Published online by Cambridge University Press: 22 November 2021
We report measurements of the temperature frequency spectra $P(\,f, z, r)$, the variance
$\sigma ^2(z,r)$ and the Nusselt number
$Nu$ in turbulent Rayleigh–Bénard convection (RBC) over the Rayleigh number range
$4\times 10^{11} \underset{\smash{\scriptscriptstyle\thicksim}} { < } Ra \underset{\smash{\scriptscriptstyle\thicksim}} { < } 5\times 10^{15}$ and for a Prandtl number
$Pr \simeq ~0.8$ (
$z$ is the vertical distance from the bottom plate and
$r$ is the radial position). Three RBC samples with diameter
$D = 1.12$ m yet different aspect ratios
$\varGamma \equiv D/L = 1.00$,
$0.50$ and
$0.33$ (
$L$ is the sample height) were used. In each sample, the results for
$\sigma ^2/\varDelta ^2$ (
$\varDelta$ is the applied temperature difference) in the classical state over the range
$0.018 \underset{\smash{\scriptscriptstyle\thicksim}} { < } z/L \underset{\smash{\scriptscriptstyle\thicksim}} { < } 0.5$ can be collapsed onto a single curve, independent of
$Ra$, by normalizing the distance
$z$ by the thermal boundary layer thickness
$\lambda = L/(2 Nu)$. One can derive the equation
$\sigma ^2/\varDelta ^2 = c_1\times \ln (z/\lambda )+c_2+c_3(z/\lambda )^{-0.5}$ from the observed
$f^{-1}$ scaling of the temperature frequency spectrum. It fits the collapsed
$\sigma ^2(z/\lambda )$ data in the classical state over the large range
$20 \underset{\smash{\scriptscriptstyle\thicksim}} { < } z/\lambda \underset{\smash{\scriptscriptstyle\thicksim}} { < } 10^4$. In the ultimate state (
$Ra \underset{\smash{\scriptscriptstyle\thicksim}} { > } Ra^*_2$) the data can be collapsed only when an adjustable parameter
$\tilde \lambda = L/(2 \widetilde {Nu})$ is used to replace
$\lambda$. The values of
$\widetilde {Nu}$ are larger by about 10 % than the experimentally measured
$Nu$ but follow the predicted
$Ra$ dependence of
$Nu$ for the ultimate RBC regime. The data for both the global heat transport and the local temperature fluctuations reveal the ultimate-state transitions at
$Ra^*_2(\varGamma )$. They yield
$Ra^*_2 \propto \varGamma ^{-3.0}$ in the studied
$\varGamma$ range.