Natural convection in a differentially heated cubical cavity under the effects of wall and molecular gas radiation at Rayleigh numbers up to 3 x 10(9)

Numerical simulations are carried out to analyze the effects of wall and molecular gas radiation on weakly turbulent natural convection in a differentially heated cubical cavity in the Rayleigh number range 3 x 10(7) - 3 x 10(9). Two opposite vertical walls are maintained at different temperatures and the four other walls are adiabatic. A direct numerical simulation (pseudo-spectral Chebyshev collocation) method for flow and temperature fields has been combined with direct ray-tracing method for radiation field, and, at the highest Rayleigh number, a spectral subgrid scale radiation model has been implemented to account for the smallest, though non-optically thin, turbulent scales. The effects of radiation on mean temperature and flow fields are to substantially decrease the thermal stratification in the core of the cavity (through different mechanisms in wall and gas radiation cases), to promote the organization of the flow near the top and bottom walls in the form of relatively thin horizontal boundary layers, and to lead to significant intensification of the mean circulation in the cavity. Both wall and gas radiation are shown to increase mechanical and thermal turbulence levels and to strongly modify the spatial distribution of the most turbulent regions. In the case of wall radiation, significant turbulence production is found upstream the vertical boundary layers due to Rayleigh-Benard instabilities in unstably stratified regions along the horizontal walls. Gas radiation is shown to act as an additional dissipation mechanism for both the mean temperature variance and the variance of temperature fluctuations. However, radiative damping of temperature fluctuations decreases as the Rayleigh number increases. (C) 2016 Elsevier Inc. All rights reserved