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We investigate Rayleigh-Bénard convection in a cubical cavity under the effect of wall and gas radiation. Coupled direct numerical simulations are carried out for a radiating air / \chemform{H\_2O} / \chemform{CO\_2} mixture at room mean temperature, using a Chebyshev spectral method for the flow and a ray-tracing method for the radiation field The global absorption distribution function (ADF) model is used to represent the spectral radiative properties of the mixture. Time-averaging is then applied to compare the results, regardless of the multiple flow configurations obtained. Three different Rayleigh numbers are studied, from Ra = 1E7 to Ra = 1E9. Under the Boussinesq approximation, solutions to the uncoupled simulations only depend on the Rayleigh and Prandtl numbers and are not affected by the size of the cavity. However, it goes otherway when radiation is taken into account ; two cavitiy sizes are then considerated with edges of 1 and 3 meters long. As the cavity size increases, at fixed Rayleigh number, so does the effects of radiation on the flow. Radiation has a stabilizing effect on the large-scale circulation. The large-scale circulation settles in vertical mid-planes or diagonal planes depending on the radiation conditions, and reorientations are occasionally observed in the cavity. In the same time, the convective flux in the core, as well as the conductive flux at the walls and the kinetic energy, are increased as radiation is taken into account, and this increase is more important for the larger size of the cavity. The contribution of radiation to the potential energy balance and to the "thermal energy" balance are presented. Proper orthogonal decomposition (POD) analysis is then used in order to better understand the evolution of the flow structures and the influence of radiation. The shape of the modes, as well as their symmetries and temporal dynamics, are discussed. The influence of the modes in the reorientation process is also studied.

International audience; This paper presents a reduced-order modelling strategy for Rayleigh–Bénard convection of a radiating gas, based on the proper orthogonal decomposition (POD). Direct numerical simulation (DNS) of coupled natural convection and radiative transfer in a cubic Rayleigh–Bénard cell is performed for an air/H2O/CO2 mixture at room temperature and at a Rayleigh number of 107 . It is shown that radiative transfer between the isothermal walls and the gas triggers a convection growth outside the boundary layers. Mean and turbulent kinetic energy increase with radiation, as well as temperature fluctuations to a lesser extent. As in the uncoupled case, the large-scale circulation (LSC) settles in one of the two diagonal planes of the cube with a clockwise or anticlockwise motion, and experiences occasional brief reorientations which are rotations of π/2of the LSC in the horizontal plane. A POD analysis is conducted and reveals that the dominant POD eigenfunctions are preserved with radiation while POD eigenvalues are increased. Two POD-based reduced-order models including radiative transfer effects are then derived: the first one is based on coupled DNS data while the second one is an a priori model based on uncoupled DNS data. Owing to the weak temperature differences, radiation effects on mode amplitudes are linear in the models. Both models capture the increase in energy with radiation and are able to reproduce the low-frequency dynamics of reorientations and the high-frequency dynamics associated with the LSC velocity observed in the coupled DNS.

Models and computational schemes are presented to simulate the flow field and radiation in solid propellant rocket plumes at high altitude. Different models are developed to calculate plume radiation induced by both the gas and the dispersed phase of alumina particles. Simulations were made to reproduce a 110 km altitude rocket plume and comparisons with experimental data on the plume radiance are presented. Influence of different physical processes on the simulation of the plume flow field and radiation is discussed, such as alumina phase change, scattering and the coupling of radiation with the two-phase flow.