Application, à un cristal de béryl,d'une nouvelle méthode d'étude par réflexion

The vibrational frequencies of beryl are determined from a measurement of the optical constants with a method utilizing reflexion from samples with and without an evaporated layer of an optically known substance.     

Influence des phénomènes d'hystérésis sur les propriétés hydriques de matériaux poreux: Comparaison de deux modèles de simulation du comportement thermohydrique de parois de bâtiment

Porous materials hysteretic moisture characteristics : influence on water content distributions in buildings walls. Comparison of two simulation models. Two models for determining the temperature and moisture distributions within a wall are presented. The first one is a direct application of the classical Philip and de Vries model. The second one, using the suction potential, is derived from Milly's researches and is more efficient because it allows the hysteresis phenomena to be taken into account. First, the different equilibrium and transfer parameters used in the models are reviewed. Indications are given on how they can be experimentally determined and results for two cement pastes and a cellular concrete are illustrated. Then, calculation results of water content distributions for the two models are presented for an homogeneous cellular concrete wall in steady environmental conditions. In that particular case, the two models estimations are in good agreement despite different numerical treatments and experimental procedures. Using Milly's model with cyclic boundaries conditions, a significant influence of hysteresis phenomena is clearly shown.     

Modélisation d'un échangeur de chaleur compact à courants croisés séparés par des couches de matériau à changement de phase

A mathematical model was developed to predict the thermal behaviour of a crossflow compact heat exchanger with layers of phase change material (PCM) operating under winter conditions. The model is validated and a parametric study is conducted to determine the design and operating conditions that promote condensation and may lead eventually to the frosting of the exchanger. The transient and steady-state thermal efficiency of the exchanger are also predicted as a function of the number of thermal units NTU_f, of the ratio of the products of the mass flow rate by the heat capacity on the cold and hot sides C and of the Biot number Bi. Results indicate that for Bi = 0.6 (which corresponds to a 3 mm thick PCM layer), condensation occurs when NTU_f ≥ 2.0 for C = 1.0 and when NTU_f ≥ 5.0 for C = 0.5. If the thickness of the PCM layer increases, condensation is avoided and the duration of the heat recovery period is prolonged. At the same time however, the steady state thermal efficiency diminishes. A heat exchanger for which Bi = 0.6, C = 0.5 and NTU_f = 4.0 appears to be a good compromise for acceptable heat recovery and efficiency. In this case, the heat recovery period lasts 1.6 h and the steady state thermal efficiency levels off at 64 %.     

Modélisation dynamique en thermique du bâtiment

This work describes the advantages provided by applying the method of spectral analysis to solve problems in the thermal analysis of buildings. A reduced spectrum of the model is obtained with the analysis of the dynamic responses of the system. The model obtained is simple and efficient, since it describes with accuracy the main dynamic behaviour of the structure. The aim is to use the model for the control of ambiant temperatures in buildings. These temperatures do not require accurate precision but a good restitution of involved dynamics.     

Simulation numérique du comportement d'un béton cellulaire dans sa phase d'élaboration sous gradient de pression de vapeur à haute température

A numerical simulation of the coupled heat and mass transfers is carried out to analyse the thermal and moisture properties of cellular concrete during a manufacturing cycle. This cycle consists of two phases: a vacuuming phase is created first, then the material is baked under high energy and saturated vapour conditions. A 2D code was developped using a finite volume method. The numerical results were validated by means of a comparison study of the problem, in order to improve the process with a view to energy consumption and product quality.     

Modélisation de séchoirs à tapis. Utilisation des réseaux de neurones

Modelling of convective layer dryers. Using neural networks. Dryer modelling is considered in this paper. A dryer scale approach is implemented in order to write the classical differential equations through parameters such as the heat transfer coefficient or drying kinetics. The behaviour of the dryers is described by a non-linear system which integrates these equations in a transfer network using the finite difference method. The finite difference method is easy to implement, but appears to be too slow for dryer designing. So, in the second part of the study, neural networks are used to model drying process in steady state. When applying neural networks method to the design of dryers, one of the main problems is to find necessary and sufficient inputs so that the neural networks can learn transfers laws. To reduce the problem, each output is defined by a single neural network and non-dimensional numbers are used. The following step deals with the determination of the number of neurones and the minimization of output error for each efficiency (change of training points). Then, neural networks are used to simulate different configurations of dryers. Results are compared with the finite difference method and an industrial application is studied in the last chapter.     

Ecriture de l'énergie cinétique à l'aide d'opérateurs quasi-moments

Cet article concerne l'utilisation des quasi-moments ?π _m , définis par

,

pour exprimer l'opérateur correspondant à l'énergie cinétique de N particules en Mécanique Quantique. La condition de Wilson-Howard portant sur les coefficients s^ml est interprétée comme la condition pour que les opérateurs ?π _m soient hermitiques quand on utilise l'élément de volume s dq ₁ … dq ₃ N (s=[dét {s^ml }]^-1). La condition générale pour qu'il soit possible de trouver un élément de volume avec lequel les opérateurs ?π _m sont hermitiques est donnée et différentes expressions de l'opérateur énergie cinétique sont établies quand cette condition est remplie et quand elle ne l'est pas.     

Développement de lasers visibles pompés par diode à base de cristaux de Nd:GdCOB autodoubleurs de fréquence

We present the performances of a diode-pumped solid-state laser operating in the visible by self-frequency-doubling. A cw output power of 115 mW at 545 nm has been obtained in a stable concave–concave cavity by using a crystal of Nd:GdCa₄O(BO₃)₃ pumped by a 2 W high-brightness laser diode. In a plano–plano cavity, similar to a microchip laser, we have obtained, for the first time to our knowledge, an output power of 22 mW in the green.     

Dimérisation des sels de l'ion nitrosodisulfonate à l'état solide

La dimérisation de plusieurs sels du radical libre nitrosodisulfonate est étudiée par R.S.E. Les spectres d'échantillons polycristallins de deux formes cristallographiques des sels de sodium, potassium et rubidium, sont caractéristiques d'un état triplet accessible thermiquement. Ces spectres sont attribués à des paires de groupements NO radicalaires, couplés par échange et sont décrits par l'Hamiltonien de spin:

Les paramètres de déplacement en champ nul, les composantes du tenseur g le long des directions principales X, Y, Z du tenseur fin et la composante du tenseur A le long de l'axe Z (dans le cas où la structure hyperfine est résolue), ont été mesurés et discutés.

A partir de ces mesures, on montre que l'axe Z est dirigé suivant les orbitales 2p des atomes d'azote et d'oxygène, et que l'axe X est dirigé selon la liaison NO. Par suite, l'arrangement des deux groupements NO d'une paire est toujours sensiblement rectangulaire.

Dans le cas des deux formes cristallines du potassium et du rubidium, l'écart singulet triplet dépend de la température, ainsi que les paramètres de déplacement en champ nul, ces variations avec la température sont attribuées à l'expansion thermique du réseau.

La présence d'une structure hyperfine bien résolue dans certains de ces sels, et la forte anisotropie des largeurs de raies dans les autres sels, suggèrent que les excitations sont fortement localisées, et que si elle sont mobiles leur fréquence de saut est inférieure à 10⁷ Hz.     

Amélioration du coefficient de qualité de résonateurs à ondes transverses sur plaques fines de quartz

Resonators based on transverse waves propagating in thin quartz plates have been optimized using a specific fabrication step in order to improve their quality coefficient and to minimize their insertion losses. Systematic measurements have been performed, allowing to emphasize the efficiency of the proposed approach. The possibility to fabricate resonators operating in the frequency range 100–250 MHz and exhibiting quality coefficient times frequency product greater than 10 ¹³ is there demonstrated. This last figure corresponds to more than the state of the art in surface acoustic wave resonators.     

Les études Séismologiques en Tchécoslovaquie de 1951 à 1953

Ohne ZusammenfassungRapport national présenté devant l'Association Internationale de Séismologie et de Physique de l'Intérieur de la Terre pendant la 10ème Assemblée Générale de l'Union Géodésique et Géophysique Internationale (UGGI) à Rome en Septembre 1954.     

Identification des propriétés thermiques de composites fibres de verre/résines thermodurcissables: Application à l'optimisation des procédés de moulage

Identification of glass fiber/thermosetting resins composites thermal properties. Application to the optimization of molding processes. A procedure has been developed to optimize molding processes of thermosetting composite materials. Three stages have been distinguished. In the first one, some thermal and kinetic properties have been measured by differential scanning calorimetry. Thermal conductivities have been identified afterwards in thick instrumented pieces, placed in a thermally regulated press. High dependences of thermal conductivities on temperature and transformation degree have been shown. Secondly, coupled heat transfers have been numerically simulated and results have been satisfactorily compared with experimental thermograms. Finally, optimization technics based on effective inverse methods have been used.These points have been illustrated with two examples : glass fiber/epoxy resin and glass fiber/polyester. Sufficient mechanical characteristics of the first one, which is cured in oven, and good surface aspect of the second, that is made by injection in heated and closed molds, had to be obtained. The results let foresee real improvement of the corresponding molding processes.     

Comportement thermique d'un matériau de référence poreux soumis à des atmosphères de chauffe différentes

This study presents an analysis of the temperature evolution in a porous sample. Various situations have been examined: the porous solid is initially dry or water saturated, and the heating fluid is dry air or steam. This analysis showed experimentally the optimal phases of using steam to heat a material. Moreover, as humidity in the solid considerably modifies the heating kinetics, the study has also been carried out on a non-porous solid. The treatment of the temperature curves under an adimensionnal form shows a singular behaviour of those temperatures following the heating fluid nature and its temperature. Equations giving temperature versus time have been proposed. They are based on the definition of a time lag and a time constant, both being defined as a function of the single fluid temperature parameter. Finally, in the particular case of the non-porous solid heated by dry air, with additional approximations, it has been possible to check that the experimental time constant is very close to a fundamental time constant.     

Réalisabilité d'un schéma non linéaire de retour à l'isotropie de la corrélation pression-gradient d'un scalaire

The study of the equilibrium states of a homogeneous turbulence, in the absence of mean gradients, leads to an equilibrium solution with zero values for the turbulent scalar flows. Stability of this equilibrium solution is achieved by a simple condition on one of the coefficients of the model. A realisibility study of the turbulent scalar flow has been carried out an has led to a general realisibility condition imposed on the model coefficients. Moreover, it has been established that, if the return to isotropy is compatible with two supplementary constraints on the model coefficients, it is possible to substitute a sufficient yet much simpler condition for the general realisability condition. Finally, a numerical optimization on the basis of the experimental results and the direct simulation results, has proven that the proposed model ensures a better prediction of the scalar turbulence with respect to Rotta's model.     

Une nouvelle méthode calorimétrique périodique de mesure de l'émissivité des matériaux opaques à température ambiante

This paper presents a new calorimetric periodic technique for measuring emissivity of opaque materials at room temperature, without surface temperature measurement. The sample is in a vacuum chamber, so the thermal losses are only radiative. The presented technique requires thermal modulation of one side of the sample (the front side). The measured signals are the sample's front side temperature and the infrared flux of the other side (back side). Experimental data and a heat conduction model are compared, yielding the Biot number which characterizes the thermal losses. Using the identified value of the Biot number, it is possible to access the total hemispherical emissivity. Measurements have been carried out on a PVC sample (5 mm thick) coated with black paint and on a PVC sample coated with aluminium paint. The results are concordant with the expected ones ; the repeatability error is about 3 %.     

Phénomènes de glissement à l'interface liquide–solide

The problem of liquid–solid slip is described here, in a simplified manner. Today, several experiments have shown that substantial slip appears when a non-wetting liquid flows along a surface which is smooth on an atomic scale. This phenomena is characterised by a length, called the slip length, or Navier length, generally denoted by L_s. A number of experiments indicate that this quantity may be as large as several hundreds of nanometers. Numerical simulations also show the existence of slip in non-wetting conditions, but the corresponding lengths found here are much smaller than those found experimentally. A theory, based on the existence of a gas film of nanometre thickness has been proposed, but has not yet been experimentally confirmed. Experiments on this are difficult, and sometimes controversial. To cite this article: P. Tabeling, C. R. Physique 5 (2004).