Modélisation des propriétés diélectriques des nanocomposites

Dans ce travail, nous nous intéressons aux propriétés diélectriques dans le domaine des micro-ondes (2.45, 9.50 et 35 GHz) des composites constitués des inclusions de noir de carbone dans une matrice résine époxyde. Nous avons utilisé deux types de carbone : le monarch 700 et le sterling, dont les diamètres des inclusions sont respectivement 0.018 et 0.175 μm. Les résultats obtenus montrent que la taille des particules conductrices parait être un paramètre prépondérant pour définir la permittivité complexe de ces composites. Il résulte de cette étude que les modèles proposés pour la détermination de permittivité de ces milieux, devront nécessairement prendre en considération la taille des inclusions.Mots-clés : Lois de mélange, propriétés diélectriques, permittivité complexe,nanocomposites, matrice époxyde, inclusions de carbone

Heat Transport through Rough Channels

We investigate the two-dimensional transport of heat through viscous flow between two parallel rough interfaces with a given fractal geometry. The flow and heat transport equations are solved through direct numerical simulations, and for different conduction-convection conditions. Compared with the behavior of a channel with smooth interfaces, the results for the rough channel at low and moderate values of the Peclet number indicate that the effect of roughness is almost negligible on the efficiency of the heat transport system. This is explained here in terms of the Makarov's theorem, using the notion of active zone in Laplacian transport. At sufficiently high Peclet numbers, where convection becomes the dominant mechanism of heat transport, the role of the interface roughness is to generally increase both the heat flux across the wall as well as the active length of heat exchange, when compared with the smooth channel. Finally, we show that this last behavior is closely related with the presence of recirculation zones in the reentrant regions of the fractal geometry.Comment: 12 pages, 8 figure

Shared Virtual Memory and Message Passing Programming on a Finite Element Application

This paper describes the methods used and experiences made with implementing a finite element application on three different parallel computers with either message passing or shared virtual memory as the programming model. Designing a parallel finite element application using message-passing requires to find a data domain decomposition to map data into the local memory of the processors. Since data accesses may be very irregular, communication patterns are unknown prior to the parallel execution and thus makes the parallelization a difficult task. We argue that the use of a shared virtual memory greatly simplifies the parallelization step. It is shown experimentally on an hypercube iPSC/2 that the use of the KOAN/Fortran-S programming environment based on a shared virtual memory allows to port quickly and easily a sequential application without a significant degradation in performance compared to the message passing version. Results for recent parallel architectures such as the Paragon XP/S for message-passing and the KSR1 for shared virtual memory are presented, too

Influence of variable viscosity and thermal conductivity, hydrodynamic and thermal slips on magnetohydrodynamic micropolar flow: a numerical study

Thermophysical and wall slip effects arise in many areas of nuclear technology. Motivated by such applications, in this article the collective influence ofvariable viscosity, thermal conductivity, velocity and thermal slipseffects on a steady two-dimensional magnetohydrodynamic microplar fluid over a stretching sheet are analyzednumerically. The governing nonlinear partial differential equations have been converted into a system of non-linear ordinary differential equations using suitable coordinate transformations. The numerical solutions of the problem are expressed in the form of non-dimensional velocityand temperature profiles and discussed from their graphical representations. Nachtsheim-Swigert shooting iteration technique together withthesixth order Runge-Kutta integration scheme has been applied for the numerical solution.A comparison with the existing results has been done and an excellent agreement is found.Further validation with adomian decomposition method is included for the general model. Interesting features in the heat and momentum characteristics are explored. It is found that greater thermal slip and thermal conductivity elevate thermal boundary layer thickness. Increasing Prandtl number enhances Nusselt number at the wall but reduces wall couple stress (micro-rotation gradient). Temperatures are enhanced with both magnetic field and viscosity parameter. Increasing momentum (hydrodynamic) slip is found to accelerate the flow and elevate temperatures

Trade-Offs Between Shared Virtual Memory And Message-Passing On An Ipsc/2 Hypercube

: This paper presents the results of an experiment which evaluates the performance of shared virtual memory running on a distributed memory parallel architecture (iPSC/2 hypercube) . Two parallel versions of the modified Gram-Schmidt algorithm are presented. Each uses two disparate distribution schemes: data distribution; and control distribution. In the first case, cooperation between processes is achieved by sending messages, whereas shared variables are applied in the second case. This latter technique is made possible by implementing a shared virtual memory as part of the operating system. We present briefly an example of such a mechanism, called KOAN, which has been designed specifically for the iPSC/2 hypercube. This shared virtual memory allows us to compare experimentally the performances of the two distribution schemes. Theoretical execution models are also addressed in order to extrapolate expected performance on a large number of processors. (R'esum'e : tsvp) CENTRE NATIONAL..

Caracterisation de la structure des sillages MHD amont et aval d'un cylindre a petit nombre de Reynolds magnetique

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc