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

Fluctuation-Dissipation Relations and statistical temperatures in a turbulent von K\'arm\'an flow

We experimentally characterize the fluctuations of the non-homogeneous non-isotropic turbulence in an axisymmetric von K\'arm\'an flow. We show that these fluctuations satisfy relations analogous to classical Fluctuation-Dissipation Relations (FDRs) in statistical mechanics. We use these relations to measure statistical temperatures of turbulence. The values of these temperatures are found to be dependent on the considered observable as already evidenced in other far from equilibrium systems.Comment: four pages 2 figures one tabl

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 comparison of high Reynolds number turbulence in both HeI and HeII: First results

International audienceTSF experiment for comparison of high Reynolds number turbulence in both HeI and HeII: First result

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

Développement d'anémomètres pour l'étude de la turbulence cryogénique

L'étude a pour objet le dimensionnement et la réalisation d'anémomètres spécifiques aux conditions cryogéniques. Le travail principal concerne l'adaptation du principe des anémomètres à fil chaud. A cet effet nous utilisons des filaments de Niobium Titane supraconducteurs (T c=9 K) dont le diamètre est compris entre 0,5 pm et 1 pm. La zone sensible du détecteur est réalisée en réduisant localement la section du filament sur une longueur typique d'une fraction de micromètre. Cette opération, qui a pour but de créer un point chaud (par réduction ponctuelle du courant critique du fil), est effectuée au moyen d'un canon à ions Gal1ium (FIB). L'étalonnage du détecteur est obtenu dans une boucle de circulation d'hélium liquide conçue et réalisée pour cet usage. La sensibilité du détecteur à la vitesse est inférieure d'un facteur 1,5 à celle des fils chauds classiques. Les premières étapes du développement d'une sonde locale de pression (visant à terme un fonctionnement en tube de Pitot) sont présentées: il est montré que la fabrication d'une membrane de petite taille, pouvant être intégrée directement dans l'écoulement, ne pose pas de problème particulier. Les jauges de contrainte métalliques -déposées sur la membrane pour en mesurer la déformation- ont cependant une sensibilité faible et ne permettent pas à l'heure actuelle d'atteindre les performances visées.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Controlled spherical deuterium droplets as Lagrangian tracers for cryogenic turbulence experiments

International audienceThe study of the smallest scales of turbulence by (Lagrangian) particle tracking faces two major challenges: the requirement of a 2D or 3D optical imaging system with sufficiently high spatial and temporal resolution and the need for particles that behave as passive tracers when seeded into the flow. While recent advances in the past decade have led to the development of fast cameras, there is still a lack of suitable methods to seed cryogenic liquid helium flows with mono-disperse particles of sufficiently small size, of the order of a few micrometers, and a density close enough to that of helium. Taking advantage of the surface tension, we propose two different techniques to generate controlled liquid spherical droplets of deuterium over a liquid helium bath. The first technique operates in a continuous mode by fragmenting a liquid jet, thanks to the Rayleigh–Taylor instability. This results in the formation of droplets with a diameter distribution of 2 ± 0.25DN, where DN is the diameter of the jet nozzle (DN = 20 μm in the present experiment). This method offers a high production rate, greater than 30 kHz. The second technique operates in a drop-on-demand mode by detaching droplets from the nozzle using pressure pulses generated using a piezoelectric transducer. This approach yields a much narrower diameter distribution of 2.1 ± 0.05DN but at a smaller production rate, in the range 500 Hz–2 kHz. The initial trajectories and shapes of the droplets, from the moment they are released from the nozzle until they fall 3 mm below, are investigated and discussed based on back-light illumination images

Multiscale energy budget of inertially driven turbulence in normal and superfluid helium

20 pages, 15 figures, submitted to Peer journalInternational audienceIn this paper we present a novel hydrodynamic experiment using liquid $^4$He.Lagrangian trajectories are obtained using two-domensional particle trackingon hollow glass microspheres in a cryogenic liquid helium turbulent flow.The flow is forced inertially by a canonical oscillating grid, both belowand above the superfluid transition. This allows for a direct comparison ofthe Lagrangian statistics in the normal (He I), and superfluid (He II) phase.The high temporal resolution allows us to resolve the velocity fluctuations atintegral and inertial scales, and most importantly assess the noisecontribution.The careful analysis of velocity fluctuations, acceleration fluctuations,and pair dispersion, allows us to extract estimates of the energy injectionrate at large scale, the energy flux cascading through inertial scales,and the dissipation rate at small scale, and therefore build the energybudget in both the normal and superfluid phase.We find that, within experimental uncertainty, the statistical features ofturbulence and the energy budget in superfluid helium is indistinguishablefrom those of normal helium, highlighting the importance of conductingexperiments in both He I and He II to draw meaningful conclusions,because deviations from the theoretical predictions may arise from noisecontributions or deviation from the homogeneous and isotropic approximations

Turbulent cascade of a quantum fluid at finite temperature

International audienceThe turbulent inertial cascade undergone by a quantum fluid at small but finite temperature above absolute zero is simulated by DNS of 2 coupled Navier-Stokes equations. Following the 2-fluid model of Landau and Tisza, one equation accounts for the viscous normal fluid, while the other equation models the superfluid dynamics on scales larger than the inter-vortex spacing. An artificial superfluid viscosity is introduced - as a turbulence closure - to model non-viscous processes taking place on scales smaller than the inter-vortex spacing, such as phonon emission. Three outcomes are presented. The first is the strong locking of the superfluid and normal fluid along the turbulent cascade, including the overlapping of the vorticity structures. The second is the peaking at small scales of the residual slip velocity between the two fluids. The third is a temperature dependence of the Reynolds number - defined using the separation of large and small scales. We argue that these three features evidenced in the simulations should also be present in real turbulent quantum fluids