101 research outputs found
Simulations numériques 3D d'écoulements de convection naturelle à haut nombre de
Nous prĂ©sentons une Ă©tude numĂ©rique 3D d'Ă©coulements de convection naturelle instationnaires en cavitĂ© fermĂ©e parallĂ©lĂ©pipĂ©dique dans laquelle est placĂ© un barreau chauffant cylindrique de section carrĂ©e, alignĂ© suivant l'axe longitudinal. Le fluide considĂ©rĂ© est de l'air. Deux configurations gĂ©omĂ©triques sont traitĂ©es, l'une oĂč la position du barreau est centrĂ©e, l'autre dĂ©centrĂ©e. Le nombre de Rayleigh associĂ© au flux de chaleur et Ă la hauteur du barreau est Ra= 6,04.108. Afin de minimiser les coĂ»ts en temps de calcul, le domaine de simulation peut ĂȘtre restreint Ă la zone mĂ©diane de la cavitĂ©. La configuration d'Ă©tude est alors dĂ©finie comme un domaine pĂ©riodique de longueur caractĂ©ristique Ly. Nous montrons l'influence de Ly sur les caractĂ©ristiques dynamiques intrinsĂšques de l'Ă©coulement. Par ailleurs, Nous discutons de la pertinence des rĂ©sultats obtenus dans le cadre de conditions de simulation spĂ©cifiques (non prise en compte du rayonnement, adiabaticitĂ© des parois, flux de chaleur fixĂ©), par comparaison avec des mesures expĂ©rimentales
Modelling of natural convection flows with large temperature differences : a benchmark problem for low Mach number solvers. Part 1, Reference solutions
There are very few reference solutions in the literature on non-Boussinesq natural convection flows. We propose here a test case problem which extends the well-known De Vahl Davis differentially heated square cavity problem to the case of large temperature differences for which the Boussinesq approximation is no longer valid. The paper is split in two parts: in this first part, we propose as yet unpublished reference solutions for cases characterized by a non-dimensional temperature difference of 0.6, (constant property and variable property cases) and (variable property case). These reference solutions were produced after a first international workshop organized by CEA and LIMSI in January 2000, in which the above authors volunteered to produce accurate numerical solutions from which the present reference solutions could be established
Seuils de stabilité pour un écoulement engendré dans une cavité cylindrique avec surface libre et fond tournant
Par une étude de stabilité linéaire nous
nous proposons d'analyser un écoulement dans une cavité cylindrique fixe remplie de
fluide, dont le fond tourne. Le rayon de la cavité est grand devant la hauteur de
fluide. Ce travail s'inspire d'une étude expérimentale récente, réalisée à Marseille,
(Poncet & Chauve 2007) qui a mis en évidence une instabilité aux motifs
particuliÚrement intéressants. La rotation et la présence d'une surface libre rendent
difficile la simulation numérique car la déformation de cette derniÚre impose d'utiliser
des codes de calculs permettant de prendre en compte des géométries complexes. Dans une
premiÚre approche, nous avons utilisé une modélisation de la surface libre plane
(conditions de symétrie) afin de comparer les résultats expérimentaux et notre étude de
stabilité linéaire. Nous discuterons de la pertinence de cette modélisation c'est à dire
de la nécessité de prendre en compte la déformation de la surface
Simulation de l'écoulement dans un canal différentiellement chauffé
Nous Ă©tudions numĂ©riquement l'Ă©coulement de convection naturelle dans un canal vertical soumis Ă une stratification. Des conditions aux limites pĂ©riodiques sont imposĂ©es dans la direction verticale. L'influence de la stratification et du nombre du Rayleigh est examinĂ©e. Un modĂšle peut ĂȘtre utilisĂ© pour le rĂ©gime faiblement nonlinĂ©aire autour de la premiĂšre bifurcation. Au-delĂ d'un nombre de Rayleigh plus Ă©levĂ© on observe l'apparition d'ondes solitaires
Global carbon budget 2019
Accurate assessment of anthropogenic carbon dioxide (CO2) emissions and their redistribution among the atmosphere, ocean, and terrestrial biosphere â the âglobal carbon budgetâ â is important to better understand the global carbon cycle, support the development of climate policies, and project future climate change. Here we describe data sets and methodology to quantify the five major components of the global carbon budget and their uncertainties. Fossil CO2 emissions (EFF) are based on energy statistics and cement production data, while emissions from land use change (ELUC), mainly deforestation, are based on land use and land use change data and bookkeeping models. Atmospheric CO2 concentration is measured directly and its growth rate (GATM) is computed from the annual changes in concentration. The ocean CO2 sink (SOCEAN) and terrestrial CO2 sink (SLAND) are estimated with global process models constrained by observations. The resulting carbon budget imbalance (BIM), the difference between the estimated total emissions and the estimated changes in the atmosphere, ocean, and terrestrial biosphere, is a measure of imperfect data and understanding of the contemporary carbon cycle. All uncertainties are reported as ±1Ï. For the last decade available (2009â2018), EFF was 9.5±0.5âGtCâyrâ1, ELUC 1.5±0.7âGtCâyrâ1, GATM 4.9±0.02âGtCâyrâ1 (2.3±0.01âppmâyrâ1), SOCEAN 2.5±0.6âGtCâyrâ1, and SLAND 3.2±0.6âGtCâyrâ1, with a budget imbalance BIM of 0.4âGtCâyrâ1 indicating overestimated emissions and/or underestimated sinks. For the year 2018 alone, the growth in EFF was about 2.1â% and fossil emissions increased to 10.0±0.5âGtCâyrâ1, reaching 10âGtCâyrâ1 for the first time in history, ELUC was 1.5±0.7âGtCâyrâ1, for total anthropogenic CO2 emissions of 11.5±0.9âGtCâyrâ1 (42.5±3.3âGtCO2). Also for 2018, GATM was 5.1±0.2âGtCâyrâ1 (2.4±0.1âppmâyrâ1), SOCEAN was 2.6±0.6âGtCâyrâ1, and SLAND was 3.5±0.7âGtCâyrâ1, with a BIM of 0.3âGtC. The global atmospheric CO2 concentration reached 407.38±0.1âppm averaged over 2018. For 2019, preliminary data for the first 6â10 months indicate a reduced growth in EFF of +0.6â% (range of â0.2â% to 1.5â%) based on national emissions projections for China, the USA, the EU, and India and projections of gross domestic product corrected for recent changes in the carbon intensity of the economy for the rest of the world. Overall, the mean and trend in the five components of the global carbon budget are consistently estimated over the period 1959â2018, but discrepancies of up to 1âGtCâyrâ1 persist for the representation of semi-decadal variability in CO2 fluxes. A detailed comparison among individual estimates and the introduction of a broad range of observations shows (1) no consensus in the mean and trend in land use change emissions over the last decade, (2) a persistent low agreement between the different methods on the magnitude of the land CO2 flux in the northern extra-tropics, and (3) an apparent underestimation of the CO2 variability by ocean models outside the tropics. This living data update documents changes in the methods and data sets used in this new global carbon budget and the progress in understanding of the global carbon cycle compared with previous publications of this data set (Le QuĂ©rĂ© et al., 2018a, b, 2016, 2015a, b, 2014, 2013). The data generated by this work are available at https://doi.org/10.18160/gcp-2019 (Friedlingstein et al., 2019)
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