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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.

International audience; In spite of the crash of Schiaparelli, the Exomars Descent Module in October 2016, following a nevertheless successful aerothermodynamic entry, data from the infrared radiometers ICOTOM embedded of the COMARS modules was sent to the orbiter before and after the blackout phase. Three pairs of radiometers were located in line on the back shield of the probe in order to monitor the infrared radiation received by the thermal protection system from CO and CO2 molecules. Within a pair, one ICOTOM (B1) was dedicated to the range 4,17-5 µm (2000-2400 cm-1) and the other one (B2) was dedicated to the range 2,6-3,36 µm (2950-3850 cm-1) in order to collect cold and CO2 bands as well as CO rovibrational radiation. Flight data provided radiative heat flux densities in both ranges for three locations in the shield, especially at the end of the hot phase. Lower housing temperatures than expected led to recalibration of the ICOTOM depending on the incoming flux and on the sensor own temperatures.In order to derive of maximum of information about the thermodynamic status of the gas behind the spacecraft, CNES gathered laboratories and companies to rebuild the aerodynamic field, the chemical composition, the non-equilibrium status and the radiative transfer within the back body plasma from numerical simulation and on-ground experiments.The main purpose is to obtain a comprehensive numerical tool able to recover the flight measurements and to deal with similar entry situations in the future. Only few hypotheses were made especially about the equilibrium status of the plasma and about the angle of attack of the probe. Then the implicit-in-time code has been developed for 3D geometry and includes partial state-to-state chemistry. Kinetic chemical models developed for 0D and 1D geometry were reduced in order to make them compatible with reasonable computation times. However, a full (vibrational and electronic) model is used along lines in the flow with a Lagrangian approach. A specific scheme has been established to describe the vibrational structure of CO2. That model has been made compatible with chemical and radiative calculations. Solving the radiative transfer equation within the back body plasma implies to take into account possible non-equilibrium effect in CO2 vibrational populations. Following a line-by-line model working out of equilibrium, a statistical narrow-band model was developed in order to reduce the time cost of calculations involving aerodynamics, chemistry and radiation. However, radiation transfer calculations were not coupled to the aerothermochemistry calculations and are carried out as a post-treatment on the fields of densities and temperatures (when applicable).Another important part of the project consists of ground tests of ICOTOM radiometers on plasmas close to those encountered by Schiaparelli. That experimental approach is to confront the signals given by the ICOTOM radiometers and the heat flux densities derived from them with laboratory measurements obtained by optical and laser spectroscopy and interferometry. Ground plasma flows are then used as test cases both for ICOTOM measurements and for numerical simulation. Various facilities such as ICP and arc-jet plasmas were used within that study.

I - Physique et applications atmosphériques des filaments de plasma générés par des impulsions laser ultrabrèves (Jérôme Kasparian, Jean Pierre Wolf) Spectroscopie d'émission optique sur plasma induit par laser : Principe et applications (Patrick Mauchien, Catherine Gallou, Jean Luc Lacour, Laurent Salmon) II-2. Spectroscopie de fluorescence induite par diodes laser : Application au diagnostic des plasmas (Stéphane Mazouffre) II-3. Un capteur de flux d'énergie dans les plasmas (A.-L. Thomann, N. Semmar, R. Dussart, L. Bedra, J. Mathias, Y. Tessier et S. Mazouffre) III-1. Modélisation de la photoionisation dans les plasmas d'air non-thermiques à la pression atmosphérique (Anne Bourdon, Sébastien Célestin, Julien Capeillère et Pierre Ségur) III-2. Modélisation des plasmas produits par interaction laser-matière (Tatiana E. Itina) IV-1. Propriétés et transferts radiatifs dans les plasmas (M.Y. Perrin, A. Soufiani, Ph. Riviere) IV-2. Modifications induites en surface de polymères par traitement laser (Patricia Laurens, Sophie Petit) IV-3. Lampes fluorescentes sans mercure, à la recherche de l'efficacité énergétique (Eric Robert, Sébastien Point, Sébastien Dozias, Christophe Cachoncinlle, Raymond Viladrosa et Jean Michel Pouvesle) IV-4. Sources plasmas de rayonnement X créées par laser (Fabien Dorchies) Importance du rayonnement dans les procédés thermiques industriels (Yann Cressault, Philippe Teulet, Jean-Jacques Gonzalez, Alain Gleizes) La spectroscopie optique appliquée à l'étude des disjoncteurs (Dunpin Hong, Jean-Marc Bauchire et François Gentils) Exemples de micro plasmas à haute densité : Électroérosion et ouverture de contacts électriques (Jean-Luc Dorier, Antoine Descoeudres, Christoph Hollenstein) Chauffage et génération de courant par ondes dans les tokamaks (Gérard Bonhomme)

International audience; In this work, we study quantitatively the efficiency of a Monte Carlo technique using a space partitioning recursive oct-tree algorithm applied to radiative transfer calculations. The obstacles are embedded in finer parallelepipedic volumes than the free space left for photon propagation. This enables faster propagation of the photons inside the obstacle-free space but, for absorbing media, this does not allow the progressive deposit of the absorbed energy in each crossed cell. Three algorithms have been implemented for general geometries together with fast intersection methods. Comparisons for a complex geometry are made first for hypothetical gray media to assess clearly the effects of optical thickness and then for an electric discharge-like air plasma at local chemical and thermal equilibrium using a global spectral model. When compared to standard forward and energy partitioning approach, the oct-tree algorithm is shown to reduce the CPU time for wall flux calculations but not for radiative volumetric powers at equivalent statistical convergence.

International audience; The spectroscopic data required for radiative transfer calculations in CO2–N2 plasmas are investigated. For diatomic electronic systems, we extend the systematic approach previously developed in Chauveau et al. [Contributions of diatomic molecular electronic systems to heated air radiation. JQSRT 2002;72:503–30], to the carbonaceous molecules C2, CN, CO, and . The calculated vibrational transition moments are systematically compared to experimental data or other calculations when available. The positions and intensities of the rotational lines have been determined in intermediate a/b Hund's coupling case with up-to-date molecular constants. The other important mechanisms include triatomic (CO2) rovibrational transitions, atomic and ion lines, and continuum radiation. The analysis of the optically thin emission of pure CO2 and CO2(97%)–N2(3%) mixture plasmas at chemical and thermal equilibrium in the temperature range 2000–15 000 K shows a persistence of CO2 IR radiation predominance up to about 3800 K in spite of its small molar fraction. The analysis confirmed the importance of the CO fourth positive system for temperatures between 5000 and 7000 K and of atomic lines above. The contribution of continuum radiation to the optically thin emission is found to be weak. Vibrational Einstein coefficients and Franck–Condon factors of the investigated diatomic molecules are provided as supplementary material and the corresponding rotational line lists are available under request.

International audience; The technique which makes it possible to simultaneously determine the temperature T and the partial pressure of carbon dioxide in a vibrationally equilibrium gas mixture at atmospheric pressure by using the spectral distribution of the unsaturated absorption coefficient at the oscillation lines of a tunable CO2 laser is presented. The results of model calculations of the influence of H2O molecules upon the precision of determining temperature and partial pressure of CO2 in CO2:N-2:H2O=10:80:10 mixture at full pressure 1 atm and temperature T=1000 K have given.