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Planar pulsating traveling wave solutions of non-cooperative Fisher--KPP systems in space-time periodic media
No full textNon-cooperative Fisher-KPP systems with space-time periodic coefficients are motivated for instance by models for structured populations evolving in periodic environments. This paper is concerned with entire solutions describing the invasion of open space by a persistent population at constant speed. These solutions are important in the understanding of long-time behaviors for the Cauchy problem. Adapting methods developed for scalar equations satisfying the comparison principle as well as methods developed for systems with homogeneous coefficients, we prove, in each spatial direction, the existence of a critical speed such that: there exists no almost planar generalized transition waves with a smaller speed; if the direction is rational, each rational speed not smaller than the critical speed is the speed of a weak planar pulsating traveling wave; if the coefficients are homogeneous in space, each speed not smaller than the critical speed is the speed of a strong planar pulsating traveling wave
Constraining the Low-Temperature Oxidation Mechanism of n-Hexanol through the Detection and Identification of C<sub>6</sub> Elusive Intermediates
No full textInternational audienceAlcohol-based fuels are currently considered viable alternatives to reduce the carbon footprint of the transportation sector. Consequently, a comprehensive mechanistic understanding of the lowtemperature oxidation of alcohols is essential for application in advanced low-temperature compression engines. In this work, a multidimensional approach involving experimental investigations, kinetic modeling, and theoretical calculations was used to provide new insights into the low-temperature oxidation mechanism of a C6 alcohol, n-hexanol (C3(CHC2)C5OH), through the detection and identification of elusive C6 intermediates. The oxidation of n-hexanol was investigated in a jet-stirred reactor (JSR) under stoichiometric conditions (ϕ = 1.0), an initial fuel concentration of 2%, a residence time of 2 s, a temperature range between 500 K -660 K, and a pressure of 700 Torr. The reactants, intermediates, and final products were detected and identified by means of molecular-beam mass spectrometry coupled with single-photon ionization employing tunable synchrotron-generated vacuum ultraviolet radiation. Chemical kinetic simulations were performed using a previously published kinetic model (Togbé et al., Energy Fuels 2010, 11, 5859-5875) to predict the reactivity of n-hexanol and elucidate the predominant formation pathways of the observed low-temperature species. Experimental photoionization efficiency curves in conjunction with ab initio calculations, enabled the identification of important low-temperature species, such as C6 unsaturated alcohols, C6 olefinic hydroperoxides, C6 cyclic ethers, C6 diones, and C6 ketohydroperoxides. The results of this study provide valuable insight into the mechanism of low-temperature oxidation chemistry of n-hexanol, contributing to the development of kinetic models for the low-temperature oxidation of n-hexanol and other long-chain linear alcohols
Portage sur GPU du calcul des forces internes d'un code basé sur la méthode des éléments finis spectraux
No full textNational audienceLa méthode des éléments finis spectraux est utilisée dans de nombreux codes de simulations. Ces codes sont très coûteux en terme de calculs et nécessitent donc d'être parallélisés. Dans le cadre de la simulation de la propagation d'ondes sismiques, nous proposons de faciliter la parallélisation sur GPU du cœur de cette méthode en s’appuyant sur une approche existante vectorisée sur CPU. L’attention est portée sur l’utilisation de la mémoire partagée et les éventuels conflits d’accès. La méthode proposée pour le portage sur GPU permet d'obtenir un speedup de 7.7 par rapport à la version vectorisée sur CPU utilisant un cœur
On the characterization of highly oxidized products from cool flames in a continuously-stirred tank reactor
No full textInternational audienceTerpenes could be used as renewable fuel in the future. However, these chemicals are relatively complex. Therefore, model-fuels could be used to better understand their oxidation pathways. Methylcyclohexenes were selected for that purpose in this work
A transparent IHPV for the in-situ geochemical characterization of magmatic volatile phases and melts
No full textInternational audienceChemical exchanges between magmas and volatile-rich fluids and gases are fundamental processes of magmatic and volcanic activity, but also play a critical role in the formation of various ore deposits. Yet, the composition and properties of the so-called magmatic volatile phases (MVPs) remain elusive, due to difficulties in their sampling, both in natural systems and in experimental laboratories.Here, we present a novel ‘transparent’ internally-heated high-pressure vessel (T-IHPV) that enables the geochemical characterization of coexisting hydrous melts and MVPs in-situ, under typical shallow magmatic conditions. The experimental design is validated through the observation of haplogranite and rhyodacite melting to 900 °C and 130 MPa and the in-situ X-ray absorption (XAS) study of bromine and strontium distribution and speciation in the haplogranite-H2O system to 800 °C and 100 MPa. These preliminary experiments confirm the efficient partitioning of Br in MVPs in differentiated systems (around 41 for haplogranite) and reveal the stability of hydrated Br species instead of HBr in the MVPs. Coupled to other spectroscopic methods (Raman, SAXS/WAXS, XRD), we expect the T-IHPV to shed a new light on the thermodynamics and kinetics of chemical reactions at stake in shallow magmatic and hydrothermal reservoirs
Investigating Lubricants Base Oils Reactivity in Ammonia Engines via Accelerated Aging
No full textInternational audienceAs a carbon-free molecule, ammonia is more and more considered as a relevant fuel for long distance and off-road applications. However, this gas has different combustion characteristics compared to conventional fuels, challenging the suitability of lubricants to such engines. In this work, the evolution of lubricants under conditions mimicking ammonia combustion was assessed. Mineral and polyester lubricant base oils were exposed to oxygen, nitrogen oxides, and ammonia in a pressurized reactor under stirring. Oil aliquots were sampled at regular intervals, and characterized using Fourier Transform Infrared Spectroscopy (FTIR), viscosity and total oxygen and nitrogen contents measurements. Exposure to air containing nitrogen oxides resulted in quicker accumulation of oxidation products compared to neat air, for both the mineral and complex polyester base oil. Besides, exposure to gaseous ammonia in air resulted in a slower oxidation rate for both oils, compared to neat air. A global measurement of the total nitrogen content after 2 h showed a significant increase for both oils. Under similar conditions, the total nitrogen content of the polyester base oil was higher than the mineral oil, indicating a higher affinity of the ammonia with this matrix. Chromatograms obtained with a Gas Chromatography (GC) column associated to a Nitrogen Chemiluminescence Detector (NCD) showed that nitrogen speciation evolved from nitrogen contained in very light molecules, likely dissolved ammonia, to molecular nitrogen distributed across the entire range of hydrocarbon chain lengths. This demonstrates that ammonia reacted with the base oils and their degradation products.Two-dimensional gas chromatography and mass spectrometry will be performed in the upcoming months to identify these new kinds of degradation products. This study paves the way towards a better understanding of oil reactivity when exposed to ammonia combustion byproducts, including outcomes on engine operation and emissions of pollutants
"Ce roman a toute l'allure d'un mélodrame de seconde classe". Usages et fonctions de mélodramatique dans la critique romanesque (1829-1857)
No full textInternational audienceVers 1830, la critique littéraire s’empare des étiquettes mélodrame et mélodramatique dans sa croisade anti-romanesque. Ce crochet transgénérique constitue un trait phraséologique récurrent, dont seront étudiés les usages et les fonctions entre 1829 et 1857. Vidés de leur sens générique, ces termes s’intègrent à la terminologie biaisée d’une critique qui, faute de poétique propre au roman, prend appui sur l’imaginaire des genres du XIXe siècle pour jouer son rôle descriptif, démarcatif et normatif
Caractérisation et identification des étangs de pisciculture actuels et potentiels des Pays de la Loire
No full textFinancements intermédiés et fragilité en Afrique
No full textNonobstant le grand intérêt sur la question du financement des Etats fragiles, très peu d'études se sont penchées jusqu'ici sur l'impact des ressources externes sur la fragilité dans les pays. Cette étude vient compléter la littérature et se penche sur l'impact spécifique des financements intermédiés sur la fragilité des pays africains. L'étude utilise la méthode de l'équilibrage entropique et la méthode de l'appariement par score de propension appliquée à un panel de 50 pays africains sur la période 2009-2019, pour estimer l'effet des financements intermédiés sur la fragilité. Les résultats sont significatifs et montrent que le financement intermédié réduit la fragilité des pays concernés. Le test de robustesse confirme cet impact positif et significatif indépendamment de la source de financement. Ces résultats devraient intéresser les décideurs politiques, les institutions financières internationales et les banques centrales, de même que les chercheurs dans la formulation des politiques et stratégies d'inclusion financière et de renforcement de la résilience dans les Etats fragiles.</div