Published online by Cambridge University Press: 01 February 2013
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
 $\mathit{Pr}= 19. 4$  (and also   $\mathit{Pr}= 7. 8$ ) and Rayleigh number
 $\mathit{Pr}= 7. 8$ ) and Rayleigh number   $\mathit{Ra}$  varying from
 $\mathit{Ra}$  varying from   $8. 3\times 1{0}^{8} $  to
 $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-
 $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
 $\mathit{Pr}$  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
 $\mathit{Pr}$  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
 $\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} $  (
 $\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
 ${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
 $\mathit{Ra}$  as a power law, which is   $\langle \vert \Omega \vert \rangle {T}_{d} \propto {\mathit{Ra}}^{0. 36\pm 0. 01} $ . The
 $\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
 $\mathit{Re}$  number based on the oscillation frequency of the LSC is found to scale with   $\mathit{Ra}$  as
 $\mathit{Ra}$  as   $\mathit{Re}= 0. 13{\mathit{Ra}}^{0. 43\pm 0. 01} $ . It is also found that the normalized flow strength
 $\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-
 $\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.
 $\mathit{Pr}$  regime as well.
 $\Gamma = 0. 50$
                     
                   and Prandtl number 
               . J. Fluid Mech.
               676, 5–40.Google Scholar
                        $\Gamma = 0. 50$
                     
                   and Prandtl number 
               . J. Fluid Mech.
               676, 5–40.Google Scholar