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skelette en ondelettes fluctuations vitesse jet
above : wavelet analysis of velocity fluctuations on the axis of a round helium gaz jet.

 

High Reynolds number in a cryogenics mini-jet

 

 

Photos

 

Mini-jet pictures

 

 

Introduction

 

Gaseous helium near 4.2 K is used as a working fluid for developed turbulence at high Reynolds numbers in a jet experiment. Thanks to its very small viscosity (10-7 m2/s), a large range of Reynolds numbers (1000-1000000) can be explored by varying the pressure in a 20-cm-cell.

 

 

Results

 

A preliminary study in a water jet has been conducted at scale 1:1. A picture of the flow in provided below.

Jet d'eau
Water jet (see also air jet)

The Reynolds numbers attainable in an cryogenic helium jet are high and -besides- can be varied over a large range. In counterpart, dissipative length-scales can as small as a few microns. To perform temperature and velocity measurements down to those scales, nonperturbative micron-size sensors have been developed. A high performance 10 MHz lock in amplifier detection scheme and an analogic to digital converter have also been developed, to mesure and acquire velocity fluctuations up to 1 MHz and with 16 bits of accuracy.

The data analysis shows that a 10 µm space resolution and a sub-microsecond time response was obtained. The statistic analysis of the velocity structure functions after correction of noise sources, shows a universal behaviour at large scales, independent of the Reynolds number and of the experiment geometry. The cumulant method reveals an autosimilarity cascade process of velocity differences, on an extended scale larger than the traditional inertial range. Furthermore, the cascade process converges toward a log-normal behaviour at high Reynolds. Finally, we observe at large scales a decrease in the dissymmetry of probability density functions, that can be explained by a relative reduction of the dissipation with increasing Reynolds numbers.

Spectre de vitesse du mini jet cryogenique
Velocity spectrum in the cryogenics mini jet

 

 

For more informations

 

Intermittency in a turbulent low temperature gaseous helium jet.
Chanal, O., B. Chabaud, B. Castaing and B. Hébral (2000).
European Physical Journal B 17(2): 309-317.

 

 


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VASE
Ph.R. Jan, 2012