Comment on "Turbulent heat transport near critical points: Non-Boussinesq effects" (cond-mat/0601398)

In a recent preprint (cond-mat/0601398), D. Funfschilling and G. Ahlers describe a new effect, that they interpret as non-Boussinesq, in a convection cell working with ethane, near its critical point. They argue that such an effect could have spoiled the Chavanne {\it et al.} (Phys. Rev. Lett. {\bf 79} 3648, 1997) results, and not the Niemela {\it et al.} (Nature, {\bf 404}, 837, 2000) ones, which would explain the differences between these two experiments. We show that:-i)Restricting the Chavanne's data to situations as far from the critical point than the Niemela's one, the same discrepancy remains.-ii)The helium data of Chavanne show no indication of the effect observed by D. Funfschilling and G. Ahlers.Comment: comment on cond-mat/060139

Prandtl and Rayleigh numbers dependences in Rayleigh Bénard convection

International audienceUsing low-temperature gaseous helium close to the critical point, we investigate the Prandtl-number dependence of the effective heat conductivity (Nusselt number) for a 1/2 aspect ratio Rayleigh-Bénard cell. Very weak dependence is observed in the range 0.7 < Pr < 21; 2 × 10^8 < Ra < 2 × 10^10: the absolute value of the average logarithmic slope δ = (∂ln Nu/∂ln Pr)Ra is smaller than 0.03. A bimodality of Nu, with 7% difference between the two sets of data, is observed, which could explain some discrepancies between precise previous experiments in this range

Turbulent velocity spectra in superfluid flows

International audienceWe present velocity spectra measured in three cryogenic liquid 4He steady flows: grid and wake flows in a pressurized wind tunnel capable of achieving mean velocities up to 5 m/s at temperatures above and below the superfluid transition, down to 1.7 K, and a "chunk" turbulence flow at 1.55 K, capable of sustaining mean superfluid velocities up to 1.3 m/s. Depending on the flows, the stagnation pressure probes used for anemometry are resolving from one to two decades of the inertial regime of the turbulent cascade. We do not find any evidence that the second order statistics of turbulence below the superfluid transition differ from the ones of classical turbulence, above the transition

TSF experiment for comparison of high Reynolds number turbulence in both HeI and HeII: First results

International audienceTSF experiment for comparison of high Reynolds number turbulence in both HeI and HeII: First result

Observation of the 1/2 power law in Rayleigh-Bénard convection

International audienceThe 1/2 power law is reported in a {R}ayleigh-{B}énard experiment: Nu~?Ra/sup 1/2/, where Ra and Nu are the {R}ayleigh and {N}usselt numbers. This observation is coherent with the predictions of the ultimate convection regime, characterized by fully turbulent heat transfers. Ordered rough boundaries are used to cancel the correction due to the thickness variation of the viscous sublayer, and the observation of the asymptotic regime is therefore possible. This result supports the interpretation of a laminar-turbulent boundary-layer transition to account for the observation of Chavanne et al. of a new regime [X. Chavanne et al., Phys. Rev. Lett. 79, 3648 (1997)]

From Hard Turbulence to the Ultimate Regime in a Rough Rayleigh-Bénard cell

International audienceFrom Hard Turbulence to the Ultimate Regime in a Rough Rayleigh-Bénard cel

Ultimate regime of convection: search for a hidden triggering parameter

International audienceIn 1962, R. Kraichnan predicted a transition to a new regime of convection for high Rayleigh number, the so called ultimate regime of convection. This regime is characterized by the most efficient thermal transfer of all those predicted in convection. Recently, Rayleigh-Bénard experiments in cryogenics conditions report apparently contradictory results on the existence of a transition compatible with this regime for Rayleigh numbers above 10^12. Here, we report the first steps of a systematic investigation on the conditions in which the transition can be obtained

Prandtl and wall effects in hard turbulence convection

International audienceUsing low-temperature gaseous helium close to the critical point, we investigate the {P}randtl-number dependence of the effective heat conductivity ({N}usselt number) for a 1/2 aspect ratio {R}ayleigh-{B}énard cell. Very weak dependence is observed in the range 0.7 < Pr < 21; 2 * 10/sup 8/ < Ra < 2 * 10/sup 10/: the absolute value of the average logarithmic slope delta = ( delta ln Nu/ delta ln Pr)/sub Ra/ is smaller than 0.03. A bimodality of Nu, with 7{\%} difference between the two sets of data, is observed, which could explain some discrepancies between precise previous experiments in this range

Cryogenic turbulence experiments

International audienceCryogenic turbulence experiments are presente