Étude statistique et dynamique de la propagation d'épidémies dans un réseau de petit monde

We study numerically in this work the statistical and dynamical properties of the clusters in a one dimensional small world model. The parameters chosen correspond to a realistic network of children of school age where a disease like measles can propagate. Extensive results on the statistical behavior of the clusters around the percolation threshold, as well as the evoltion with time, are discussed. To cite this article: N. Zekri, J.P. Clerc, C. R. Physique 3 (2002) 741–747.     

Étude d'une nouvelle technique de mesure des températures dans le sol

The measure of soil temperatures is generally achieved by implanting thermocouples at different depths. For the setting up of these thermocouples, and in order to resist the constraints of shearing, some rigid pegs were employed as supports. The utilization of these pegs, which are generally in metal, introduces some soil thermal perturbations. In order to minimize this kind of perturbation, a new technique of temperature measurements in soil is presented. The principle of this technique consists of replacing the peg by a removable stem which could be removed after setting up thermocouples in the soil. Apart from the thermal aspect, some mechanical tests of the new technical device are achieved. In order to notice the effects of the pegs, a qualitative study is carried out. It concerns, in particular, depths of 20, 30 and 50 cm and shows gaps in temperatures between the two techniques.     

Transfert de chaleur par rayonnement dans un four d'incinération industriel: application de la méthode des zones

In the context of resource conservation, incineration of domestic wastes is a technique allowing an interesting potential in energy valorization. However, because of the variability (annual, monthly) in quality and quantity of raw material, one of the characteristics of waste incineration cogeneration power plants is their frequent partial or excess loading. In order to describe their behaviour at these non-nominal loads, there is a need for simulation tools which predict with accuracy the behaviour of the principal components of the plant at these loads. This paper presents an original approach for predicting the heat transfer in a waste incinerator. In particular, the model developed is applied to and validated with measurements from a waste incinerator located at the Cottendart waste incineration cogeneration power plant in Switzerland. The part of the incinerator studied is characterized by high gas temperatures (1 000-500 °C) for which radiative heat transfer represents the most important part of the global heat exchange. A three-dimensional model using nodal analysis is proposed. The radiative heat exchange is treated using the zone method. With a fine enough mesh, the behaviour of the system in regions far from the nominal point can be predicted with very high accuracy. This paper describes the model and the simulation results.     

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 %.     

Mise en œuvre d'une méthode d'approximation diffuse adaptative pour la résolution des équations de diffusion et de transport

An adaptative diffuse approximation method for convection diffusion problems has been developed. The grid refinement method is first described and two examples are reported to illustrate the validity of the proposed method. The first one is a two-dimensionnal diffusion problem and the second one is a one-dimensionnal convection-diffusion problem.     

Étude tridimensionnelle du transfert radiatif dans un milieu semi-transparent diffusant anisotrope par la méthode des transferts discrets modifiée

The development of the modified discrete transfer method (MDT) in a three-dimensional rectangular configuration allowed us to simulate the thermal behaviour of a semi-transparent, grey, absorbing emitting and anisotropically scattering medium at the radiative equilibrium. An internal source distributes heat uniformly in the medium while the walls of the enclosure that surround it, opaque, grey, diffuse for emission and reflection, are submitted to prescribed temperatures. A linear variation law of the temperature, as well as the scattered radiation intensity, within a grid cell associated with the direction set of the discrete ordinates method has been adopted. A grid close enough to each inner wall was necessary for a better estimation of the incident flux near the singularities of the considered system. These global improvements led to a new version of the stable MDT method, as accurate as the zonal method and as flexible as the discrete ordinates one.     

Contrôle de matériaux d'origine biologique par méthode photothermique: intérêt de la détection pyroélectrique

Photothermal methods are well adapted to thin multilayer material analysis. In the particular case of biological materials, which are thermally fragile, low excitation power is required. We have studied whole human blood sedimentation by using photothermal radiometry and a photopyroelectric technique. Results obtained with random- and sine-modulated excitation are discussed. Evolution laws of plasma thicknesses and optical absorption coefficients during the process have been determined by identifying the parameters using different estimation methods. It appears that results are consistent with theoretical predictions of our 2D-thermophysical model and also with the values measured by the well-known Westergreen reference method, usually used in clinical analysis. Finally, in order to extend the investigation field of the pyroelectric method, particularly in the domain of biological materials, we have analysed the possibility of using the sensitive pyroelectric sensors without direct thermal contact with the sample and detecting the temperature changes at the sample surface through a thin air monolayer.     

Conductivité thermique de mousses cellulaires de carbone: Mesures par méthode flash et interprétation

This paper deals with the thermal diffusivity measurement of cellular carbon foams by the flash method. The thermal conductivity is obtained from the thermal diffusivity by using specific heat and density of carbon foam. These materials constitute non-homogeneous porous semitransparent media, hence the classic experimental set-up of the flash method must be adapted. A study is carried out in order to show the influence of the sample thickness, of the temperature, of the enclosed gas pressure and of the gas composition on the effective thermal conductivity. Identification functions are established in the monodimensional and bidimensional cases. Results indicate the dependence of the equivalent thermal conductivity upon the sample thickness. This result, attributed to radiative transfer, is confirmed by modelling. Radiative thermal conductivities are obtained with a Monte-Carlo code. Experimental and theoretical results are compared. It is demonstrated that the concept of effective conductivity is not relevant to these materials.     

Commande floue et thermique du bâtiment. Proposition d'une loi de chauffe adaptée à la régulation des bâtiments de type tertiaire à occupation discontinue

Fuzzy control and thermal properties of buildings: proposition of a heating law fitting the regulation of tertiary buildings with intermittence. In this article, we start with a general presentation of thermal regulation in buildings and of fuzzy logic applied to the control. Then, thanks to the numerical simulation of a building, we show why regulators based on the use of a straight line heating law are not adapted to the practice of intermittence. The command law we propose is based on fuzzy control. It takes into account both the exterior temperature and the thermal state of the building we estimate by means of the same function whatever the building. Simulating the opening of a window, we show that the closing degree of the regulation loop is very weak. The inference system incorporates constraints upon the conclusions which allow to do an off-line learning from the second week (respect of the open-loop) without any risk of obtaining an aberrant power because of a non-learned rule. The regulation we obtain limits considerably the disfunctioning caused by the open-loop control with intermittent regime. We conclude upon the interest of fuzzy control as regards to the temperature regulation of complex dynamic systems.     

Application de la méthode des ordonnées discrètes au transfert radiatif dans un milieu bidimensionnel gris á géométrie complexe

Application of the discrete method to the radiative heat transfer in a two-dimensional grey medium of complex geometry. This paper describes a new approach in determining the radiative intensity and the temperature fields in a semi-transparent medium enclosed in a two-dimensional cavity the boundary surfaces of which are uniformly grey and purely isotropic diffuse reflectors, with the help of a new combination of ray tracing, finite volumes and discrete ordinates method. Since the grid used can be unstructured, the technique is applicable to the calculation of radiative transfer in enclosures of complex geometry. The basic equations are given, followed by results for cases of simple geometry compared with the exact solutions and the treatment of other cases of more complex geometry. The method eliminates oscillations in the intensity field and yields accurate results.     

Étude de deux cas d'écoulements thermoconvectifs le long d'une plaque verticale non uniformément chauffée dans une cavité

Study for two types of thermoconvective flows along a non-isothermal vertical wall in a cavity. For the study of the natural convection along a non-uniformly heated vertical wall in a cavity, we experimentally defined thermal curves. These curves were corrected by our semi-analytical corrective model. The two configurations, the first one instable, with a warm part lower to a cold part, and the other one, with warm and cold parts inverted are studied. We calculated convective flux densities and we showed a different comportment according to the configuration. We proposed some values for the convective exchange coefficient in the case of regular profiles with no extreme. We recommended some correlations for the stable areas of the flow close to the active wall.     

Facteur de bruit d'une chaı̂ne de cinématographie ultrarapide : application à la fusion par confinement inertiel

Inertial confinement fusion simulates in a laboratory the thermodynamic state of the center of stars, thus leading to the determination of stellar parameters. In order to reach that aim, high-speed cinematography brings up instruments specifically adapted to picosecond measurement, for which it is necessary to know the final precision. A model of the noise factor of the instruments under study is introduced and confronted to the experimental results obtained.     

Étude expérimentale de la résistance thermique de contact entre l'étain en cours de fusion et un creuset refroidi

This paper deals with the analysis of the thermal contact conditions during the melting of tin on different cooled walls, for different heat situations and for different cooling flows. Two experimental set ups are studied. A polished nickel substrate covers the melting-pot of the first one. Semi intrinsic thermocouples are implemented to measure the temperature of the substrate. This will allow a better study of the thermal contact resistance distribution on the wall. The second melting-pot is made of copper. Its surface roughness is variable. The heat system is stronger. The results obtained with the first experimental set-up show that the thermal contact resistance is time-dependent and non-uniformly spread. However, the heat evacuation is relatively uniformly spread while the heat power increases. Tests carried out with the second experimental set-up point out huge temperature oscillations which are attributed to unstable thermal contact conditions. This thermal behaviour can be explained by the buckling of the tin crust. The apparition of buckling seems to be favoured by a higher dissipated power and by a higher thermal conductivity of the melting pot.     

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.     

Theorie statistique des alliages metalliques binaires liquides a l'aide d'une methode de groupes de sites—Analyse de l'ordre local: II. Application a des alliages présentant une courbe d'enthalpie libre de mélange dissymétrique par rapport à la concentration

The model proposed in the preceding article is modified as to explain the non-symmetry of the enthalpy of mixing. Studied alloys are supposed to be compound-forming alloys so that the local order is modified. The structure of the compound is assumed to be A₃B.     

Modèle correctif de la mesure de la température d'un gaz par couple thermoélectrique. Application à une zone à fort gradient thermique

Corrective model of the gas temperature thermocouple measure. Application to an important thermal gradient zone. The gas temperature measurement with a thermocouple in important thermal gradient zones requires a corrective model. For example, such zones exist for thermal boundary layers near active walls. We have calculated these thermocouple thermal exchanges where connection wires are considered as fins with a variable ambient gas temperature. The heat exchanges by convection and radiation on the thermocouple head are analytically calculated, then a numerical method is used for fins where the space increment is the same as for the experimental measure. The corrective model in steady state is semi-analytical. Its validation is made with experimental results from studies of flows along a non-isothermal vertical wall in a cavity filled with wet air. Several applications are offered for many thermal curves, for more important gradient zones corrections are larger than 1.5 K for a K type thermocouple of 0.08 mm wire diameter.     

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.