Browsing by Author "Funfschilling, Denis"

We report on the presence of the boundary zonal flow in rotating Rayleigh–Bénard convection evidenced by two-dimensional particle image velocimetry . Experiments were conducted in a cylindrical cell of aspect ratio $\varGamma =D/H=1$ between its diameter ( $ ) and height ( $ ). As the working fluid, we used various mixtures of water and glycerol, leading to Prandtl numbers in the range .6 \lesssim \textit {Pr} \lesssim 76$ . The horizontal velocity components were measured at a horizontal cross-section at half height. The Rayleigh numbers were in the range 0^8 \leq \textit {Ra} \leq 3\times 10^9$ . The effect of rotation is quantified by the Ekman number, which was in the range .5\times 10^{-5}\leq \textit {Ek} \leq 1.2\times 10^{-3}$ in our experiment. With our results we show the first direct measurements of the boundary zonal flow (BZF) that develops near the sidewall and was discovered recently in numerical simulations as well as in sparse and localized temperature measurements. We analyse the thickness $\delta _0$ of the BZF as well as its maximal velocity as a function of Pr , Ra and Ek , and compare these results with previous results from direct numerical simulations.

We report experimental results for heat-transport measurements, in the form of the Nusselt number Nu, by turbulent Rayleigh-Benard convection (RBC) in a cylindrical sample of aspect ratio Gamma equivalent to D/L = 1.00 (D = 1.12m is the diameter and L = 1.12m the height) and compare them with previously reported results for Gamma = 0.50. The measurements were made using sulfur hexafluoride at pressures up to 19 bars as the fluid. They are for the Rayleigh-number range 4 x 10(11) less than or similar to Ra less than or similar to 2 x 10(14) and for Prandtl numbers Pr between 0.79 and 0.86. For Ra < Ra-1 similar or equal to 2 x 10(13) we find Nu = N0Ra gamma eff with gamma(eff) = 0.321 +/- 0.002 and N-0 = 0.0776, consistent with classical turbulent RBC in a system with laminar boundary layers (BLs) below the top and above the bottom plate and with the prediction of Grossmann and Lohse. For Ra > Ra-1 the data rise above the classical-state power-law and show greater scatter. In analogy to similar behavior observed for Gamma = 0.50, we interpret this observation as the onset of the transition to the ultimate state. Within our resolution this onset occurs at nearly the same value of Ra-1 as it does for Gamma = 0.50. This differs from an earlier estimate by Roche et al (2010 New J. Phys. 12 085014), which yielded a transition at Ra-U similar or equal to 1.3 x 10(11) Gamma(-2.5 +/- 0.5). A Gamma-independent Ra-1 would suggest that the BL shear transition is induced by fluctuations on a scale less than the sample dimensions rather than by a global Gamma-dependent flow mode. Within the resolution of the measurements the heat transport above Ra-1 is equal for the two Gamma values, suggesting a universal aspect of the ultimate-state transition and properties. The enhanced scatter of Nu in the transition region, which exceeds the experimental resolution, indicates an intrinsic irreproducibility of the state of the system. Several previous measurements for Gamma = 1.00 are re-examined and compared with the present results. None of them identified the ultimate-state transition.

We report on the experimental results for heat-transport measurements, in the form of the Nusselt number Nu, by turbulent Rayleigh-Benard convection (RBC) in a cylindrical sample of aspect ratio Gamma equivalent to D/L = 0.50 (D = 1.12m is the diameter and L = 2.24m the height). The measurements were made using sulfur hexafluoride at pressures up to 19 bar as the fluid. They are for the Rayleigh-number range 3 x 10(12) less than or similar to Ra less than or similar to 10(15) and for Prandtl numbers Pr between 0.79 and 0.86. For Ra < Ra-1 similar or equal to 1.4 x 10(13) we find Nu = N-0 Ra-gamma eff with gamma(eff) = 0.312 +/- 0.002, which is consistent with classical turbulent RBC in a system with laminar boundary layers below the top and above the bottom plate. For Ra-1 < Ra < Ra-2 (with Ra-2 similar or equal to 5 x 10(14)) gamma(eff) gradually increases up to 0.37 +/- 0.01. We argue that above Ra-2 the system is in the ultimate state of convection where the boundary layers, both thermal and kinetic, are also turbulent. Several previous measurements for Gamma = 0.50 are re-examined and compared with our results. Some of them show a transition to a state with gamma(eff) in the range from 0.37 to 0.40, albeit at values of Ra in the range from 9 x 10(10) to 7 x 10(11) which is much lower than the present Ra-1 or Ra-2 . The nature of the transition found by them is relatively sharp and does not reveal the wide transition range observed in this work. In addition to the results for the genuine Rayleigh-Benard system, we present measurements for a sample which was not completely sealed; the small openings permitted external currents, imposed by density differences and gravity, to pass through the sample. That system should no longer be regarded as genuine RBC because the externally imposed currents modified the heat transport in a major way. It showed a sudden decrease of gamma(eff) from 0.308 for Ra < Ra-t similar or equal to 4 x 10(13) to 0.25 for larger Ra. A number of possible experimental effects are examined in a sequence of appendices; none of these effects is found to have a significant influence on the measurements.

We report experimental results for the heat transport, as expressed by the Nusselt number Nu, by turbulent Rayleigh-Bénard convection in a cylindrical sample of aspect ratio Γ = D/L = 0.50 (D = 1.12 m is the diameter and L = 2.24 m the height). The measurements are for the Rayleigh-number range 1012 ≲ Ra ≲ 1015 and for a Prandtl number Pr ≃ 0.86. At these large Ra the results were exceptionally sensitive to details of the experiment. Near Ra = 1015 the Nusselt number could be caused to vary over the range 3500 ≲ Nu ≲ 5800 by minor changes in the apparatus or operating procedure.

We report results for the temperature profiles of turbulent Rayleigh-Benard convection (RBC) in the interior of a cylindrical sample of aspect ratio Gamma equivalent to D/L = 0.50 (D and L are the diameter and height, respectively). Both in the classical and in the ultimate state of RBC we find that the temperature varies as A X ln(z/L) + B, where z is the distance from the bottom or top plate. In the classical state, the coefficient A decreases in the radial direction as the distance from the side wall increases. For the ultimate state, the radial dependence of A has not yet been determined. These findings are based on experimental measurements over the Rayleigh-number range 4 X 10(12) less than or similar to Ra less than or similar to 10(15) for a Prandtl number Pr similar or equal to 0.8 and on direct numerical simulation at Ra = 2 X 10(12), 2 X 10(11), and 2 X 10(10), all for Pr = 0.7.

Measurements of the Nusselt number Nu and of a Reynolds number Re(eff) for Rayleigh-Benard convection (RBC) over the Rayleigh-number range 10(12) less than or similar to & Ra less than or similar to 10(15) and for Prandtl numbers Pr near 0.8 are presented. The aspect ratio Gamma equivalent to D/L of a cylindrical sample was 0.50. For Ra less than or similar to 10(13) the data yielded Nu alpha Ra(gamma eff) with gamma(eff) similar or equal to 0.31 and Re(eff) alpha Ra(zeta eff) with zeta(eff) similar or equal to 0.43, consistent with classical turbulent RBC. After a transition region for 10(13) less than or similar to Ra less than or similar to 5x 10(14), where multistability occurred, we found gamma(eff) similar or equal to 0: 38 and zeta(eff) = zeta similar or equal to 0.50, in agreement with the results of Grossmann and Lohse for the large-Ra asymptotic state with turbulent boundary layers which was first predicted by Kraichnan.

We describe a pressure vessel for conducting experiments in helium (He), air, nitrogen (N-2) or sulfur hexafluoride (SF6) under pressures of up to 19 bars, and facilities for the study of Rayleigh-Benard convection inside this pressure vessel. The convection cells, known as the high pressure convection facilities (HPCFs), can have interior heights up to L = 2.3 m and diameters up to D = 1.2 m. Measurements of the Nusselt number Nu for Rayleigh numbers Ra up to Ra = 4 x 10(13) and a Prandtl number Pr similar or equal to 0.8 gave Nu proportional to Ra-gamma eff with gamma(eff) similar or equal to 0.308. At Ra there was a sharp transition to a new regime. The Nusselt number was continuous at Ra , but the exponent characterizing its dependence on Ra changed suddenly to gamma(eff) = 0.25. Near Ra = Ra similar or equal to 3 x 10(14), there was a further change in the Ra-dependence of Nu. A new state with gamma(eff) similar or equal to 0.17 evolved and there was bistability of the gamma(eff) = 0.25 and the gamma(eff) = 0.17 branches.