Evaluation de la fraction volumique locale de la phase solide dans un écoulement diphasique

Local particle volume fraction measurements in two-phase flows are rare. Generally, the concentration is supposed to be spatially homogeneous in the sedimentation flows fundamental experiments. This is far to be realistic in regimes different from Stokes' regime, particularly near the walls. This paper compares two methods of evaluation of the volume fraction in a three-dimensional two-phase flow.     

Une technique de suivi d'interfaces pour la modélisation de la cavitation par poche

A tracking method is presented for the modeling of partial and supercavitation. The velocity and pressure fields in the cavitating flow are computed by a Navier–Stokes solver using a pseudo-compressibility method. The cavity flow is computed from the velocity field by a tracking method based on a volume of fluid technique (VOF). The method is illustrated by several computations, two cases of partial cavitation on a hydrofoil and a case of a cavitating body emerging at a free surface.     

Représentation de la ligne de contact mobile dans les équations de la mécanique des fluides

Although the contact angle at equilibrium has a well understood theory for the case of flat homogeneous solid surfaces, the displacement of the contact line is still not well understood. We propose to introduce in the dynamics of the contact line a mobility relation between the deviation of the contact angle out of its equilibrium value and the speed of the line on the solid. When the line slides on the solid thanks to an evaporation/condensation process, this introduces a dynamical Arrhenius factor that may be sufficiently small to make the mobility of the contact line the limiting factor of the dynamics in many physical situations. Then, the shape of the liquid/vapor interface will be in quasi equilibrium giving a contact angle that will define ultimately the speed of the contact line.     

Nouveau fondement fonctionnel de la Mécanique

Sans résumé Mémoire transmis par C. Truesdell     

Effet de la rugosité sur un fluide laminaire avec conditions de Fourier

The effect of tiny asperities covering a wall on a flow governed by Stokes equations with Fourier boundary conditions is investigated. We calculate the limit flow and we give estimates of the deviations of the drag, velocity field and pressure, in terms of the size ε of the asperities. In the particular case of a plate, the limit drag is larger than the drag of the smooth wall, in contrast with the situation found for Dirichlet boundary conditions.     

Extension de la DRBEM à la propagation guidée en écoulement cisaillé

The present study aims to extend the Dual Reciprocity Boundary Element Method in order to solve acoustic wave propagation equations in the frequency domain for a parallel shear flow. The Linearized Euler Equations are written as a coupled pair of equations, which are second-order in terms of acoustic pressure and first-order in terms of normal acoustic velocity. Good agreement between numerical results and analytical solutions for a low Mach number shear flow (M<0.1) shows the interest of the method.     

Application de la méthode des approximations successives à la résolution d'un problème aux limites de l'hydrodynamique

Sans résumé Mémoire transmis par R. Finn     

Fonction de distribution pour les gaz polyatomiques dans la couche de Knudsen. Solution approchée de l'équation de Boltzmann

Polyatomic gases in strong non equilibrium vibrational state are studied in the Knudsen layer. A kinetic equation is derived from the Boltzmann equation for a stationary gas without macroscopic velocity. The simplification are basically deduced from the order of magnitude of adimmensional gradient terms. The approximate solution of this equation is deduced from a recurrent algorithm on the adimensional space variable power. Furthermore, the boundary condition allows us to obtain density and temperature jumps at the wall.     

Effet du couplage thermomécanique sur la propagation d'un front de changement de phase

Calorimetric effects related to the propagation of a phase change front in a monocrystalline shape memory alloy CuZnAl were derived from thermographic data analysis. At constant load, the displacement of the front induces a creep of the sample strongly depending on thermal exchanges with the surroundings. The main role played by the thermomechanical couplings can be pointed out by reversing the heat flux at the boundary of the sample: this leads to an inversion of the front propagation way associated with a recovery of the creep strain.     

Sur la nature de la transition à l'instationnaire d'un écoulement de convection naturelle en cavité différentiellement chauffée à grands écarts de température

Natural convection of air inside a rectangular cavity, differentially heated under large temperature gradients, is considered. The low Mach approximation equations are those obtained by Paolucci allowing for filtering of sound waves. Transition to unsteadiness is studied with numerical simulation, with a finite volume code based on a fractional time step method derived from projection methods used for incompressible flows. When the fluid physical properties are prescribed constants, transition to unsteadiness follows the classical scheme of a Hopf bifurcation. The transition is quite different when viscosity obeys Sutherland's law while the Prandtl number is kept constant. There is evidence of hysteresis, therefore the transition seems to be subcritical. In the vicinity of the transition, on the large amplitude branch, an intermittent solution is observed, with periodic bursts separating quasi-steady states.     

Variation de la vitesse de propagation d'ondes ultrasoniques au cours du passage de l'état liquide à l'état solide

Résumé Une étude théorique de la vitesse de propagation des ondes ultrasoniques dans les liquides et solides rend plausible l'hypothèse que l'évolution de l'état liquide à l'état solide doit être accompagnée d'une variation de la vitesse de propagation des ondes ultrasoniques.Il résulte des essais d'orientation relatés dans ce mémoire que la variation de la vitesse de propagation des ondes ultrasoniques est susceptible de caractériser la vitesse de passage de l'état liquide à l'état solide d'une matière ou d'un matériau.La méthode est applicable dans un domaine étendu et est sensible même assez loin du point de solidification. Elle se prête à l'étude du processus de l'évolution de la solidification des suspensions, des solutions, des matériaux organiques à longue chaîne, etc.

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Zusammenfassung Die theoretische Überlegung über die Geschwindigkeit der Ausbreitung von Ultraschallwellen in Flüssigkeiten und festen Körpern kann die Hypothese angenommen werden, daß die Umwandlung vom flüssigen in den festen Zustand von einer Geschwindigkeitsänderung der Ultraschallwellen begleitet wird.Aus den in diesem Bericht gebrachten Einführungsversuchen ergibt sich, daß die Variation der Ausbreitung von Ultraschallwellen dazu geeignet ist, die Umwandlungsgeschwindigkeit der Materialien von flüssigem in festen Zustand zu bewerten.Die Methode ist in einem weitem Bereiche brauchbar und ist auch weit vom Erstarrungspunkt empfindlich. Sie kann angewendet werden, um den Erstarrungsprozeß von Suspensionen, Lösungen und organischen Stoffen zu untersuchen.

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Ingénieur des constructions civiles. Docteur spécial en connaissance des matériaux. Agrégé près la Faculté des Sciences Appliquées de l'Université de Liège (Belgique).     

Des modèles d'arcs élastiques suivant l'ordre de la courbure et les types de chargement

After reconsidering a classification of the homogeneous isotropic elastic arches, based on the curvature of the middle line, we determine a set of asymptotic models for each category of arch in terms of the assumptions made regarding the work of applied efforts.     

Phénomènes transitoires au cours de la déformation plastique à chaud de l'acier

Sans résumé     

Étude numérique des instabilités de la phase fluide et de l'interface de solidification en croissance dirigée horizontale

Convective instabilities of low Prandtl number fluids are subject to various studies. The purpose here is an extension to hydrodynamic transitions in the fluid phase with a free or rigid surface under phase change and to construct the corresponding bifurcation diagram. Investigations interesting the process control by using the latent heat and the solid/liquid thermal conductivity difference are considered. The solution methodology is based on a front localisation approach showing a very good ability to describe convection regimes coupled to the phase change transition in the case of vertical solidification with confinement. To cite this article: E.A. Semma et al., C. R. Mecanique 331 (2003).     

Mécanique de la rupture fragile en présence de plasticité : définition d'un taux de restitution d'énergie

This study aims at defining an energetic parameter which characterises progressive crack growth in a brittle elastoplastic medium. First, Francfort and Marigo's elastic fracture theory, based on a minimum energy principle, is recalled. Then, a link with the framework of generalised standard materials is exhibited and allows to introduce the new energetic contributions due to plasticity.     

Localisation de la déformation sous sollicitations lentes : une approche dynamique

For materials exhibiting a non-linear behaviour (instantaneous elasticity and viscid non-linearities), strain-localization is associated with the occurrence of dynamic effects, even at slow loadings. A criterion is proposed for testing the relevance of the usual equilibrium hypothesis associated with slow loadings; at its fulfilment, the boundary problem is treated as dynamic, leading, by combination of inertia and viscosity terms, to localization. A 1D example of this approach is eventually given.     

Approximation of poisson integrals by repeated de la Vallée Poussin sums

We obtain asymptotic equalities for upper bounds of deviations of the repeated de la Vallée Poussin sums on classes of Poisson integrals. Under certain conditions, these equalities guarantee the solvability of the Kolmogorov–Nikol’skii problem for the repeated de la Vallée Poussin sums and classes of Poisson integrals. We indicate conditions under which the repeated de la Vallée Poussin sums guarantee a better order of approximation than ordinary de la Vallée Poussin sums.     

Analyse de la répartition du potentiel électrique dans les flammes

Distribution of electrical potential is investigated for two kinds of flames: Bunsen burner and counterflow flames. Different electrode materials have been tested to determine these potentials. Measurable potentials and organized structures are found. Premixed Bunsen burner flames show a zero potential cone at their base just under the most intense potential region. One-dimensional counterflow diffusion flames are characterized by the stretch rate. The potential reaches its minimum value in the reaction zone and its absolute value decreases when stretch increases.