Prandtl and wall effects in hard turbulence convection

International audienceUsing low-temperature gaseous helium close to the critical point, we investigate the {P}randtl-number dependence of the effective heat conductivity ({N}usselt number) for a 1/2 aspect ratio {R}ayleigh-{B}énard cell. Very weak dependence is observed in the range 0.7 < Pr < 21; 2 * 10/sup 8/ < Ra < 2 * 10/sup 10/: the absolute value of the average logarithmic slope delta = ( delta ln Nu/ delta ln Pr)/sub Ra/ is smaller than 0.03. A bimodality of Nu, with 7{\%} difference between the two sets of data, is observed, which could explain some discrepancies between precise previous experiments in this range

Nano-shaped hot-wire for ultra-high resolution anemometry in cryogenic helium

International audienceWe present the principle, modelling, and the first implementation of a new type of high resolution hot-wire anemometer designed to operate at cryogenic temperatures and very high Reynolds numbers. Its spatial resolution of a few micrometers is comparable to the most spatially-resolved hot-wires reported in the literature. Compared to existing designs, its fabrication involves a limited number of steps, essentially the shaping at nanoscales of a superconducting NbTi wire of sub-micron diameter. The velocity spectra in the far wake of a centimeter-sized grid is measured as a proof of concept in stringent flow conditions

Cryogenic High-Reynolds Turbulence Experiment at CERN

The potential of cryogenic helium flows for studying high-Reynolds number turbulence in the laboratory has been recognized for a long time and implemented in several small-scale hydrodynamic experiments. With its large superconducting particle accelerators and detector magnets, CERN, the European Laboratory for Particle Physics, has become a major world center in helium cryogenics, with several large helium refrigerators having capacities up to 18 kW @ 4.5 K. Combining a small fraction of these resources with the expertise of three laboratories at the forefront of turbulence research, has led to the design, swift implementation, and successful operation of GReC (Grands Reynolds Cryogéniques) a large axisymmetric turbulent-jet experiment. With flow-rates up to 260 g/s of gaseous helium at ≈5 K and atmospheric pressure, Reynolds numbers up to 10 7 have been achieved in a 4.6 m high, 1.4 m diameter cryostat. This paper presents the results of the first runs and describes the experimental set-up comprehensively equipped with “hot” wire micro-anemometers, acoustic scattering vorticity measurements and a large-bandwidth data acquisition system