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Dynamics of the large-scale circulation in high-Prandtl-number turbulent thermal convection
Published online by Cambridge University Press: 01 February 2013
Abstract
We report experimental investigations of the dynamics of the large-scale circulation (LSC) in turbulent Rayleigh–Bénard convection at high Prandtl number $\mathit{Pr}= 19. 4$ (and also
$\mathit{Pr}= 7. 8$) and Rayleigh number
$\mathit{Ra}$ varying from
$8. 3\times 1{0}^{8} $ to
$2. 9\times 1{0}^{11} $ in a cylindrical convection cell with aspect ratio unity. The dynamics of the LSC is measured using the multithermal probe technique. Both the sinusoidal-fitting (SF) and the temperature-extrema-extraction (TEE) methods are used to analyse the properties of the LSC. It is found that the LSC in high-
$\mathit{Pr}$ regime remains a single-roll structure. The azimuthal motion of the LSC is a diffusive process, which is the same as those for
$\mathit{Pr}$ around 1. However, the azimuthal diffusion of the LSC, characterized by the angular speed
$\Omega $ is almost two orders of magnitude smaller when compared with that in water. The non-dimensional time-averaged amplitude of the angular speed
$\langle \vert \Omega \vert \rangle {T}_{d} $ (
${T}_{d} = {L}^{2} / \kappa $ is the thermal diffusion time) of the LSC at the mid-height of the convection cell increases with
$\mathit{Ra}$ as a power law, which is
$\langle \vert \Omega \vert \rangle {T}_{d} \propto {\mathit{Ra}}^{0. 36\pm 0. 01} $. The
$\mathit{Re}$ number based on the oscillation frequency of the LSC is found to scale with
$\mathit{Ra}$ as
$\mathit{Re}= 0. 13{\mathit{Ra}}^{0. 43\pm 0. 01} $. It is also found that the normalized flow strength
$\langle \delta \rangle / \mrm{\Delta} T\times \mathit{Ra}/ \mathit{Pr}\propto {\mathit{Re}}^{1. 5\pm 0. 1} $, with the exponent in good agreement with that predicted by Brown & Ahlers (Phys. Fluids, vol. 20, 2008, p. 075101). A wealth of dynamical features of the LSC, such as the cessations, flow reversals, flow mode transitions, torsional and sloshing oscillations are observed in the high-
$\mathit{Pr}$ regime as well.
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