Published online by Cambridge University Press: 17 February 2016
We present measurements of the orientation   ${\it\theta}_{0}$  and temperature amplitude
 ${\it\theta}_{0}$  and temperature amplitude   ${\it\delta}$  of the large-scale circulation in a cylindrical sample of turbulent Rayleigh–Bénard convection (RBC) with aspect ratio
 ${\it\delta}$  of the large-scale circulation in a cylindrical sample of turbulent Rayleigh–Bénard convection (RBC) with aspect ratio   ${\it\Gamma}\equiv D/L=1.00$  (
 ${\it\Gamma}\equiv D/L=1.00$  (  $D$  and
 $D$  and   $L$  are the diameter and height respectively) and for the Prandtl number
 $L$  are the diameter and height respectively) and for the Prandtl number   $Pr\simeq 0.8$ . The results for
 $Pr\simeq 0.8$ . The results for   ${\it\theta}_{0}$  revealed a preferred orientation with up-flow in the west, consistent with a broken azimuthal invariance due to the Earth’s Coriolis force (see Brown & Ahlers (Phys. Fluids, vol. 18, 2006, 125108)). They yielded the azimuthal diffusivity
 ${\it\theta}_{0}$  revealed a preferred orientation with up-flow in the west, consistent with a broken azimuthal invariance due to the Earth’s Coriolis force (see Brown & Ahlers (Phys. Fluids, vol. 18, 2006, 125108)). They yielded the azimuthal diffusivity   $D_{{\it\theta}}$  and a corresponding Reynolds number
 $D_{{\it\theta}}$  and a corresponding Reynolds number   $Re_{{\it\theta}}$  for Rayleigh numbers over the range
 $Re_{{\it\theta}}$  for Rayleigh numbers over the range   $2\times 10^{12}\lesssim Ra\lesssim 1.5\times 10^{14}$ . In the classical state (
 $2\times 10^{12}\lesssim Ra\lesssim 1.5\times 10^{14}$ . In the classical state (  $Ra\lesssim 2\times 10^{13}$ ) the results were consistent with the measurements by Brown & Ahlers (J. Fluid Mech., vol. 568, 2006, pp. 351–386) for
 $Ra\lesssim 2\times 10^{13}$ ) the results were consistent with the measurements by Brown & Ahlers (J. Fluid Mech., vol. 568, 2006, pp. 351–386) for   $Ra\lesssim 10^{11}$  and
 $Ra\lesssim 10^{11}$  and   $Pr=4.38$ , which gave
 $Pr=4.38$ , which gave   $Re_{{\it\theta}}\propto Ra^{0.28}$ , and with the Prandtl-number dependence
 $Re_{{\it\theta}}\propto Ra^{0.28}$ , and with the Prandtl-number dependence   $Re_{{\it\theta}}\propto Pr^{-1.2}$  as found previously also for the velocity-fluctuation Reynolds number
 $Re_{{\it\theta}}\propto Pr^{-1.2}$  as found previously also for the velocity-fluctuation Reynolds number   $Re_{V}$  (He et al., New J. Phys., vol. 17, 2015, 063028). At larger
 $Re_{V}$  (He et al., New J. Phys., vol. 17, 2015, 063028). At larger   $Ra$  the data for
 $Ra$  the data for   $Re_{{\it\theta}}(Ra)$  revealed a transition to a new state, known as the ‘ultimate’ state, which was first seen in the Nusselt number
 $Re_{{\it\theta}}(Ra)$  revealed a transition to a new state, known as the ‘ultimate’ state, which was first seen in the Nusselt number   $Nu(Ra)$  and in
 $Nu(Ra)$  and in   $Re_{V}(Ra)$  at
 $Re_{V}(Ra)$  at   $Ra_{1}^{\ast }\simeq 2\times 10^{13}$  and
 $Ra_{1}^{\ast }\simeq 2\times 10^{13}$  and   $Ra_{2}^{\ast }\simeq 8\times 10^{13}$ . In the ultimate state we found
 $Ra_{2}^{\ast }\simeq 8\times 10^{13}$ . In the ultimate state we found   $Re_{{\it\theta}}\propto Ra^{0.40\pm 0.03}$ . Recently, Skrbek & Urban (J. Fluid Mech., vol. 785, 2015, pp. 270–282) claimed that non-Oberbeck–Boussinesq effects on the Nusselt and Reynolds numbers of turbulent RBC may have been interpreted erroneously as a transition to a new state. We demonstrate that their reasoning is incorrect and that the transition observed in the Göttingen experiments and discussed in the present paper is indeed to a new state of RBC referred to as ‘ultimate’.
 $Re_{{\it\theta}}\propto Ra^{0.40\pm 0.03}$ . Recently, Skrbek & Urban (J. Fluid Mech., vol. 785, 2015, pp. 270–282) claimed that non-Oberbeck–Boussinesq effects on the Nusselt and Reynolds numbers of turbulent RBC may have been interpreted erroneously as a transition to a new state. We demonstrate that their reasoning is incorrect and that the transition observed in the Göttingen experiments and discussed in the present paper is indeed to a new state of RBC referred to as ‘ultimate’.
 $Pr=0.8$
                     
                   and
                        $Pr=0.8$
                     
                   and 
                     
                         $10^{11}\leqslant Ra\leqslant 10^{15}$
                     
                  
               . New J. Phys. 
               11, 123001.Google Scholar
                        $10^{11}\leqslant Ra\leqslant 10^{15}$
                     
                  
               . New J. Phys. 
               11, 123001.Google Scholar $Pr\simeq 0.8$
                     
                   and
                        $Pr\simeq 0.8$
                     
                   and 
                     
                         $3\times 10^{12}\lesssim Ra\lesssim 10^{15}$
                     
                  : aspect ratio
                        $3\times 10^{12}\lesssim Ra\lesssim 10^{15}$
                     
                  : aspect ratio 
                     
                         ${\it\gamma}=0.50$
                     
                  
               . New J. Phys. 
               14, 103012.Google Scholar
                        ${\it\gamma}=0.50$
                     
                  
               . New J. Phys. 
               14, 103012.Google Scholar $Pr\simeq 0.8$
                     
                   and
                        $Pr\simeq 0.8$
                     
                   and 
                     
                         $4\times 10^{11}\lesssim Ra\lesssim 2\times 10^{14}$
                     
                  : ultimate-state transition for aspect ratio
                        $4\times 10^{11}\lesssim Ra\lesssim 2\times 10^{14}$
                     
                  : ultimate-state transition for aspect ratio 
                     
                         ${\it\gamma}=1.00$
                     
                  
               . New J. Phys. 
               14, 063030.Google Scholar
                        ${\it\gamma}=1.00$
                     
                  
               . New J. Phys. 
               14, 063030.Google Scholar $f^{-1}$
                     
                   power spectra of temperature fluctuations in turbulent Rayleigh–Bénard convection. Phys. Rev. Lett. 
               112, 174501.Google Scholar
                        $f^{-1}$
                     
                   power spectra of temperature fluctuations in turbulent Rayleigh–Bénard convection. Phys. Rev. Lett. 
               112, 174501.Google Scholar