Calcul direct du rayonnement acoustique d'une couche de mélange par macrosimulation

A Large Eddy Simulation code (ALESIA for Appropriate Large Eddy Simulation for Aeroacoustics) was developed to compute the noise radiated by vortex pairing in a 2-D mixing layer. The code uses numerical techniques specific to Computational AeroA-coustics (CAA) in order to obtain the compressible field with high precision. We show that it is possible to determine directly the acoustic radiation, and that vortex pairing noise corresponds to a rotating quadrupôle source.     

Construction d'un opérateur de propagation à partir des équations d'Euler linéarisées

Linearized Euler equations based on a stationary mean flow are used to calculate the propagation of acoustic waves. This method allows one to take into account the mean flow effects on the radiated sound field. The multipolar feature of a source clearly appears in the wave equation. It is shown that these sources are numerically identified in solving directly the linearized Euler equations. The case of a quadrupolar distribution in a jet flow is then investigated.     

Calcul de flammes turbulentes non prémélangées à l'aide d'une approche probabiliste et d'une cinétique chimique réduite

The emergence of methods allowing the simplification of detailed chemical kinetics enables detailed modeling of turbulent diffusion flames. One of these methods, the “Intrinsic Law-Dimensional Manifolds” (ILDM) method and its application to the computation of turbulent reactive flows are described here. A CFD solver and a Monte-Carlo code solving the transport equation of the scalar joint pdf are coupled and two jet-flames stabilized by a pilot flame are calculated. It is shown that this coupled, solver leads to a good prediction of the partial extinction of the flame as well as to a correct estimation of the main variables describing the flame. © 1999 Académie des sciences/Éditions scientifiques et médicales Elsevier SAS     

Application de la dynamique des solutions concentrées de polymères à la résonance acoustique de réticulation

One can characterize the large-scale displacements of a macromolecule by the time required for the chain to renew completely its conformation. These displacements are displayed by changing the topological orientation of the condensation reactions. This effect is produced by putting the medium through a sound wave or an alternating electric field that is in resonance with the Brownian motion of the macromolecule. The physical contact between the chains is enhanced and the intermolecular reactions are more frequent than the intramolecular reactions. The application of a sound wave or an electric field of a characteristic frequency on the elastomers during chemical crosslinking produces a noticeable change in the crosslinking density.     

Sur une méthode de projection vectorielle pour la résolution des équations de Navier-Stokes

We propose a method of solving the Navier-Stokes equations in incompressible flow. It is based on the projection of the velocity field, approached by a prediction step on a zero divergence field. The novelty of this method concerns how the projection is made, directly operating on all the components of the velocity field through a coupling. A highly implicit algorithm allows us to maintain all physical boundary conditions of the problem during the solution steps.     

Nouvelle méthode de dosage colorimétrique d'une très petite quantité de vanadium à l'aide de la formaldoxime

Résumé Là s'achève notre étude sur la coloration de vanadium quadrivalent avec la formaldoxime, et son application à l'analyse chimique. Cette coloration est la plus sensible parmi les réactions colorées de vanadium connues jusqu'à présent.L'influence de l'excès du réactif et de la température ainsi que celle de diverses substances ont été étudiées. On peut sans inconvénients, réduire la quantité d'échantillon, le réactif employé étant d'une très grande sensibilité.

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Summary This completes our study of the coloration produced by quadrivalent vanadium with formaldoxime and its application in analytical chemistry. This color reaction is the most sensitive among those of vanadium discovered hitherto. The influence of the excess of the reagent, of the temperature, as well as that of various substances have been investigated. Since the reagent is so very sensitive, there is no disadvantage in reducing the size of the sample.

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Zusammenfassung Die Farbreaktion von Vanadium (IV) mit Formaldoxim und ihre analytische Anwendung wurden untersucht. Sie ist unter den bisher bekannten Farbreaktionen des Vanadiums die empfindlichste. Der durch einen Überschuß an Reagens, durch die Temperatur und durch verschiedene Substanzen auf die Reaktion ausgeübte Einfluß wurde geprüft. Infolge der großen Empfindlichkeit des verwendeten Reagens läßt sich die Probemenge ohne Nachteil sehr gering halten.

     

Contribution à l'étude du mécanisme de mûrissement de résines polyesters insaturées en présence de MgO

The thickening process for unsaturated polyester resin in styrene monomer and with magnesium oxide (MgO) has been evaluated by gel permeation chromatography (GPC). The GPC analysis n tetrahydrofuran (THF) shows that with time the distribution shifts towards higher molecular weight. This effect disappears when the products are analysed in a polar solvent. The process of thickening is attributed to the formation of reversible ionic association. The existence of strong ionic bonds in the final products is confirmed by the presence of a rubber-like plateau in dynamic mechanical properties. The effects of water and magnesium oxide concentrations are given.     

Contribution à l'étude des méthodes de dosages électrotitrimétriques

The author establishes a general equation for the polarisation curve of an oxidation-reduction system taking into account the kinetics of reactions and concentration polarisation, and shows how it can be applied to the interpretation of titration curves obtained by the dead-stop end-point method of Foulk and Bawden and by the derivative method of Reilley, Cooke and Furman.     

Etude par spectrographie d'absorption infrarouge des produits de décomposition du mélange chlorate de potassium-bioxyde de manganèse

Résumé Le chlorate de potassium, chauffé avec du bioxyde de manganèse, donne de l'oxygène et du chlorure de potassium, sans formation transitoire de perchlorate et de permanganate de potassium.

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Summary Potassium chlorate when heated with manganese dioxide yields oxygen and potassium chloride without transitory formation of potassium perchlorate and permanganate.

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Zusammenfassung Kaliumchlorat zerfällt beim Erhitzen mit Mangandioxyd in Sauerstoff und Kaliumchlorid, ohne daß Perchlorat und Permanganat als Zwischenprodukte gebildet werden.

     

Traitement électrochimique d' eaux usées chargées de phénol : étude comparative sur des électrodes de dioxyde de plomb et de platine

Electrochemical treatment of waste water containing phenol: a comparative study on lead dioxide and platinum electrodes. The objective of this work was to study the efficiency of the Pb/PbO₂ electrode for decomposing the molecule of phenol, then to compare it to a platinum model electrode. Preliminary investigations by cyclic voltammetry showed that the Pb/PbO₂ anode presents a good chemical and electrochemical stability and possesses a high oxygen overvoltage. The study also showed that the electrochemical oxidation of phenol on Pt and PbO₂ in acidic media is a complex process. Long-time electrolysis was carried out using a three potential-plateau program with different values of the oxidation potentials and different concentrations of phenol. The obtained results showed that the transformation of phenol is total on the Pb/PbO₂ anode and that it is partial on Pt. On the other hand an increase in the phenol concentration decreases the rate of its conversion on the electrodes.     

Transformations de N-acylaziridines catalysées par des supports à base de silice et d’alumine : une élucidation mécanistique

The silica gel, neutral alumina and acid activated clays transform the N-acyl-2,2-dimethylaziridines 1 into a mixture of N-methallylamides 2, oxazolines 3 and amidoalcohols 4. The reaction of N-benzoyl-2,2-dimethylaziridine with benzylcyanid anion leads to a mixture of amidopyrroline 5 and iminopyrrolidine 6. The product 5 was converted into its tautomer 6 by chromatogaphy on silica gel. Ionic mechanisms were then proposed to explain the formation of products 2–6 resulting from the regioselective ring opening on the more substituted C2 carbon side.     

Détermination du spectre de puissance acoustique émis par une flamme turbulente

This note presents the elaboration of a predictive tool for the acoustic generation of a flame. First, volumetric expansion occurring in turbulent combustion is identified as the main acoustic source term. Following an approach similar to Lighthill's aeroacoustic analogy, a pressure wave equation is derived. By way of a reduced energy relation we express source terms as a function of the temperature fluctuations. Determination of the sound pressure autocorrelation leads to the sound power spectrum. This consists of a double volumetric integration of the production terms weighted by a spatio-frequential coherence function. By means of two characteristic times, this coherence function takes into account combustion and turbulence effects on the acoustic generation. The aerothermoacoustic model is then applied to the turbulent flame produced by an experimental burner and results are compared with experimental spectra.     

Application de la méthode de Melnikov pour l'étude du roulis non linéaire des navires dans la houle

For studying ship rolling within a global analysis more and more mathematical tools from chaos theory are available. For this purpose, however, drastic simplifications must be applied: (i) rolling is the single degree of freedom, (ii) motion is represented by an ordinary differential equation, and (iii) the study is achieved in the vicinity of vanishing stability. Here the Melnikov method is used in this application. The focus is on ships whose restoring moment is represented with an odd polynomial. Above the fifth degree, a harmonically excited system exhibits frequencies which cancel the fluctuating part of the Melnikov function. Hence the heteroclinic orbits still exist whatever the ‘smallness’ of the perturbation as soon as the system is undamped. The existence of such cancellation is otherwise confirmed from an analysis of the erosion of the attraction basin.     

Réactivité du groupement époxyde vis à vis d'une amine aromatique en fonction de l'environnement moléculaire

Analysis of the reaction (mechanistic and kinetic) between diepoxy aromatic compounds and aniline, taken as a model for the curing of epoxy resins with diamines, shows (a) the network build-up is dependent on the reactivity of functional groups and their position on the aromatic ring, (b) the curing proceeds by competitive mechanisms viz. an autocatalytic process characterized by an activation energy of ∼ 62.7 kJ/mol and a non-catalytic path characterized by a second order reaction.     

Comportement d'une couche limite thermique en écoulement cisaillé avec chauffage spatio-périodique à la paroi

The evolution in time of a thermal boundary layer generated at the interface shear flow/flat plate is studied by means of an integral method, when the plate is suddenly heated with a periodical flux density in space. From the deduced analytical steady state solution based on the use of a no-thickness plate hypothesis, it is shown that the thermal boundary layer is more influenced by spatial cooling phases than by heating phases.     

Calcul de chiralité quantitative par la méthode des moindres carrés

It is shown how the root mean square chiral index is computed without enumerating all the permutations of a set of n atoms. As an example, the quantitative measure of chirality is achieved for various helicene derivatives. It is also shown that the chiral index of a protein backbone is computable without enumeration.     

Contribution à l'étude des émissions radioélectriques aurorales de Jupiter

We study the radioelectric emissions of Jupiter in the frequency range 1–40 MHz with the decametric observatory of Nançay and the Wind spacecraft. A wake in front of Io explains the Φ_Io/CML chart. An appropriate coordinate system shows that the Ioontrolled emissions occur mainly when Io is located in the northern magnetic hemisphere of Jupiter.