Julien Salort

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-Tiszas 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 Karman-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.

Thermal boundary layer near roughnesses in turbulent Rayleigh-Benard convection: Flow structure and multistability

We present global heat-transfer and local temperature measurements, in an asymmetric parallelepiped Rayleigh-Benard 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.

Cantilever anemometer based on a superconducting micro-resonator: Application to superfluid turbulence

We present a new type of cryogenic local velocity probe that operates in liquid helium (1 K < T < 4.2 K) and achieves a spatial resolution of ≈ 0.1 mm. The operating principle is based on the deflection of a micro-machined silicon cantilever which reflects the local fluid velocity. Deflection is probed using a superconducting niobium micro-resonator sputtered on the sensor and used as a strain gauge. We present the working principle and the design of the probe, as well as calibration measurements and velocity spectra obtained in a turbulent helium flow above and below the superfluid transition.

Investigation of intermittency in superfluid turbulence

This paper reports new experimental and simulation velocity data for superfluid steady turbulence above 1K. 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 & Tabeling (1998) and the first numerical evidence of intermittency in superfluid turbulence.

Superfluid high REynolds von Kármán experiment

The Superfluid High REynolds von Kármán experiment facility exploits the capacities of a high cooling power refrigerator (400 W at 1.8 K) for a large dimension von Kármán flow (inner diameter 0.78 m), which can work with gaseous or subcooled liquid (He-I or He-II) from room temperature down to 1.6 K. The flow is produced between two counter-rotating or co-rotating disks. The large size of the experiment allows exploration of ultra high Reynolds numbers based on Taylor microscale and rms velocity [S. B. Pope, Turbulent Flows (Cambridge University Press, 2000)] (Rλ > 10000) or resolution of the dissipative scale for lower Re. This article presents the design and first performance of this apparatus. Measurements carried out in the first runs of the facility address the global flow behavior: calorimetric measurement of the dissipation, torque and velocity measurements on the two turbines. Moreover first local measurements (micro-Pitot, hot wire,…) have been installed and are presented.

Probing quantum and classical turbulence analogy in von Kármán liquid helium, nitrogen, and water experiments

SHREK Collaboration: B. Saint-Michel, E. Herbert, J. Salort, C. Baudet, M. Bon Mardion, P. Bonnay, M. Bourgoin, B. Castaing, L. Chevillard, F. Daviaud, P. Diribarne, B. Dubrulle, Y. Gagne, M. Gibert, A. Girard, B. Hébral, Th. Lehner, and B. Rousset Laboratoire SPHYNX, CEA/IRAMIS/SPEC, CNRS URA 2464, F-91191 Gif-sur-Yvette, France Laboratoire FAST, CNRS UMR 7608, Université Paris-Sud, Université Pierre-et-Marie-Curie, Bât. 502, Campus universitaire, 91405 Orsay, France Laboratoire de Physique de l’ÉNS de Lyon, CNRS/Université Lyon F-69364 Lyon cedex 7, France Laboratoire des Écoulements Géophysiques et Industriels, CNRS/UJF/INPG, F-38041 Grenoble Cedex 9, France Service des Basses Températures, INAC/SBT, UMR CEA-UJF 9004, CEA Grenoble, 17 rue des Martyrs 38054 Grenoble Cedex France Université Grenoble Alpes, Institut NÉEL, F-38042 Grenoble, France, CNRS, Institut NÉEL, F-38042 Grenoble, France LUTH, Observatoire Paris-Meudon, 5 Pl. Jules Janssen, F-92195 Meudon Cedex, FranceWe report measurements of the dissipation in the Superfluid helium high REynold number von Karman flow experiment for different forcing conditions. Statistically steady flows are reached; they display a hysteretic behavior similar to what has been observed in a 1:4 scale water experiment. Our macroscopical measurements indicate no noticeable difference between classical and superfluid flows, thereby providing evidence of the same dissipation scaling laws in the two phases. A detailed study of the evolution of the hysteresis cycle with the Reynolds number supports the idea that the stability of the steady states of classical turbulence in this closed flow is partly governed by the dissipative scales. It also supports the idea that the normal and the superfluid components at these temperatures (1.6 K) are locked down to the dissipative length scale.

Simultaneous temperature and velocity Lagrangian measurements in turbulent thermal convection

We report joint Lagrangian velocity and temperature measurements in turbulent thermal convection. Measurements are performed using an improved version (extended autonomy) of the neutrally-buoyant instrumented particle that was used by to performed experiments in a parallelepipedic Rayleigh-Benard cell. The temperature signal is obtained from a RFtransmitter. Simultaneously, we determine particles position and velocity with one camera, which grants access to the Lagrangian heat flux. Due to the extended autonomy of the present particle, we obtain well converged temperature and velocity statistics, as well as pseudo-eulerian maps of velocity and heat flux. Present experimental results have also been compared with the results obtained by a corresponding campaign of Direct Numerical Simulations and Lagrangian Tracking of massless tracers. The comparison between experimental and numerical results show the accuracy and reliability of our experimental measurements. Finally, the analysis of lagrangian velocity and temperature frequency spectra is shown and discussed. In particular, we observe that temperature spectra exhibit an anomalous f^2.5 frequency scaling, likely representing the ubiquitous passive and active scalar behavior of temperature

Turbulent velocity profiles in a tilted heat pipe

In this paper, we analyze the mean velocity profile and the Reynolds shear stress in a turbulent, inclined, heat pipe. We show that the simplest version of a mixing length model is unable to reproduce the evolution of the velocity profile shape with the inclination angle ψ. An improvement of this model, taking into account some buoyancy effects, gives nice qualitative agreement with the observations. The agreement implies a low value for the gradient Richardson number Ric above which the flow is laminar. While such a low value (Ric ≃ 0.05) is surprising, we found it in agreement with published experimental data, when the information given allowed to calculate the gradient Richardson number Ri.

TSF Experiment for comparision of high Reynold’s number turbulence in He I and He II : first results.

Superfluid 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.

Convection at very high Rayleigh number: signature of transition from a micro-thermometer inside the flow

In 2001, a change in the statistics of temperature fluctuations in Rayleigh-Benard convection was reported at very high Rayleigh number (Ra ~ 1012) [1]. This change was concomitant with an enhancement of the heat transfer which had been interpreted [2] as the triggering of Kraichnan Convection Regime [3]. But a systematic study of finite probe size effect showed that the 200 µm probe used in the 2001 study was about three times too large to be free from finite size correction [4], calling for a confirmation of these results. We report new measurements of temperature fluctuations performed with a probe ten times smaller than the one used in 2001. In this proceeding, we discuss experimental aspects of this experiment and complementary measurements made in the “Barrel of Ilmenau”. The first experiment was conducted with cryogenic helium and the second with air. Once combined, those two experiments lead to evidence of a signature of a transition in the local temperature fluctuations, supporting the conclusion of the 2001 study.