Expérience à Grands Reynolds Cryogéniques : GReC

We have developed at CERN, an hydrodynamic experiment using Helium at low temperature as the working fluid (5 K). The very small kinematic viscosity of Helium allows to create highly turbulent flows at Reynolds numbers never reached, within well controlled laboratory conditions (RlÅ 6000). The chosen geometry, known to produce high turbulent rate (30 %), is an open round jet which develops within a 1 m diameter and 2, 5 m high cell. In order to mesure flow-rates from 20 g/s to 300 g/s at 5 K with an accuracy better than10 %, we have built a cryogenic mass flowmeter. To characterize the smallest flow structures, we have manufactured micrometric anemometers. These sensors are made of a 5 um diameter glass fiber covered by a Cr layer. To limit the fiber sensitive area, a superconducting Pb-In alloy is sputtered over the Cr layer, but on a small length (2 to 5 um). Vertical and horizontal moving systems, working at low temperature, allow to make velocity measurements everywhere in the jet. A high performance 10 MHz lock in amplifier has been developed and associated with an analogic to digital converter to mesure and acquire velocity fluctuations up to 5 MHz and with 18 bits accuracy for records up to 512 Mpts. The Rl and the n evolutions with the Reynolds number show that our measurements are in agreement with those previously obtained in the Helium jet experiment developed at CRTBT for lower Rl (<1200). The data analysis, based on the velocity structure functions, exhibits an inertial area ranging over more than four decades. A 20 um spatial resolution for the sensor has been measured which corresponds to the Kolmogorov scale of the lowest flow-rate (RlÅ1750). The spectra reveal an asymptotic increase of the inertial range plateau mean value with the Reynolds. In an other hand, we have characterized the intermittency phenomenon by the study of C1 and C2 cumulants. Since the C2 is more stable than the first one, we use it to describe the intermittency process.