On the influence of radiation on the large-scale circulation in a Rayleigh–Bénard cubic cell

Radiative transfer effects on the Large-Scale Circulation (LSC) in a Rayleigh–Bénard cubic cell are analyzed from coupled Direct Numerical Simulation data for an air/H${}_2$O/CO${}_2$ mixture in the range $Ra\in [10^6,10^8]$. The local effects of radiation are first studied using conditional averaging. It is found that radiation coupling accentuates the temperature field asymmetry between the ejecting and the impinging sides. The mean kinetic energy increase is also higher on the ejecting side of the horizontal entrainment zone. To better understand the impact of these variations on the reorientation frequency of the LSC, a clustering analysis based on Latent Dirichlet Allocation (LDA) is carried out in the vertical mid-planes of the cell. The dynamics of the LSC can then be tracked through local characteristic patterns called motifs. We show that the contributions of the dominant heat flux motifs associated with plume ejection increase in the presence of radiation, while those of the dominant temperature motifs associated with impinging plumes tend to decrease, which is consistent with a reinforcement of the LSC with respect to the corner structures. Using a motif-based model, we show that the frequency decrease associated with this reinforcement is offset by the increase due to kinetic effects. The model predictions are found to be in good agreement with numerical data.