Calcul approché de la trainée d'une bulle sphérique de gaz mise brusquement en ascension dans un liquide

An analysis is made for a gas bubble impulsively started to rise with a constant velocity in a quiescent liquid of infinite extent. Satisfactory results for the total drag force are obtained for spherical air bubbles in water, provided that the method developed here is applied at early times, the flow separation is negligible and the Reynolds number R_e is sufficiently large.     

Vérification expérimentale de la relation de réciprocité d'Onsager pour l'électro-osmose et l'électro-filtration dans une argile naturelle

This Note is devoted to the experimental verification of the Onsager's reciprocal relations in the particular case of electro-osmosis and electro-filtration. A special set up has been designed to carry out the measurements of both the electro-osmotic permeability and the streaming potential. This has been performed by using a natural material i.e., saturated kaolinite. To cite this article: K. Beddiar et al., C. R. Mecanique 330 (2002) 893–898.     

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.     

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.     

Quelques questions de la théorie de la stabilité pour les systèmes aux différences finies

Sans résumé Présenté par C.Icob     

Sur l'intégrale de ligne et les fréquences irrégulières dans les calculs de tenue à la mer

The first results obtained using two seakeeping computation codes developed at Ecole Centrale de Nantes (Aquaplus) and at the LEA-Poitiers (Poseidon) are presented. Both use the diffraction–radiation with forward speed Green function, but with two different formulations, in a panel code in the frequency domain. Results obtained with both codes are in good agreement and also retrieve test measurements or other calculations available. However, irregular frequencies and their links with the waterline integral are made evident.     

Interprétation de la double structure observée dans l'onde de détonation du nitrométhane gazeux

Calculations of the reaction zone of the detonation of gaseous nitromethane, either pure or diluted with oxygen, in the range of equivalence ratio Φ between 0.1 and 1.75, show that for 1.75⩾Φ⩾1.3 the chemical energy is released in two main successive reaction steps characterized by very different induction times. These results corroborate the experimental observations of two levels of cellular structures in the same range of equivalence ratios. To our knowledge this work is the first which deals with the problem of nonmonotonous chemical energy release behind the leading shock of a detonation wave.     

Mise en évidence par granularité en lumière blanche de phénomènes de multilocalisation de la déformation pour les bétons de poudres réactives sollicités en traction

From kinematic observations by granularity in whiting light mode based on a technique of digital speckle images correlation, the behaviour in tension of a reactive powders concrete is studied. The measures show several localisation areas of the deformation fields and the role of the fibres in the transmission of the stresses in the neighbourhood of a damaged area.     

Kinematic regimes of convection at high Prandtl number in a shallow cavityRégimes cinématiques de la convection à haut nombre de Prandtl dans une cavité allongée.

We consider free convection in a horizontal shallow cavity with different end temperatures, filled with a high Prandtl number fluid. From scaling analysis, we find two kinematic regimes resulting from the competition of heat transfer by conduction and by convection. Numerical simulations realized for a large range of Rayleigh number and aspect ratio confirm the phenomenological analysis and provide the threshold between the two regimes. The conductive and convective regimes occur at $\mathit{Ra}{A}^2$ smaller and larger than 443 respectively, where Ra is the Rayleigh number and A is the aspect ratio. In the convective regime, the characteristic velocity is independent of depth of the cavity. To cite this article: J.-M. Flesselles, F. Pigeonneau, C. R. Mecanique 332 (2004).     

Homogenization limit for electrical conduction in biological tissues in the radio-frequency rangeLimite d'homogénéisation pour la conduction électrique dans les tissus biologiques dans le domaine des radiofréquences

We study an evolutive model for electrical conduction in biological tissues, where the conductive intra-cellular and extracellular spaces are separated by insulating cell membranes. The mathematical scheme is an elliptic problem, with dynamical boundary conditions on the cell membranes. The problem is set in a finely mixed periodic medium. We show that the homogenization limit u₀ of the electric potential, obtained as the period of the microscopic structure approaches zero, solves the equation $\text{?}\text{div}\text{(σ}{}_0\text{?}{}_{\mathrm{x}}\text{u}{}_0\text{+A}{}^0\text{?}{}_{\mathrm{x}}\text{u}{}_0\text{+∫}{}_0{}^{\mathrm{t}}\text{A}{}^1\text{(t?τ)?}{}_{\mathrm{x}}\text{u}{}_0\text{(x,τ)}\text{d}\text{τ?}\text{F}\text{(x,t))=0}$ where σ₀>0 and the matrices A⁰, A¹ depend on geometric and material properties, while the vector function $\text{F}$ keeps trace of the initial data of the original problem. Memory effects explicitly appear here, making this elliptic equation of non standard type. To cite this article: M. Amar et al., C. R. Mecanique 331 (2003).