Flow organisation in laterally unconfined Rayleigh-Benard turbulence

We investigate the large-scale circulation (LSC) of turbulent Rayleigh-Benard convection in a large box of aspect ratio Gamma = 32 for Rayleigh numbers up to Ra = 10(9) and at a fixed Prandtl number Pr = 1. A conditional averaging technique allows us to extract statistics of the LSC even though the number and the orientation of the structures vary throughout the domain. We find that various properties of the LSC obtained here, such as the wall-shear stress distribution, the boundary layer thicknesses and the wind Reynolds number, do not differ significantly from results in confined domains (Gamma approximate to 1). This is remarkable given that the size of the structures (as measured by the width of a single convection roll) more than doubles at the highest Ra as the confinement is removed. An extrapolation towards the critical shear Reynolds number of R-es(crit) approximate to 420, at which the boundary layer (BL) typically becomes turbulent, predicts that the transition to the ultimate regime is expected at Ra-crit approximate to O(10(15)) in unconfined geometries. This result is in line with the Gottingen experimental observations (He et al., Phys. Rev. Lett., vol. 108, 2012, 024502; New J. Phys., vol. 17, 2015, 063028). Furthermore, we confirm that the local heat transport close to the wall is highest in the plume impacting region, where the thermal BL is thinnest, and lowest in the plume emitting region, where the thermal BL is thickest. This trend, however, weakens with increasing Ra.

Automated summary

In this study, large-scale circulation of turbulent Rayleigh-Benard convection was investigated in a large container with high Rayleigh number conditions. The results showed that the properties of LSC remain consistent even with varying boundaries, and the transition to the ultimate regime is predicted to occur in unconstrained geometries. Additionally, observations were in line with experimental data from Gottingen.