Un phénomène de concentration évanescente pour des flots non-stationnaires incompressibles en dimension deux

We consider a sequencev of non-stationary solutions of the incompressible 2D-Euler equation, locally bounded inL ². We prove that if the defect measure is supported in a one-dimensional set (³) of some special type (which we call finite type), the weak limitv ofv is a solution of the Euler equations: our theorem is of the type concentration-cancellation.     

Calcul de fonctions de corrélation pour des modèles réorientationnels par sauts instantanes,en utilisant la théorie des groupes

On développe un formalisme permettant le calcul des fonctions de corrélations rotationnelles pour des modèles de réorientation moléculaires par sauts instantanés, en utilisant le formalisme de la théorie des groupes. Les mouvements sont décrits par des rotations appartenant à un sous-groupe (continu ou fini) du groupe des rotations, mais les probabilités par unité de temps ne sont pas nécessairement des fonctions centrales (constantes sur les classes du groupe). On traite un cas où le mouvement est décrit par la superposition d'une diffusion rotationnelle et de sauts discrets autour d'axes cristallographiques. Ce cas est appliqué à la diffusion inélastique incohérente de neutrons sur le néopentane plastique.     

Mise en œuvre d'une méthode d'approximation diffuse adaptative pour la résolution des équations de diffusion et de transport

An adaptative diffuse approximation method for convection diffusion problems has been developed. The grid refinement method is first described and two examples are reported to illustrate the validity of the proposed method. The first one is a two-dimensionnal diffusion problem and the second one is a one-dimensionnal convection-diffusion problem.     

Etude numérique des effets de densité dans un jet de mélange turbulent en microgravité

Numerical study of density effects on a turbulent mixing jet in microgravity. The aim of this paper is to present density effects on a turbulent heterogeneous mixing free jet in microgravity. The influence of the inlet density ratio S between the jet and the surrounding gas is pointed out from numerical simulations based on second order single point closure schemes. The mean quantities, entrainment, expansion and second order moments are presented. The density ratio varies from 0,14 to 5,11. Results are discussed in comparison with both experimental data and asymptotical considerations. It is shown that the density ratio effects on the jet development vanish far from the exit leading to self-similar profiles (full self-preservation according to George [1]).     

Effet isotopique en relaxation magnétique nucléaire. Cas du système AX

La comparaison des vitesses de relaxation du proton dans les couples de molécules isotopomères ¹³C-¹HBr₃ et ¹²C-¹HBr₃, ²⁹Si-¹HCl₃ et ²⁸Si-¹HCl₃, C₆H₅ ⁷⁷Se-¹H et C₆H₅ ⁷⁶Se-¹H permet de déterminer indirectement le temps de relaxation spin-milieu des noyaux de ¹³C, ²⁹Si et ⁷⁷Se. Dans le cas de spins ½ couplés (A-X), l'analyse des courbes d'évolution des raies permet de préciser les mécanismes de relaxation qui interviennent. La méthode présente sur l'observation directe, l'avantage de la sensibilité du proton mais l'évolution des raies au cours du temps est assez compliquée. Toutefois, nous avons vérifié que les deux méthodes concordaient bien dans le cas du carbone 13 du bromoforme.     

Effet de la température sur la taille des gouttes d'un jet diesel

The fuel spray from a one-hole injector is characterized by experimental conditions close to those of the Diesel engine. Droplet sizes and velocity histories were measured using a phase Doppler analyzer in a transparent constant volume combustion chamber. This bomb was filled with nitrogen at a pressure of 2 MPa, in a temperature range of 20 to 325 °C. The combined effects of temperature coalescence and aerodynamic drag lead to an increase of the Sauter mean diameter with the penetration of the spray. At room temperature, the coalescence process predominates. At higher temperatures, this growth is mainly due to the evaporation of the smallest drops.     

Sur le pouvoir thermoélectrique de l’alliage Cu3Au en fonction de l’ordre

The thermoelectric power of Cu₃Au alloy is calculated as a function of the order with the hypothesis of free electrons, assuming that the Brillouin zones do not change at the critical point. By assuming a square-well scattering potential it is found that the thermoelectric power decreases with increasing temperature and exhibits a discontinuity at the critical point.     

Relaxation nucléaire de l'azote et du proton dans le cyanoacétylène liquide

On a étudié les temps de relaxation spin-réseau de l'azote et du proton du cyanoacétylène en phase liquide à différentes températures. La vitesse de relaxation du proton est décomposée en différentes contributions : spin-rotation, intra et intermoléculaire dipole-dipole. Les mouvements moléculaires de translation et de réorientation sont analysés en utilisant différents modèles (conditions aux limites de ‘glissement total’ et de ‘frottement total’, correction de Perrin …).     

Termes de signe positif et négatif en analyses exergétiques des flux de chaleur

Positive and negative terms in the exergetic analysis of heat fluxes. In various technical plants heat transfer rates at temperatures above that of the environment and heat transfer rates at temperatures below the temperature of the environment can be simultaneously observed. This is the case in cryogenic industrial processes where a cooling effect is produced from a heat source. This is also the case for all absorption refrigeration plants or absorption plants that provide refrigeration and heat pumping, in particular for air-conditioning of buildings. In these cases exergetic analysis leads to equations in which some terms are positive and others are negative. This can generate difficulties in the definition of exergetic, or rational, efficiencies. In this paper ways of resolving these difficulties are discussed.     

Polarisabilités optiques moléculaires de dérivés benzèniques en solution déterminées par des mesures d'effet Kerr statique

Une expression théorique simplifiée de la constante de Kerr permet une comparaison cohérente des mesures d'effet Kerr statique et de diffusion Rayleigh dépolarisée. Cette comparaison ainsi que des mesures de moments dipolaires sont effectuées sur une série de composés benzèniques en solution diluée dans le tétrachlorure de carbone. La méthode d'exploitation des résultats expérimentaux conduit à la détermination du tenseur des polarisabilités optiques moléculaires α de ces composés. Un accord satisfaisant est obtenu en comparant la constante spécifique de Kerr S _k et l'anisotropie optique moléculaire γ², calculées à partir de ces polarisabilités, à leurs valeurs expérimentales.     

Mesure du chaos dans les systèmes conservatifs en vue de l'étude des transferts dans les systèmes ouverts

Assessment of such a measure has been revealed to be especially difficult due to the lake of mathematical criteria applicable to conservative systems. Existing tools are mainly based on the characterization of the ‘strangeness’ of the attractors. These asymptotic measures are thus limited to dissipative systems. We have adapted some of these tools in order to apply them to conservative systems, based on short time observations of the system instead of asymptotic observations. In this study we have used the alternating Dean flow as a benchmark and on the basis of which the mathematical tools have been constructed. Instead of observing an attractor, we suggest observing the image formed by the cross section of a tracer filament injected upstream of the flow. Such an image is simulated by using a numerical model for the flow. With the image, we evaluate the ‘information dimension’ as well as the ‘integral correlation dimension’. Since we use short time observations, the dimensions depend on the initial position of the injected filament. However, their evolution follows the apparent disorder observed on the images. As a global measure of the chaotic behavior, we suggest calculating the mean value of the dimensions for all injection positions defined as ‘mean integral correlation dimensions’.     

Influence des phénomènes d'hystérésis sur les propriétés hydriques de matériaux poreux: Comparaison de deux modèles de simulation du comportement thermohydrique de parois de bâtiment

Porous materials hysteretic moisture characteristics : influence on water content distributions in buildings walls. Comparison of two simulation models. Two models for determining the temperature and moisture distributions within a wall are presented. The first one is a direct application of the classical Philip and de Vries model. The second one, using the suction potential, is derived from Milly's researches and is more efficient because it allows the hysteresis phenomena to be taken into account. First, the different equilibrium and transfer parameters used in the models are reviewed. Indications are given on how they can be experimentally determined and results for two cement pastes and a cellular concrete are illustrated. Then, calculation results of water content distributions for the two models are presented for an homogeneous cellular concrete wall in steady environmental conditions. In that particular case, the two models estimations are in good agreement despite different numerical treatments and experimental procedures. Using Milly's model with cyclic boundaries conditions, a significant influence of hysteresis phenomena is clearly shown.     

Sur les possibiliti es de l'analyse par activation appliquée en médecine

An fur et à mesure que l'analyse par activation se développe dans toutes les branches de l,industric et de la science, en tant que méthode analytique actuelle, son application devient det plus en plus importante dans les domaines de la médecine, de la biologie et de la biochimie, ainsi que dans les domaines des recherches et des contrôles des radioéléments destinés à usage médical.     

Etude du désenfumage par extraction mécanique dans un local siège d'un feu: expériences en grandeur et simulations numériques par modèles de zones et de champs

This study regards conditions of control of fire-induced smoke in a 7-m × 7-m rectangular cell, for an ‘intermediate configuration’ of a volume larger than that of an apartment room but smaller than that of an entrance hall, of a fire source a little less powerful than ‘design fires’ considered in standard approaches, of a fan-powered exhaust rate a little higher than rates typically required in regulations.From case to case were experimentally modified the following parameters: ceiling height (3 m, or 6.4 m), source type and power (between 100 and 500 kW), exhaust volume flow rate (from a few tenths of m³.s⁻¹ to a few m³.s⁻¹). Each experiment was supposed to reach a steady-state configuration, which has proved more or less true in practice. Smoke stratification within the cell has proved less sharp for a fire source non located in the centre of the cell, and the mean temperature of gases in the exhaust inlet has proved lower than under the cell ceiling, which suggested a phenomenon of ‘plugholing’ (air mixing upstream). In spite of the weaknesses of the computer models, and of a certain unaccuracy of measurements, plugholing phenomenon could be recognized in simulation results yielded by the zone model (which were in a better accordance with test results for higher exhaust rates), since the field model gave rise to a better agreement between computed and measured results for lower heat release rates and/or higher rates of smoke exhaust.     

Polarisation de la lumière diffusée par des particules agrégées fractales

Some of the dust particles present in the solar system are likely to be aggregates. The study of polarization of the light scattered by such aggregated particles is performed for the six main formation processes. The analysis of the results shows that an irregular and fluffy structure of the particles allows a better understanding of the light scattering observations.     

Etude des mouvements moléculaires en milieu liquide à partir du profil des bandes de vibration dans l'infrarouge et des fonctions de corrélation

We have studied the fundamental bands of chloroform and bromoform, both pure and in solution in various solvents; also the first harmonics of v ₁, and a few other harmonics and combination bands. The correlation functions of v ₁ and v ₂ and band moments of v ₁ have been calculated. The comparison of our results with those obtained in microwave and far IR and Raman spectra offers an opportunity to discern, in widths and correlation functions, what can be attributed to vibration and what originates in rotational diffusion. Our results are interpreted with the assumption that rotational diffusion is produced by small angles jumps and that the vibrational effect is very important. The v ₁ band shows an additional widening not accounted for in existing theories.     

Comportement thermique d'un matériau de référence poreux soumis à des atmosphères de chauffe différentes

This study presents an analysis of the temperature evolution in a porous sample. Various situations have been examined: the porous solid is initially dry or water saturated, and the heating fluid is dry air or steam. This analysis showed experimentally the optimal phases of using steam to heat a material. Moreover, as humidity in the solid considerably modifies the heating kinetics, the study has also been carried out on a non-porous solid. The treatment of the temperature curves under an adimensionnal form shows a singular behaviour of those temperatures following the heating fluid nature and its temperature. Equations giving temperature versus time have been proposed. They are based on the definition of a time lag and a time constant, both being defined as a function of the single fluid temperature parameter. Finally, in the particular case of the non-porous solid heated by dry air, with additional approximations, it has been possible to check that the experimental time constant is very close to a fundamental time constant.