Continuum dynamics of the intention field under weakly cohesive social interaction

We investigate the long-time dynamics of an opinion formation model inspired by a work by Borghesi, Bouchaud and Jensen. First, we derive a Fokker–Planck-type equation under the assumption that interactions between individuals produce little consensus of opinion (grazing collision approximation). Second, we study conditions under which the Fokker–Planck equation has non-trivial equilibria and derive the macroscopic limit (corresponding to the long-time dynamics and spatially localized interactions) for the evolution of the mean opinion. Finally, we compare two different types of interaction rates: the original one given in the work of Borghesi, Bouchaud and Jensen (symmetric binary interactions) and one inspired from works by Motsch and Tadmor (non-symmetric binary interactions). We show that the first case leads to a conservative model for the density of the mean opinion whereas the second case leads to a non-conservative equation. We also show that the speed at which consensus is reached asymptotically for these two rates has fairly different density dependence

Energy relaxation and heating in the afterglow of high electric field ns-discharges in ambient air using spontaneous Raman scattering

The spatio-temporal rovibrational excitation and relaxation mechanisms of N2(X) in the post-discarge of a 10 ns high-voltage diffuse discharge are studied by Spontaneous Raman Scattering. It is shown that the vibrational excitation of nitrogen molecules remains high despite the strong electric fields applied during the discharge itself and the relaxation processes are similar to lower voltage ns discharges. The main differences with the lower field discharges are rather visible at the beginning of the discharge with a specific spatial volume distribution and a significant vibrational non-equilibrium between v=0,1 and v>1. The spatial distribution of the rovibrational excitation of the diffuse discharge is very wide radially, consistent with the sustainability of fields greater than 100 Td over nearly 8 mm during propagation. The initial rovibrational excitation is inhomogeneous along the axis. The gas temperature reaches up to about 1200 K close to the pin (85 kV, ambient air) while it remains below 500 K in the rest of the volume. It is possible to control the heating of the discharge without greatly modifying the energy transfer mechanisms by adjusting the duration of the voltage pulse. In terms of reactivity, high atomic oxygen densities seem to be very localized in the vicinity of the pin (10^24 m-3 at 1.5 mm from the pin, corresponding to about 20 % dissociation). This inhomogeneity reflects the distribution of energy in the volume of the discharge. The main effects of humidity are also studied. It amplifies the fast heating and accelerates the decay of atomic oxygen in the post-discharge. No significant acceleration of the V-T relaxation of nitrogen due to the addition of water vapour was observed for the studied conditions. A shock wave was identified which is triggered at around 500 ns

Influence of water vapour on the propagation speed and mean energy of an atmospheric non-equilibrium diffuse discharge in air

We report results on the influence of humidity on the propagation and the energy of a pin-to-plane nanosecond pulse discharge at atmospheric pressure. Water vapour only impacts discharges in saturated gas mixtures, for which propagation is first slowed down, but accelerates faster than usual close to the plane. Energy is unchange

Contribution à l'étude du déclenchement de la combustion par décharge électrique en milieu diphasique

Cette étude tente de comprendre les mécanismes de déclenchement de la combustion d'un mélange d'air et de carburant à partir d'une décharge électrique en géométrie inhomogène de type pointe-plan. Les paramètres pris en compte sont d'une part la pression et d'autre part la nature diphasique du mélange. Dans une première partie, nous montrons que l'augmentation de la pression modifie le développement de la décharge en confinant la chaleur au cœur du filament. A pression atmosphérique, il est nécessaire de transiter du streamer vers l'arc pour obtenir un plasma réactif propice à l'allumage. A pression élevée, ce plasma thermique peut être obtenu beaucoup plus rapidement pendant la propagation de la décharge dans l'intervalle. Dans une deuxième partie, une étude expérimentale sur des maillages isolants et des simulations tridimensionnelles de distribution spatiale du champ électrique permettent de mettre en évidence les interactions électrostatiques d'une décharge électrique et d'un brouillard de gouttelettes isolantes. Les effets des hétérogénéités du mélange sur la décharge sont précisés dans une troisième partie par l'analyse électro-optique du développement spatio-temporel d'une décharge électrique de type streamer en présence d'un jet de gouttelettes liquides isolantes. La mise en place d'un dispositif synchronisé de caméras ultra-rapides à balayage et d'une technique de découplage spatio-temporel des courants permet de confirmer les effets observés dans la deuxième partie, à savoir une augmentation des tensions seuil de streamer et une diminution de la tension d'arc en présence de gouttelettes, et de caractériser précisément l'augmentation des vitesses de propagation de la décharge et l'effet de guidage sur son développement spatial dans les mêmes conditions. Les gouttelettes de carburant peuvent être considérées comme des réservoirs d'énergie et d'électrons germes favorisant la propagation de la décharge, le passage à l'arc et le déclenchement de la combustion.This study is an attempt to understand the mechanisms involved in the release of the combustion of an air-fuel mixture by means of an electrical discharge in a non-uniform geometry like a point-to-plane gap. The parameters taken into account are the pressure and the two-phase nature of the mixture. In a first part, it is shown that the increase of pressure modifies the discharge development, confining the heat in the core of the filament. At atmospheric pressure, it is necessary to pass from the streamer to the spark in order to obtain a reactive plasma favorable to combustion triggering. When pressure is higher, this thermal plasma can be reached much more quickly during the propagation of the discharge into the interval. In a second part, an experimental study on isolating meshes and three-dimensional simulations of electrical field space distributions reveal the electrostatic interactions between an electrical discharge and a cloud of isolating droplets. The effects of the mixture heterogeneities on the discharge can be specified in a third part by the electro-optical analysis of the space-time development of a streamer discharge in the presence of a jet of isolating liquid drops. Using a synchronised set-up with ultra-fast cameras (streak and frame) and a space-time decoupling technique of the current impulses, the effects observed in the second part can be confirmed: The increase of the streamer threshold voltages, the reduction of the spark critical voltages, a guiding effect on the spatial advancement of the discharge and an increase of its propagation velocity. The fuel droplets can be considered as local energy sources, providing primary electrons promoting the discharge propagation, the transition towards the spark and the combustion triggering.PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Etude du déclenchement de combustion de mélanges air-propane et air-heptane par décharge mono-impulsionnelle nanoseconde

De nombreuses études sont menées pour la compréhension et l'utilisation de plasmas hors équilibre pour les procédés industriels capables d'améliorer la combustion, de stabiliser des flammes et de réduire les polluants. En effet, dans le cadre des nouvelles normes européennes, il devient indispensable de pouvoir maîtriser la qualité de la combustion et de réduire ainsi les émissions polluantes. Même si le principe de l'allumage classique par étincelle est depuis longtemps connu et utilisé dans l industrie automobile, ce système présente néanmoins quelques limites. En effet, le caractère localisé de l étincelle créée réduit la probabilité de rencontre entre l étincelle et une zone de mélange inflammable ce qui conduit à des ratés d allumages et spécialement en mélanges pauvres. Ainsi, l utilisation de systèmes différents reposant sur des plasmas non-thermiques fournit des avantages significatifs, dont les propriétés de forte réactivité chimique et de faible coût énergétique. L objet principal de ce travail de thèse est l étude de l allumage de mélanges combustibles par un certain type de décharges pulsées nanosecondes. En effet, un des intérêts du déclenchement de combustion par décharges nanosecondes est le développement d une zone spatiale d allumage nettement plus étendue que celle obtenue par l étincelle de la bougie standard. Enfin, un autre avantage des décharges nanosecondes est la création de nombreux radicaux dans le milieu combustible nécessaires à l initiation directe des cinétiques de combustion en limitant la contribution thermique, souvent impliquées dans les pertes de rendement des allumeurs. Dans notre étude, la décharge nanoseconde pulsée utilisée est caractérisée par l application d une surtension très élevée donnant un pulse de tension très court (12 ns), d amplitude très élevée (50 kV) et un front de montée très raide (2 ns). Au cours de cette étude, nous avons d abord caractérisé la décharge nanoseconde pulsée dans des mélanges air/propane et air/heptane à pression atmosphérique. Ensuite, nous avons appliqué la décharge au déclenchement de combustion dans les mélanges air/propane et air/heptane dans les proportions stœchiométriques mais aussi en mélanges pauvres et ce toujours à pression atmosphérique, ce qui a montré la réduction des délais de combustion. De plus, les résultats en mélanges stœchiométriques montrent qu il existe trois modes d allumage : un ponctuel, un double et un mode cylindrique et ce en fonction de la densité d énergie.One growing topic of interest in the field of non-thermal plasmas is the use of pulsed corona discharges for ignition purposes and more specifically the use of discharges generated under very strong overvoltages for car atmospheric engine applications. Because of strong environmental constraints on car exhaust gases, engines to be developed in the future have to run with lean air / gasoline mixtures or diluted with burnt gases. In both cases, it needs the optimization of ignition devices since classical spark gaps become inefficient in these conditions. In this context, the generation of non-equilibrium plasmas on large volumes, with high densities of active species, and the ability to induce fast gas heating is challenging. Our experimental work is dedicated to the understanding of physical mechanisms involved in the ignition of lean mixtures of air and hydrocarbons such as propane and n-heptane, at high pressure, using nanosecond range discharges. Such kind of discharges could improve the energy release in the mixtures, promoting the creation of radicals and excited species instead of direct heat through Joule effect, and thus, it could improve the ignition efficiency. A positive high voltage (50 kV) is applied between a pin electrode and a grounded plane over a short nanosecond range pulse (12 ns) with a steep rise time (2 ns). In this study, the discharge has been characterised in air/propane and air-heptane mixtures. The diffuse regime observed in pure air tends to disappear in mixtures containing few percents of propane or heptane. The experimental results show the ability of the single nanosecond pulsed discharge to ignite air-propane and air-heptane mixtures even at low equivalence ratios. It is strongly correlated to the energy density the discharge is able to release into the gas. Finally, it has been shown that for stoechiometric mixtures show that three different modes of ignition are possible, i.e. a single point, a double point or a cylindrical mode, according to the energy density. Combustion delays are strongly reduced and complete combustion of very lean mixtures can be achieved if the amount of energy is slightly increased.PARIS11-SCD-Bib. électronique (914719901) / SudocSudocFranceF

N° 59 — TARDIVEAU Jean

Tardiveau Jean, Stern Marie, Mignaval Pierre. N° 59 — TARDIVEAU Jean. In: Témoins et acteurs des politiques de l'éducation depuis la Libération. Tome 4 - Inventaire de soixante-quatre entretiens. Paris : Institut national de recherche pédagogique, 2005. pp. 166-167. (Témoins et acteurs des politiques de l'éducation, 1

Lessons Learned from a Knowledge-driven Search Application on-top of Large Data Sets

International audienceThe Web stores huge amounts of data. Additionally the number of stored data is increasing permanently. Hence, while using a search application, after some time it is likely that users are confronted with unknown instances or properties while triggering their search questions. From this observation the research challenge is derived, how data can automatically be visualized for a user without leaving the search application. Here, we will present observations from a search-driven application from the field of Question Answering on-top of the Web of Data. This Web application is using Wikidata-a data source derived from Wikipedia-and several other to provide access to general knowledge and specific knowledge from particular domains. Hence, the size of the data set is very large. The problem is how to tackle the sheer amount of available instances and properties (volume), the high variety due to the ambiguity of natural language questions, and the broad field represented by a general-purpose knowledge base. Data (instances and properties) need to be visualized so that it can be explored with respect to different dimensions and allowing different granularity. Additionally, feedback interaction points were required to make the system learn over time and deal with the ambiguity of natural language questions. Concluding, in this paper we will provide an overview of the challenges we have identified and the derived solutions

Spatio-temporal evolutions of rotational and vibrational temperatures of nitrogen and oxygen of a high field atmospheric discharge in dry air by spontaneous Raman scattering

International audienc

Influence of high fields and humidity on the spatio-temporal evolutions of rotational and vibrational temperatures of N2 and O2 of a corona discharge in atmospheric air by spontaneous Raman scattering

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