Asymmetry of Two-Dimensional Thermal Convection at High Rayleigh Numbers

While thermal convection cells exhibit left–right and top–bottom symmetries at low Rayleigh numbers (Ra), the emergence of coherent flow structures, such as elliptical large-scale circulation in Rayleigh–Bénard convection (RBC), breaks these symmetries as the Rayleigh number increases. Recently, spatial double-reflection symmetry was proposed and verified for two-dimensional RBC at a Prandtl number of 6.5 and Ra values up to 1010. In this study, we examined this new symmetry at a lower Prandtl number of 0.7 and across a wider range of Rayleigh numbers, from 107 to 1013. Our findings reveal that the double-reflection symmetry is preserved for the mean profiles and flow fields of velocity and temperature for Ra<109, but it is broken at higher Rayleigh numbers. This asymmetry at high Ra values is inferred to be induced by a flow-pattern transition at Ra=109. Together with the previous study, our results demonstrate that the Prandtl number has an important influence on the symmetry preservation in RBC.