Energy cascade and the four-fifths law in superfluid turbulence

The 4/5-law of turbulence, which characterizes the energy cascade from large to small-sized eddies at high Reynolds numbers in classical fluids, is verified experimentally in a superfluid 4He wind tunnel, operated down to 1.56 K and up to R_lambda ~ 1640. The result is corroborated by high-resolution simulations of Landau-Tisza's two-fluid model down to 1.15 K, corresponding to a residual normal fluid concentration below 3 % but with a lower Reynolds number of order R_lambda ~ 100. Although the K\'arm\'an-Howarth equation (including a viscous term) is not valid \emph{a priori} in a superfluid, it is found that it provides an empirical description of the deviation from the ideal 4/5-law at small scales and allows us to identify an effective viscosity for the superfluid, whose value matches the kinematic viscosity of the normal fluid regardless of its concentration.Comment: 6 pages, 7 figure

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

Investigation of intermittency in superfluid turbulence

International audienceThis paper reports new experimental and simulation velocity data for superfluid steady turbulence above 1 K. We present values for the scaling exponent of the absolute value of velocity-increment structure functions. In both experiments and simulations, they evidence that intermittency occurs in superfluid flows in a quite comparable way to classical turbulence. In particular, the deviation from Kolmogorov 1941 keeps the same strength as we cross the superfluid transition. To the best of our knowledge, this is the first confirmation of the superfluid 4He experimental results from Maurer et al. EPL 1998 and the first numerical evidence of intermittency in superfluid turbulence

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

Des familles au tribunal: Séparations conjugales et reproduction sociale des inégalités de sexe et de classe

International audienceInstance de production de normes par excellence, la justice apparaît comme un prisme de choix pour appréhender l’institution familiale et les mécanismes de domination qui s’y jouent. Les résultats d’une enquête sociologique menée par le Collectif Onze dans des tribunaux de grande instance montrent ainsi comment la justice familiale contribue, par son intervention brève mais néanmoins décisive, à la reproduction d’un ordre hiérarchisé entre les sexes et entre les classes

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

Probing Vortex Density Fluctuations in Superfluid Turbulence

International audienceQuantum fluids -such as superfluid helium- are recognized as a model-fluids for turbulence research because their dynamics result from the tangle of quantized vortex lines. We report a time-resolved measurements of the local vortex line density in a turbulent quantum flow, here 4He at 1.6 K. The sensor is a micromachined second sound resonator inserted across the flow. Over one decade of inertial range, the spectrum exhibits a -1.6 power law scaling, compatible with a -5/3 exponent

Velocity intermittency in turbulence : how to objectively characterize it ?

The evolution of $\lambda^2$, a parameter whose definition is recalled in the paper, has been used in order to test various intermittency models in turbulence. This paper aims to validate an experimental, model independent, method for extracting this parameter from the velocity difference probability density functions (pdf). We also discuss the characteristics of an experiment which could push further the analysis of these pdf.L'évolution d'un certain paramètre $\lambda^2$ défini dans l'article a été utilisée pour tenter de trancher entre différents modèles de l'intermittence en turbulence. Le but de cet article est de valider une méthode expérimentale de détermination de ce paramètre, indépendamment de tout modèle, à partir des densités de probabilité de différences de vitesse (pdf). Les conditions expérimentales qui permettraient de pousser plus loin la caractérisation de ces pdf sont également discutées