Transition from convection rolls to large-scale cellular structures in turbulent Rayleigh-Bénard convection in a liquid metal layer

Turbulent Rayleigh-Bénard convection was investigated within a liquid metal layer, Prandtl number Pr = 0.03, in a square vessel having a moderate aspect ratio, Γ = 5. Laboratory experiments were performed at moderate Rayleigh numbers, 7.9 × 10^3 < Ra < 3.5 × 10^5. Ultrasonic velocity profiling (UVP) was used to visualize the spatio-temporal flow structure in two horizontal planes, while temperature fluctuations were monitored simultaneously in the fluid layer. Oscillatory roll-like structures were observed at Ra ≥ 10^4, while the transition to a fully three-dimensional cell-like structure occurs around Ra = 6 × 10^4. The transition from laminar convection to thermal turbulence manifests itself in the occurrence of unstable intermediate regimes accompanied by a stepwise increment in the horizontal scale. We propose a scaling law for the horizontal length scale as a function of the Ra number based on empirically-derived relations of the oscillation frequency and the typical flow velocity. This scaling law indicates the present results are comparable with variations of the maximum size of large scale structures in different Pr conditionsand larger aspect ratios.