Christophe Baudet

Turbulent transport of material particles: an experimental study of finite size effects.

We present experimental Lagrangian statistics of finite sized, neutrally bouyant, particles transported in an isotropic turbulent flow. The particles diameter is varied over turbulent inertial scales. Finite size effects are shown not to be trivially related to velocity intermittency. The global shape of the particles acceleration probability density functions is not found to depend significantly on its size while the particles acceleration variance decreases as it becomes larger in quantitative agreement with the classical k(-7/3) scaling for the spectrum of Eulerian pressure fluctuations in the carrier flow.

Reynolds dependence of third-order velocity structure functions

We study the experimental dependence of the third-order velocity structure function on the Taylor based Reynolds number, obtained in different flow types over the range 72⩽Rλ⩽2260. As expected, when the Reynolds number is increasing, the third-order velocity structure functions (plotted in a compensated way) converge very slowly to a possible −4/5 plateau value according to the Kolmogorov 41 theory. Actually, each of these normalized third-order functions exhibits a maximum, at a scale close to the Taylor microscale λ. In this Brief Communication, we show that experimental data are in good agreement with the recent predictions of Qian and Lundgren. We also suggest that, from an experimental point of view, a log-similar plot suits very well to study carefully the behavior of the third-order velocity structure functions with the flow Reynolds number.

Spectral Vorticity and Lagrangian Velocity Measurements in Turbulent Jets

In this paper we report an experimental investigation of various statistical properties of the spatial Fourier modes of the vorticity field in turbulent jets for a large range of Reynolds numbers (530 ≤Rλ≤ 6100). The continuous time evolution of a spatial Fourier mode of the vorticity distribution, characterized by a well-defined wavevector, is obtained from acoustic scattering measurements. The spatial enstrophy spectrum, as a function of the spatial wave-vector, is determined by scanning the incoming sound frequencies. Time-frequency analysis of the turbulent vorticity fluctuations is also performed for different length scales of the flows. Vorticity time-correlations show that the characteristic time of a Fourier mode behaves as the sweeping time. Finally, we report preliminary Lagrangian velocity measurements obtained using acoustic scattering by soap bubbles inflated with helium. Gathering a large number of passages of isolated bubbles in the scattering volume, one is able to compute the Lagrangian velocity PDF and velocity spectrum. Despite the spatial filtering due to the finite size of the bubble, the latter exhibits a power law, with the -2 exponent predicted by the Kolmogorov theory, over one decade of frequencies.

Superconducting instrumentation for high Reynolds turbulence experiments with low temperature gaseous helium

Turbulence is of common experience and of high interest for industrial applications, despite its physical grounds is still not understood. Cryogenic gaseous helium gives access to extremely high Reynolds numbers (Re). We describe an instrumentation hosted in CERN, which provides a 6 kW @ 4.5 K helium refrigerator directly connected to the experiment. The flow is a round jet; the flow rates range from 20 g/s up to 260 g/s at 4.8 K and about 1.2 bar, giving access to the highest controlled Re flow ever developed. The experimental challenge lies in the range of scales which have to be investigated: from the smallest viscous scale η, typically 1 μm at Re=107 to the largest L∼10 cm. The corresponding frequencies: f=v/η can be as large as 1 MHz. The development of an original micrometric superconducting anemometer using a hot spot and its characteristics will be discussed together with its operation and the perspectives associated with superconducting anemometry.

Dynamics of spatial Fourier modes in turbulence Sweeping effect, long-time correlations and temporal intermittency

We report an experimental investigation of the statistical time dynamic of spatial Fourier modes in a fully developped turbulent jet flow. Measurements rely on an original acoustic scattering technique, allowing the direct and continuous probing in a time of spatial Fourier modes of the turbulent vorticity field at well defined spatial wavevectors. From the recordings of the amplitude of the spatial Fourier modes corresponding to different wavevectors in the inertial range, we compute the time correlation functions of the modal amplitude. The modal vorticity dynamics exhibits two well separated time scales: a short one which is scale dependant and related to the well known sweeping effect (random advection), and a much longer one, of order the integral time scale, which is not scale dependent. By computing the cross-correlation of the amplitude of two different wavevectors, we evidence a clear statistical dependance between scales, whatever their separation, at variance with the original Kolmogorov 41 theory. We ascribe our experimental results to a manifestation of a new type of statistical intermittency. We call this new statistical feature a large scale time intermittency effect, in contrast with the usual small scale spatial intermittency reported in Eulerian velocity measurements. We propose to attribute this time intermittency, to the fluctuations in time of the injected energy at large scale, in the spirit of the objection formulated by Landau against the K41 theory.

Turbulent transport of finite sized material particles

Turbulent transport of material inclusions plays an important role in many natural and industrial situations. In the present study, we report an exhaustive experimental investigation of Lagrangian dynamics of material particles in a turbulent air flow, over a wide range of sizes and densities. For fixed carrier flow conditions, we find that (i) velocity statistics are not affected by particles inertia ; (ii) acceleration statistics have a very robust signature, where only acceleration variance is affected by inertia ; (iii) inertial particles always have an intermittent dynamics ; (iv) intermittency signature depends on particles inertia ; (v) particles actual response time to turbulent forcing remains essentially of the order of the carrier flow dissipation time rather than any particles dependent time (as the Stokes time for instance). These observations are in contrast with usual predictions from Stokesian models for point particles.

Hot-wire anemometry for superfluid turbulent coflows

We report the first evidence of an enhancement of the heat transfer from a heated wire to an external turbulent coflow of superfluid helium. We used a standard Pt-Rh hot-wire anemometer and overheat it up to 21 K in a pressurized liquid helium turbulent round jet at temperatures between 1.9 K and 2.12 K. The null-velocity response of the sensor can be satisfactorily modeled by the counterflow mechanism, while the extra cooling produced by the forced convection is found to scale similarly as the corresponding extra cooling in classical fluids. We propose a preliminary analysis of the response of the sensor and show that-contrary to a common assumption-such sensor can be used to probe local velocity in turbulent superfluid helium.

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.

Liquid helium inertial jet for comparative study of classical and quantum turbulence

We present a new cryogenic wind tunnel facility developed to study the high Reynolds number developed classical or quantum turbulence in liquid (4)He. A stable inertial round jet flow with a Reynolds number of 4 × 10(6) can be sustained in both He I and He II down to a minimum temperature of 1.7 K. The circuit can be pressurized up to 3.5 × 10(5) Pa. The system has been designed to exploit the self-similar properties of the jet far field in order to adapt to the spatial resolution of the existing probes. Multiple and complementary sensors can be simultaneously installed to obtain spatial and time resolved measurements. The technical difficulties and design details are described and the system performance is presented.