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

Natural cross-ventilation of buildings, an experimental study

International audienceNatural cross ventilation can be a promising passive solution for summer thermal comfort in buildings. It takes advantage of the night temperature of the air to cool the walls of the building. Although this technique is well-known in hot climate, its use in new buildings requires being able to predict the quantity of heat that can be dissipated. There is indeed a lack of experimental data either to build design rules for engineers or to validate numerical code dedicated to the design. In this study, experimental research is carried out on a full-scale dwelling and a small-scale model, and evidence is given on the relevance of such a comparison. The flow has been estimated with spatially discrete local sensors in both systems, and flow visualizations have been additionally performed in the small-scale controlled model. For windy periods, the indoor airflow is found to be driven by the wind, as expected. For calm periods which are critical for summer comfort, the flows look much more complex with the observation of unsteady flow reversals

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

On the triggering of the Ultimate Regime of convection

Rayleigh-B\'enard cells are one of the simplest systems to explore the laws of natural convection in the highly turbulent limit. However, at very high Rayleigh numbers (Ra > 1E12) and for Prandtl numbers of order one, experiments fall into two categories: some evidence a steep enhancement of the heat transfer while others do not. The origin of this apparent disagreement is presently unexplained. This puzzling situation motivated a systematic study of the triggering of the regime with an enhanced heat transfer, originally named the "Ultimate Regime" of convection. High accuracy heat transfer measurements have been conducted in convection cells with various aspect ratios and different specificities, such as altered boundary conditions or obstacles inserted in the flow. The two control parameters, the Rayleigh and Prandtl numbers have been varied independently to disentangle their relative influence. Among other results, it is found that i) most experiments reaching very high $Ra$ are not in disagreement if small differences in Prandtl numbers are taken into account, ii) the transition is not directly triggered by the large scale circulation present in the cell, iii) the sidewall of the cell have a significant influence on the transition. The characteristics of this Ultimate regime are summarized and compared with R. Kraichnan prediction for the asymptotic regime of convection.Comment: 25 pages, 14 figure

Vorticity scattering measurements in a superfluid inertial round jet

International audienceThe aim of this proceeding paper is twofold. First, we present a newly developed cryogenic testing facility where a steady high Reynolds liquid helium inertial round jet flow is generated allowing to address classical turbulence issues, such as statistical intermittency, and quantum turbulence when the facility is operating in superfluid helium. Secondly we present the first spatial Fourier vorticity modes measurements made both above and below the superfluid transition at different nozzle velocities. These preliminary results were obtained by probing the vorticity flow-field with the ultrasonic scattering technique

Thermal boundary layer near roughnesses in turbulent Rayleigh-Bénard convection: flow structure and multistability

We present global heat-transfer and local temperature measurements, in an asymmetric parallelepiped Rayleigh-B ́enard cell, in which controlled square-studs roughnesses have been added. A global heat transfer enhancement arises when the thickness of the boundary layer matches the height of the roughnesses. The enhanced regime exhibits an increase of the heat transfer scaling. Local temperature measurements have been carried out in the range of parameters where the enhancement of the global heat transfer is observed. They show that the boundary layer at the top of the square-stub roughness is thinner than the boundary layer of a smooth plate, which accounts for most of the heat-transfer enhancement. We also report multistability at long time scales between two enhanced heat-transfer regimes. The flow structure of both regimes is imaged with background-oriented synthetic Schlieren and reveals intermittent bursts of coherent plumes

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 comparision of high Reynold's number turbulence in He I and He II : first results

International audienceSuperfluid turbulence (TSF) project uses liquid helium for the fundamental study of turbulent phenomena behind a passive grid and is able to work both in HeI and in HeII. Local and semi-local instrumentation was developed specifically for the purpose of this experiment(e.g. sub-micrometer anemometer, total head pressure tube and second sound tweezer). The difficulties encountered with this local and fragile instrumentation are discussed. Global characterization of the flow is presented including velocity, pressure, temperature stability and turbulence intensity. Finally, first results obtained with semi local measurements (total head pressure tube and second sound tweezer) both in the two phases of helium are presented