Transition vers la turbulence dans un fluide supercritique en configuration de Rayleigh-Bénard

Direct numerical simulations of the Rayleigh-Benard convection in a fluid, slightly above its gas-liquid critical point, are presented for a Rayleigh number varying from 10 6 up to 10 8 . A spatiotemporal description of the flow is carried out since the convection onset until the establishment of the turbulent regime. This description concerns mainly the structure of the temperature field as well as the global thermal equilibrium of the cavity. We emphasize on the effect of the strong stratification of the fluid induced by its high compressibility on the penetrability of the convective structures and on the global thermal equilibrium of the cavity, and a comparison with the case of a perfect gas, at equal Rayleigh number, is presented. Mots-clefs : Fluide supercritique, convection de Rayleigh-Benard, turbulence.