm{Ra}} < 3 \times 10^{12}$ 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.