72 research outputs found
Étude de la diffusion de chromophores dans la peau humaine par la spectroscopie photoacoustique pulsée
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
Simulation EHD des précipitateurs électrostatiques
On examine le fonctionnement aérodynamique d'un précipitateur électrostatique compte tenu des connaissances actuelles sur les différents régimes d'électroconvection. On montre que la turbulence dans un précipitateur est essentiellement d'origine électrohydrodynamique et que l'intéraction entre les particules chargées de petite taille et le mouvement turbulent du gaz est tout à fait analogue à celle existant entre les ions injectés et la turbulence de la plupart des liquides diélectriques. Une simulation expérimentale, avec un liquide, d'un précipitateur à deux étages est définie. Une étude expérimentale est présentée et ses résultats sont comparés à des prédictions théoriques récentes
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..
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