Linéarisation et homogénéisation en viscoélasticité

Constitutive equations for non-linear viscoelastic materials are expressed first using a causal operator which relates a response function to any loading history. The corresponding tangent linear equations are then defined through the Fréchet derivative of this operator. This definition is used to propose a step-by-step treatment in view of the derivation of the overall behaviour of non-linear non-ageing viscoelastic heterogeneous materials.     

Fonctionnalisations régio et stéréospécifiques successives en méta et en ortho des η6-arènetricarbonylchrome

The η⁶-1-methoxy-2,3-dimethylbenzenetricarbonylchrome and the η⁶-1-dimethylamino-2,3-dimethylbenzenetricarbonylchrome are chemically functional on the methyl group 3 first; the methyl group 2, which is less reactive, is functional later. The η⁶-1-dimethylamino-5,6,7,8-tetrahydronaphthalenetricarbonylchrome is difunctional on positions 5 then 8 using two different electrophiles.     

Régimes pseudo-son et acoustique en turbulence compressible

Results of a model for compressible turbulence proposed in a companion note are presented in the case of a statistically stationary isotropic field. The model is first validated by comparison with a direct numerical simulation. The results are then analyzed at higher Reynolds numbers. At low Mach number, two regimes are detected: a pseudo-sound regime occurring in the turbulent part of the spectrum and an acoustic regime appearing at much smaller wave-numbers. In the pseudo-sound regime, the spectrum of the dilatational velocity is found to scale as K⁻³. In the acoustic part of the spectrum, the model is found to agree with classical models in acoustics. Results obtained at higher Mach number are also presented and the differences between the present model and a previous EDQNM theory (Eddy Damped Quasi-Normal Markovian theory) are shortly discussed.     

Réduction et dosage des arséniates en présence de catalyseurs,en particulier le rhénium

The reduction of arsenates to arsenites is normally slow, but is accelerated in the presence of catalysts the use of alkali perrhenate or iodide permits a quantitative reduction by stannous chloride. A method of determination using the latter reducing agent has been developed.     

Instabilité et décrochage tournant en grille d'aubes

Rotating stall is a fundamental hydrodynamic instability which is encountered in most hydraulic and aeraulic turbomachinery. It appears to exist only with cascades that considerably slow down the fluid flow and occurs in the form of rotating structures in which the flow rate may be negative. The paper analyses this instability and its transitions in a simple two-dimensional configuration using a multi-domain numerical scheme and solver. The space-time variation of the flow rates occurring for an angle of attack increasing by steps shows first standing waves, then intermittent and permanent travelling waves of limited amplitude, corresponding to blade stall. At higher angles of attack, a high amplitude wave propagates a single structure (cell) with a negative flow rate, corresponding to stalling of the cascade.     

Dosages coulométriques à courant constant et en solution non agitée

Accurate evalution of “transition times” occurring during constant current electrolysis in non-stirred media may be used analytically. An apparatus has been devised for such a purpose.The method has been applied to various practical problems and stress is laid on its potentialities and main features.Under favourable conditions, determinations are performed in less than one minute, with an accuracy of about 0.2%.     

Polymérisation et post-polymérisation de l'acrylate de méthyle en milieu précipitant

The radiation initiated polymerization of methyl acrylate was investigated under precipitating conditions in n-hexane In 10 and 25% monomer solutions, the conversion curves are auto-accelerated and post-polymerization is observed in the dark. The initial rate of this post-polymerization steadily increases with the degree of conversion reached during irradiation, suggesting that trapped growing chains are responsible for the effect. The post-polymerization was followed at 23, 40 and 60°. From the results, it is concluded that chain termination is negligible under such conditions. The addition of methanol to the monomer solutions results in decreased rates and in a reduction of the extent of auto-acceleration. No influence of any “matrix effect” could be detected in these systems.     

Étude physique et numérique de la circulation cérébrale en pathologie carotidienne

A realistic and versatile physical model was built to simulate blood flow through the circle of Willis. Experimental data were then compared to those provided by numerical simulations. When the network is symmetrical and free of stenosis, the results of a linear model closely fit the experimental data. In presence of severe obstructive lesions of internal carotid arteries, the pressure drops induced by sudden changes of section or direction at the level of the communicating arteries must be taken into account. Introducing empirical flow-pressure drop relationships in the theoretical model, leads to an excellent agreement between experimental and simulated data. © 1999 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS     

Extraction et dosage de traces de rhénium,en particulier dans les molybdénites

A simple solution to the difficult problem of the separation and determination of traces of rhenium contained in molybdenites is suggested. After the preliminary attack of the cres, rhenium is extracted quantitatively between, pH 8 and pH 9 by chloroform in the form of tetraphenylarsonium perrhenate. In this pH region the molybdenum remains in the aqueous phase.Calculation has allowed the determination of favourable conditions for the extraction of perrhenates by tetraphenylarsonium chloride and chloroform. Practical results are in satisfactory agreement with theory.The extracted rhenium, after conversion to the ions ReO₄^- in acid solution, is determined colorimetrically by reaction with thiocyanate and stannous chloride.By applying the method described it is possible to isolate and determine rhenium in molybdenites when the weight ratio rhenium/ore ecxeeds 10^-7.     

Modélisation théorique d'écoulements pulses de fluides non newtoniens en conduites viscoélastiques poreuses et anisotropes

A theoretical study concerning the pulsatile flow of an inelastic fluid through anisotropic porous viscoelastic pipes is presented. The objective is to investigate the effects of porosity, conicity in pause, anisotropy and viscoelasticity of pipe wall material for a generalized Casson fluid. An implicit difference method is used to solve the equations, and to determine the axial and radial velocity profiles, the pressure, the flow rate and the flow rate filtration distributions, the transverse rotation and the wall axial and radial displacement. This study, considered as a step in the modelling of flow in blood vessels, may also contribute to other important fields such as water desalination or gel filtration.     

Les réactions solvothermales : implications en recherche de base et dans les procédés industriels

Solvothermal reactions have been developed both in solid state chemistry and in material science. In a first approach, seven main domains can be defined for the applications of such reactions: (i) the synthesis of new materials, (ii) the development of new processes, (iii) the preparation of microcrystallites, (iv) the thin film deposition, (v) the sintering and (vi) the crystal growth at moderate temperature compared to the conventional processes, (vii) the waste-retreatment. These different domains involving either the basic research or the industrial applications, in particular in the area of ceramics, are presented and their potential developments are analysed.     

Détermination des cinétiques de croissance et de dissolution du perborate de sodium tetrahydrate en lit fluidisé

The authors present the results of an experimental study on the crystallization of tetrahydrate sodium perborate in a fluidized bed. The influences of supersaturation, seed crystal size, temperature and solution velocity on the overall growth rate were examined. The overall dissolution rate determined under comparable hydrodynamical conditions for various seed crystal sizes and temperatures is also reported.The growth rate is not very sensitive to the variations in crystal size, the temperature and the hydrodynamical conditions, but it depends on the supersaturation and the presence of some soluble impurities.The comparison between growth and dissolution rates shows that under actual working conditions the diffusional mass transfer step is not the limiting step of the growth mechanism.     

Etude de la fluorescence du 9,9′-bianthryl en solutions liquides et en milieux polymériques rigides à la température ambiante

The fluorescence of 9,9′-bianthryl (9,9′BA) is very sensitive to the polarity of the solvent. This molecule exibits an intramolecular charge transfer state in polar medium. The study of the fluorescence of 9,9′BA, in liquid solutions, shows a strong dependency of the Stokes shift versus the dipole moment m of the aprotic lowly viscous solvent. Moreover, this compound exhibits in polyvinylacohol (PVA) an original anomalous behaviour.     

Dosage complexométrique de chlorures,chlorates et perchlorates en présence les uns des autres

A method is described for the analysis of a mixture of chlorides, chlorates and perchlorates by complexometric titration. It is based on the partial reduction of chlorates and the reduction of chlorates plus perchlorates respectively; the chloride formed can then be determined.