Un modèle en deux points pour la turbulence compressible

Starting from an extension of the Direct Interaction Approximation to weakly compressible turbulence previously proposed by Marion et al., a one-time two-point closure is derived. This model differs from the standard Eddy Damped Quasi-Normal Markovian theory by the assumptions introduced when expressing the two-time correlations. It is stressed that both the new model and the EDQNM closure lead to a closed set of equations whose structure is essentially unaffected. Only the characteristic times differ. Both closures lead to very similar results in the case of incompressible turbulence, whereas in the case of compressible turbulence it will be shown in a future note that the results are strongly affected.     

Un modèle général de double porosité

We consider a parabolic equation with highly contrasted diffusion coefficients in the blocks set ℳ^(ɛ) and in the surrounding fissures system ℱ^(ɛ). Without a assumption, for a large range of contrast and by mean of variational homogenization, we find the global behavior when the numbers of blocks tends to infinity, for both the non stationary and the stationary model.     

Un modèle de prédiction de l'érosion des matériaux ductiles par la cavitation

Cavitation erosion is a complex phenomenon involving the interaction of hydrodynamical, mechanical, metallurgical and chemical factors. A mathematical model is proposed to predict the erosion rate as a function of cavitation conditions and material properties.     

Étude analytique de la viscosité turbulente à l'aide du modèle K-L

From the K-L turbulence model proposed by Lewellen [1], an analytical expression of turbulence macroscale is established. This expression, obtained with the hypothesis of a null horizontal pressure gradient, can be used for inertial or Couette flow. This formulation is better than the classical linear expression and, moreover, this analytical computation and numerical computations are in a good agreement.     

Un modèle du spectre des ondes de surface générées par le vent

A model of wind-generated surface waves is discussed. The energy equilibrum between excitation effects, nonlinearity, and dissipation are governed by a self-regulating process. The numerical results are compared with experimental data. © 1999 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS     

Un modèle de transfert énergétique rotation-translation lors de dissociations unimoléculaires à trois atomes

A model of rotational-translational energy transfer in the exit-channel of a three-atom unimolecular reaction. For a three-atom unimolecular reaction of the type ABC → AB + C performed in a supersonic molecular beam experiment, we propose an analytical model to describe how the rotational angular momentum of AB varies from the transition state onto the separated products. This model is compared with quasiclassical trajectory calculations on a model potential energy surface and the agreement found is very satisfying. Including this model in a statistical treatment could possibly extend its ability to describe processes involving non-negligible exit-channel rotational-translational energy transfers.     

Un modèle d'endommagement poroélastique pour milieux poreux saturés

A new anisotropic poroelastic damage model is proposed for saturated brittle porous materials. The damage impact on the poroelastic coupling coefficients is investigated. The contribution of pore pressure in damage growth mechanism is identified.     

Un modèle membranaire de coques faiblement courbées valable pour des charges élevées

In this paper, we present the construction of a membrane model for shallow shells using asymptotic approach. This model is valid for important applied loads where the tangential part is supposed to be more important than the normal part. To our knowledge, this membrane model obtained from three-dimensional elasticity, does not exist in the literature. Although it is nonlinear, it resolution can be split into two linear problems.     

Développement et validation d'un modèle mécanique de la tête humaine

This paper presents the development and the validation of a 3D finite-element human head model intended to better understand the head injury mechanisms in case of head impact. A very realistic skull geometry has been obtained by digitalisation of a dry adult skull. Based on the established knowledge of bone mechanical properties, an elastic brittle law was attributed to the skull simulated by composite shell elements. The dynamical model response was compared with two experimental impact tests from the bibliography concerning epidural pressure and a third impact involving skull fracture. The results of the model validation appear promising and the model could be a powerful tool to predict the aggressiveness level of a head impact.     

Application du modèle de chaînes de spins alternés sur le composé α Fe2PO5

In α Fe₂PO₅ the octahedrons containing the Fe²⁺ and Fe³⁺ ions share a common face. These octahedrons form zig-zag chains of alternating spin parallel to the b axis. Exchange interactions between Fe₂₊ and Fe³⁺ are ferromagnetic: this is confirmed by neutron diffraction. The alternating spin chain model, ignoring interactions between chains, was used. Our results show that αFe₂PO₅ exhibits ferromagnetic behavior.     

Identification de modèles tribologiques par pendule

A model is developed to calculate the period and damping of a rigid pendulum's oscillations when submitted to friction. A theoretical investigation shows that the pendulum is a sensitive accurate tool for assessing friction phenomena of very low intensity. The process of measurement and analysis method is then used to identify a real pendulum.     

Sur le microdosage de l'azote d'après le principe de Dumas

Résumé Les auteurs ont apporté diverses améliorations à la technique deShelberg etSchöniger de microdosage rapide de l'azote d'après le principe deDumas. Elles ont permis d'éliminer plusieurs causes d'erreurs systématiques, d'augmenter l'universalité de la méthode et de faciliter les manipulations.

---

Summary The authors have made some improvements in theShelberg andSchöniger rapid micro method of determining nitrogen based on theDumas procedure. Several sources of systematic errors have been eliminated, the range of the method has been widened, and the manipulations have been made easier.

---

Zusammenfassung Bei dem Verfahren vonShelberg bzw.Schöniger zur raschen Bestimmung des Stickstoffes nach dem Prinzip vonDumas wurden verschiedene Verbesserungen ausgearbeitet, womit mehrere Fehlerquellen ausgeschaltet, die Anwendbarkeit des Verfahrens erweitert und seine praktische Ausführung erleichtert wurden.

---

---

Dédié à l'occasion du 100^me anniversaire de la naissance deFriedrich Emich.     

Greffage de modèles de lignines sur polystyrène par lien imino

The synthesis of oligolignols grafted on polymer (LFP) was engaged in order to further study the properties and the biodegradation mechanisms of the different lignin sub-units in conditions more similar to those involved in the natural polymer. Simplified lignin models were grafted by an imine or amino link to a 1 % amino ethyl polystyrene resin without any protection of the hydroxyl functions. Infrared was used to characterize the supported models. © 2000 Académie des sciences / Éditions scientifiques et médicales Elsevier SASsupported lignine / synthesis / lignin models     

Etude comparative de modèles de re'acteurs catalytiques à lit fixe

This paper deals with the comparison of theoretical reactor performance and measurements in an actual non-adiabatic non-isothermal fixed bed. The experimental scheme relies mainly on the measurement of radial temperature profiles at various axial distances and of the average outlet conversion for a set of nine different experiments.The intrinsic reaction rate and internal diffusion coefficients have been determined separately on a laboratory integral reactor. Heat transfer parameters were obtained from temperature profiles without reaction in the pilot plant reactor.The results given by three different reactor models are analysed. Good agreement is observed between the experiments and results computed from a heterogeneous two-dimensional model. Two simpler models are also assessed and it is shown that the one-dimensional one fails to represent the experimental results.     

Étu de de la zone cathodique d'un arc électrique

The plasma in the neighbourhood of the cathode tip of an electric arc shows spatial dependence on its physical equilibrium state. Near the axis the plasma is in the local thermodynamic equilibrium (LTE) state. Toward the outer zones, the plasma state deviates from the LTE equilibrium. This deviation is related to the distance from the cathode tip rather than to the electron temperature and density. In our opinion, it is caused mainly by the ArI ground state overpopulation. Results of our experiments support this conclusion. This overpopulation is caused by inward transport of ground state atoms into the plasma column due to the fast flow of the plasma near the tip. Our theoretical estimations of the ArI ground state overpopulation give results consistent with the experiment.     

Approche du phénomène de galop par un modèle d'effort retardé et validation expérimentale

We consider the case of elongated cylinders submitted to a cross-flow. We propose a time delayed force model for the galloping oscillations for which the linear study allows to calculate the critical velocity with quasi-steady coefficients. The phase delay between force and position is based on the Strouhal number of the cylinder measured in static conditions. The experimental validation is presented on square and rectangular sections cylinders.     

Un nouveau système de coordonnées pour les équations de Darcy-Muskat diphasiques en milieux poreux hétérogènes isotropes à deux dimensions

We address the formulation of two-phase flows in the context of a Darcy-Muskat (DM) model in terms of a saturation/global stream-function (or global pressure) coupling in heterogeneous isotropic porous media. In two dimensions the coordinate system, associated with some infinite capillary limit global stream function-global pressure, allows the saturation evolution to be unidimensional. We show that the complete (DM) model projected on this coordinate system can be very efficiently solved numerically.