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We report measurements of the dissipation in the Superfluid Helium high REynold number von Karman flow (SHREK) experiment for different forcing conditions, through a regime of global hysteretic bifurcation. Our macroscopical measurements indicate no noticeable difference between the classical fluid and the superfluid regimes, thereby providing evidence of the same dissipative anomaly and response to asymmetry in fluid and superfluid regime. %In the latter case, A detailed study of the variations of the hysteretic cycle with Reynolds number supports the idea that (i) the stability of the bifurcated states of classical turbulence in this closed flow is partly governed by the dissipative scales and (ii) the normal and the superfluid component at these temperatures (1.6K) are locked down to the dissipative length scale., Comment: 5 pages, 5 figures

We report measurements of the dissipation in the Superfluid Helium high REynold number von Karman flow (SHREK) experiment for different forcing conditions, through a regime of global hysteretic bifurcation. Our macroscopical measurements indicate no noticeable difference between the classical fluid and the superfluid regimes, thereby providing evidence of the same dissipative anomaly and response to asymmetry in fluid and superfluid regime. %In the latter case, A detailed study of the variations of the hysteretic cycle with Reynolds number supports the idea that (i) the stability of the bifurcated states of classical turbulence in this closed flow is partly governed by the dissipative scales and (ii) the normal and the superfluid component at these temperatures (1.6K) are locked down to the dissipative length scale., 5 pages, 5 figures

International audience; We relate the second-order structure function of a time series with the power spectrum of the original variable, taking an assumption of statistical stationarity. With this approach, we find that the structure function is strongly influenced by the large scales. The large-scale contribution and the contribution range are, respectively, 79% and 1.4 decades for a Kolmogorov -5/3 power spectrum. We show numerically that a single scale influence range, over smaller scales is about 2 decades. We argue that the structure function is not a good method to extract the scaling exponents when the data possess large energetic scales. An alternative methodology, the arbitrary order Hilbert spectral analysis which may constrain this influence within 0.3 decade, is proposed to characterize the scaling property directly in an amplitude-frequency space. An analysis of passive scalar (temperature) turbulence time series is presented to show the influence of large-scale structures in real turbulence and the efficiency of the Hilbert-based methodology. The corresponding scaling exponents ζ(θ)(q) provided by the Hilbert-based approach indicate that the passive scalar turbulence field may be less intermittent than what was previously believed

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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., 4 pages, 4 figures, submitted to Physical Review Letters

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 ($530 \leq R_{\lambda} \leq 6100$). The continuous time evolution of a spatial Fourier mode of the vorticity distribution, characterized by a well defined wave-vector, 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 very large number ($\geq 13~10^{6}$) 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 Komogorov theory, over one decade of frequencies., Comment: Latex, 32 pages, 14 figures

Cryogenic gaseous helium gives access to extreme turbulent experimental conditions. The very high cooling helium flow rates available at CERN have been used to reach Reynolds numbers up to Re ~ 10 7 in a round jet experiment. First results are discussed. Turbulence is a highly challenging problem both for fundamental understanding and industrial applications. Present capabilities of the most powerful computer simulations remain far away from common situations, and experimental data are needed to test the validity of theoretical predictions. Large air wind tunnels (≥ 20 m), for example, have been used for many years; however in such large experiments the Re hydrodynamic parameter cannot vary on a wide range. The physical idea is to understand how the energy injected at the large scale L of the flow cascades down through the inertial scales to the smallest scale η where it is dissipated by viscosity. In air this behavior is investigated in large systems : L is in the range of several meters and η is in the millimetric or submillimetric range. Turbulence studies in gaseous helium at cryogenic temperatures have been achieved in the last twenty years and have opened a very attractive way to vary Re on several orders of magnitude in a single geometry and reach very high Re values. They take advantage of the very low kinematic viscosity ν of gaseous helium (ν = 7×10-8 m 2 /s at 4.2 K and 1 bar, 230 times lower than air in STP conditions). In a typical (φ = 10 cm) laboratory jet experiment, Re = VL/ν (V is the mean velocity and L the width of the jet) may easily vary over three orders of magnitude by varying pressure and flow rate [1]. A large experiment with low temperature helium flow rates is today available at CERN, in the framework of the Large Hadron Collider development. We use a large scale refrigerator producing 6 kW at 4.5 K, with flow rates from 20 g/s up to 300 g/s [2]. A specific 6 m long φ 22 cm transfer line, with concentric tubes, feeds the experiment. The inner tube drives liquid and gaseous helium out of the refrigerator; the mixture is turned to gas at controlled temperature by a powerful heater (up to 10 kW) before entering a specific flow meter, a honeycomb and finally the φ = 25 mm nozzle. The round jet develops inside the 2.5 m high experimental chamber. The investigation of the expected micrometric dissipative η scale requires an original 5 µm superconducting anemometer [3], located axially 1.25 m down the nozzle : it gives access to the velocity measurements. The helium flows back to the refrigerator through the transfer line outer tube. The grid at the nozzle entrance is a honeycomb, φ 240 mm and 20 mm thickness, with hexagons of 4 mm side dimension. The nozzle diameter has been determined to maintain the flow velocity significantly lower than the sound velocity : at 300 g/s the output velocity is half the sound velocity. The nozzle height is 280 mm. The surface contraction factor in the nozzle is 92. Thus the flow in the experimental chamber is independent from the incoming flow characteristics as confirmed by first tests with water: they showed that the outgoing flow from the nozzle is laminar for several cm and is aligned with the nozzle axis before turning to the turbulent state. At the level of the detectors the flow diameter is about 40-50 cm. The mean temperature is 4.8 K. Special care has been taken to avoid spurious heat leaks in order to limit the temperature difference ∆T between the incoming and outgoing He in the experimental chamber. For heat leaks ranging around 30 W, ∆T, including the temperature fluctuations of the flow induced by the turbulent regime, does not exceed 100 mK at a 50 g/s flow rate. With such a low ∆T, the outgoing He feeds directly the last stage of the liquefier and the He flow is in constant average temperature conditions.