In a low temperature He gas Rayleigh-Bénard experiment, Rayleigh numbers from ${10}^3$ to more than ${10}^{14}$ are explored. Local velocity is estimated through the time lag between two closeby temperature probes. This allows characterizing of the high Rayleigh regime $(\mathrm{Ra}>{10}^{11})$ as a fully turbulent one, possibly corresponding to the asymptotic regime predicted by R. Kraichnan [Phys. Fluids 5, 1374 (1962)].

• Received 8 July 1997

DOI:https://doi.org/10.1103/PhysRevLett.79.3648