Aspect-ratio dependence of the transition to the ultimate state of turbulent Rayleigh-B\'enard convection

etc15 Tracking Number 210

We report on measurements of the ultimate-state transition in turbulent Rayleigh-B\'enard convection obtained in a large facility known as the ``Uboot of G\"ottingen" and using pressurized sulfur hexafluoride as the convecting fluid. We found that the transition occurs over a range of $Ra$ which becomes more narrow as $\Gamma$ increases, ranging from $Ra_1^*$ which is at most weakly dependent on $\Gamma$ and close to $10^{13}$ to $Ra_2^*$ which varies from about $2\times 10^{15}$ for $\Gamma = 0.33$ to about $7\times 10^{13}$ for $\Gamma = 1.00$.