We present an experimental study of turbulent thermal convection with smooth and rough surface plates in various combinations. A total of five cells were used in the experiments. Both the global $\mathit{Nu}$ and the $\mathit{Nu}$ for each plate (or the associated boundary layer) are measured. The results reveal that the smooth plates are insensitive to the surface (rough or smooth) and boundary conditions (i.e. nominally constant temperature or constant flux) of the other plate of the same cell. The heat transport properties of the rough plates, on the other hand, depend not only on the nature of the plate at the opposite side of the cell, but also on the boundary condition of that plate. It thus appears that, at the present level of experimental resolution, the smooth plate can influence the rough plate, but cannot be influenced by either the rough or the smooth plates. It is further found that the scaling of $\mathit{Nu}$ with $\mathit{Ra}$ for all of the smooth plates is consistent with the classical $1/ 3$ exponent. But the scaling exponent for the global $\mathit{Nu}$ for the cell with both plates being smooth is definitely less than $1/ 3$ (this result itself is consistent with all previous studies at comparable parameter range). The discrepancy between the $\mathit{Nu}$ behaviour at the whole-cell and individual-plate levels is not understood and deserves further investigation.

Sharp transitions between potentially different turbulent states are unexpected because one might think that they should be washed out by the prevailing intense fluctuations and short coherence lengths and times. Contrary to this expectation, we found a sequence of such transitions in turbulent rotating Rayleigh-Bénard convection as the rotation rate was increased. This phenomenon became most prominent at very large Rayleigh numbers up to 2 × 10 12 where the fluctuations are extremely vigorous. It was found in the heat transport as well as in the temperature gradient near the sample center. We conjecture that the transitions are between different large-scale structures which involve changes of symmetry and thus can not be gradual [5,6,7].It has been argued that Kolmogorov's theory of turbulence [4] implies that turbulent flows become featureless when the Reynolds number is large enough (for a discussion of this issue see for instance [2]). Apparently in contradiction to this expectation several experiments recently showed a sharp transition between two different turbulent states [9,8,10,1,2]. However, all of these investigations were carried out on systems with geometrical constraints in all physical directions, and it is not clear whether the sharp transitions are caused by boundary conditions or whether they would survive in an unconstrained system. Indeed for one of these systems, turbulent rotating Rayleigh-Bénard convection, measurements were made as a function of the lateral extent (aspect ratio) of the system, and it was found that the observed transition moves toward zero rotation rate as the lateral system size approaches infinity [12,11]. Here we report measurements of the heat transport, expressed in terms of the Nusselt number N u, and of the temperature gradient near the center of a cylindrical sample of fluid heated from below and rotated about its vertical axis at a rate Ω. The Prandtl number was 12.3, and the aspect ratio Γ (diameter over height) was 1.00. The rotation rate is expressed in terms of the inverse Rossby number which is proportional to Ω. Results are shown in Fig. 1. They reveal three sharp continuous transitions. The first, identified as 1/Ro c , was found previously for P r ≃ 4 and is associated with the onset of the formation of Ekman vortices which enhance the heat transport by extracting fluid from thermal boundary layers near the plates. It was shown to approach zero as Γ → ∞ and thus it is not a feature of the laterally unbounded system. The second and third transitions are found at 1/Ro c,2 ≃ 0.492 and 1/Ro c,3 ≃ 1.55 for Ra = 2.07 × 10 11 . While 1/Ro c,2 seems independent of Ra, 1/Ro c,3 is smaller for smaller Ra.

We present an experimental study of turbulent Rayleigh-Bénard convection with polymer additives made in two convection cells, one with a smooth top and bottom plates and the other with a rough top and bottom plates. For the cell with smooth plates, a reduction of the measured Nusselt number (Nu) was observed. Furthermore, the amount of Nu reduction increases with increasing polymer concentration (c), reaching ~12% for c = 120 ppm and an apparent leveling off thereafter. For the cell with rough plates, however, an enhancement (~4%) of Nu was observed when the polymer concentration is greater than 120 ppm. This increase in Nu is corroborated by an increased large-scale circulation (LSC) velocity in the same cell when polymers are added. In contrast, the LSC velocity in the smooth cell is found to be essentially the same with and without polymers. It is further found that in the smooth cell the rms values of the global Nu, σ(Nu), and that of the local temperature, σ(T), both exhibit similar dependence on c as Nu itself. In contrast, σ(Nu) and σ(T) in the rough cell are found to be essentially independent of c.

We report measurements of logarithmic temperature profiles Θ(z, r) = A(r) × ln(z/L) + B(r) in the bulk of turbulent Rayleigh-Bénard convection (here Θ is a scaled and timeaveraged local temperature in the fluid, z is the vertical and r the radial position, and L is the sample height). Two samples had aspect ratios Γ ≡ D/L = 1.00 and 0.50 (where D = 190 mm is the diameter). The fluid was a fluorocarbon with a Prandtl number of Pr = 12.3. The measurements covered the Rayleigh-number range 2 × 10 10 Ra 2 × 10 11 for Γ = 1.00 and 3 × 10 11 Ra 2 × 10 12 for Γ = 0.50. In contradistinction to what had been found for Γ = 0.50 and Pr = 0.78 by Ahlers et al. (Phys. measurements revealed no Ra dependence of the amplitude A(r) of the logarithmic term. Within the experimental resolution, the amplitude was also found to be independent of Γ . It varied with r in a manner consistent with the function A(ξ ) = A 1 / 2ξ − ξ 2 , where ξ ≡ (R − r)/R with R = D/2 and A 1 0.0016. The results for A(r) are smaller than those obtained from experiments and direct numerical simulations (Ahlers et al., Phys. Rev. Lett., vol. 109, 2012, art. 114501) at similar values of Ra for Pr = 0.7 and Γ = 1 2 by a factor that depended slightly upon Ra but was close to 2.

We present new Nusselt-number (Nu) measurements for slowly rotating turbulent thermal convection in cylindrical samples with aspect ratio Γ=1.00 and provide a comprehensive correlation of all available data for that Γ. In the experiment compressed gasses (nitrogen and sulfur hexafluride) as well as the fluorocarbon C_{6}F_{14} (3M Fluorinert FC72) and isopropanol were used as the convecting fluids. The data span the Prandtl-number (Pr) range 0.74