Large scale structures of a turbulent Rayleigh-Bénard convection in a liquid metal layer confined by a moderate aspect ratio box

We report laboratory experiments of Rayleigh-Bénard convection with a liquid metal, Prandtl number Pr = 0.03, in a rectangular cell with a moderate aspect ratio. Rayleigh number, Ra, was set at a range from 7.9 × 10^3 to 3.5 × 10^5 at which the thermal turbulence regime is expected. Multiple horizontal velocity profiles in the fluid layer by ultrasonic velocity profiling elucidated formations of several large scale flow structures with periodic oscillations. The flow structure has transitions as increasing Ra from a quasi-two-dimensional roll-like structure to a three-dimensional cell-like structure via unstable intermediate regimes with stepwise increase of its horizontal scale. By using observed Ra dependences of the frequency of oscillation and the velocity of large scale flow, we made up a model to explain the increase of horizontal scale. We evaluated effective viscosities and diffusivities based on the turbulent fluctuations, and found that the morphology of roll-like structure can be understood by using these effective values.