Dynamics of the large-scale circulation in high-Prandtl-number turbulent thermal convection

We report experimental investigations of the dynamics of the large-scale circulation (LSC) in turbulent Rayleigh–Bénard convection at high Prandtl number Pr= 19. 4 (and also Pr= 7. 8 ) and Rayleigh number Ra varying from 8. 3× 10⁸ to 2. 9× 10¹¹ 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- Pr regime remains a single-roll structure. The azimuthal motion of the LSC is a diffusive process, which is the same as those for Pr around 1. However, the azimuthal diffusion of the LSC, characterized by the angular speed Ω is almost two orders of magnitude smaller when compared with that in water. The non-dimensional time-averaged amplitude of the angular speed Ω Td ( Td = L² / κ is the thermal diffusion time) of the LSC at the mid-height of the convection cell increases with Ra as a power law, which is Ω Td ∝ Ra0. 36± 0. 01 . The Re number based on the oscillation frequency of the LSC is found to scale with Ra as Re= 0. 13Ra0. 43± 0. 01 . It is also found that the normalized flow strength δ / Δ T× Ra/ Pr∝ Re1. 5± 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- Pr regime as well.