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Influence of fin size and distribution on solid-liquid phase change in a rectangular enclosure
No full textInternational audienceThis paper details a numerical study of solid-liquid phase change heat transfer in a rectangular enclosure with the aim of optimizing the number, dimension, and positioning of fins in the enclosure. The enclosure is exposed on one side to a constant heat flux of 1000 W/m2 for 3 h and both the total fin mass and the PCM mass are kept constant in all simulated cases. The heat diffusion equation in the PCM uses the equivalent heat capacity method while natural convection driven PCM motion in the enclosure is modeled through the buoyancy force, using a modified viscosity and an additional volume force term in the Navier-Stokes momentum conservation equation. Three efficiency assessment parameters are used: (i) the height-averaged temperature of the front hot plate which should remain as low as possible for the longer possible time, (ii) the energy stored inside the PCM as a function of time, and (iii) the heat transfer rate and heat flux between the front plate and the PCM. For each simulated case, complementary data such as the standard deviation of temperature along the heated plate and the ratio of sensible heat to latent accumulation are also calculated. Results show that increasing the number of fins diminishes both the stabilization temperature and the stabilization time of the front plate during phase change, and accelerates the sensible and latent storage of energy in the PCM. Using thinner but longer fins provides the same impact except that the stabilization time also increases as the fin length is increased. Besides, at constant fin mass, varying fin spacing have marginal impact on the latent heat storage performance and on the hot plate temperature stabilization. The study also shows that the surface area plays the largest role in the increase of the heat transfer rate between the front plate and the PCM, while configurations which can promote stronger natural convection lead to higher heat flux. Finally, it was observed that systems allowing easier heat transfer to the back plate during melting provide a higher ratio of sensible heat to latent accumulation, while the standard deviation of the front plate temperature decreases as the number of fins or the spacing between fins increases
Algorithmes d'approximation pour le placement de chaînes de fonctions de services avec des contraintes d'ordre
No full textA Service Function Chain (SFC) is an ordered sequence of network functions, such as load balancing, content filtering, and firewall. With the Network Function Virtualization (NFV) paradigm, network functions can be deployed as pieces of software on generic hardware, leading to a flexibility of network service composition. Along with its benefits, NFV brings several challenges to network operators, such as the placement of virtual network functions. In this paper, we study the problem of how to optimally place the network functions within the network in order to satisfy all the SFC requirements of the flows. Our optimization task is to minimize the total deployment cost.We show that the problem can be seen as an instance of the Set Cover Problem, even in the case of ordered sequences of network functions. It allows us to propose two logarithmic factor approximation algorithms which have the best possible asymptotic factor. Further, we devise an optimal algorithm for tree topologies. Finally, we evaluate the performances of our proposed algorithms through extensive simulations. We demonstrate that near-optimal solutions can be found with our approach.Le modèle des réseaux programmables (Software Defined Networks), permet de centraliser la gestion du réseau sur un ou plusieurs contrôleurs et par conséquent de découpler la fonction de contrôle des flux de données. Ce paradigme permet aux opérateurs de réseaux de télécommunications d'offrir des services réseaux complexes et flexibles. Un service se modélise alors comme une chaîne de fonctions réseaux (firewall, compression, contrôle parental ...) qui doivent être appliquées séquentiellement à un flot de données. Dans cet article, nous étudionsle problème du placement de fonctions de services qui consiste à determiner sur quels noeuds localiser les fonctions afin de satisfaire toutes les demandes de service, de façon à minimiser le coût de déploiement.Nous montrons que le problème peut être ramené à un problème de Set Cover, même dans le cas de séquences ordonnées de fonctions réseau. Cela nous permet de proposer deux algorithmes d'approximation à facteur logarithmique, ce qui est le meilleur facteur possible. De plus, nous proposons un algorithme optimal dans le cas particulier ou la topologie des demandes est un arbre. Finalement, nous évaluons les performances de nos algorithmes par simulations. Nous montrons ainsi qu'en pratique, des solutions presque optimales peuvent être trouvées avec notre approche
Beyond Time-Triggered Co-simulation of Cyber-Physical Systems for Performance and Accuracy Improvements
No full textInternational audienceCyber-Physical Systems consist of cyber components controlling physical entities. Their development involves different engineering disciplines, that use different models, written in languages with different semantics. A coupled simulation of these models is of prime importance to rapidly understand the emerging system behavior. The coupling of the simulations is realized by a coordinator that conveys data and ensures time consistency between the different models/simulators. Existing coordinators are usually time triggered. In this paper we show that time-triggered coordinators may introduce poor performance and accuracy (both temporal and functional) when used to co-simulate cyber-physical models. Therefore, we propose a new coordinator mixing time-and event-triggered mechanisms. We validated the approach in the context of the FMI standard for co-simulation. To make possible the writing of the new coordinators , we implemented backward compatible extensions to the FMI API. Also, we implemented a new FMI exporter in an industrial tool
Innovation, Credit Constraints and National Banking Systems: A Comparison of Developing Nations
No full textInternational audienc
Modified scattering for the critical nonlinear Schrödinger equation.
No full textInternational audienceWe consider the nonlinear Schr\"o\-din\-ger equation \begin{equation*} iu_t + \Delta u= \lambda |u|^{\frac {2} {N}} u \end{equation*} in all dimensions , where and . We construct a class of initial values for which the corresponding solution is global and decays as , like if and like if . Moreover, we give an asymptotic expansion of those solutions as .We construct solutions that do not vanish, so as to avoid any issue related to the lack of regularity of the nonlinearity at . To study the asymptotic behavior, we apply the pseudo-conformal transformation and estimate the solutions by allowing a certain growth of the Sobolev norms which depends on the order of regularity through a cascade of exponents
An implicit hybridized discontinuous Galerkin method for the 3D time-domain Maxwell equations
No full textInternational audienceWe present a time-implicit hybridizable discontinuous Galerkin (HDG) method for numerically solving the system of three-dimensional (3D) time-domain Maxwell equations. This method can be seen as a fully implicit variant of classical so-called DGTD (Discontinuous Galerkin Time-Domain) methods that have been extensively studied during the last 10 years for the simulation of time-domain electromagnetic wave propagation. The proposed method has been implemented for dealing with general 3D problems discretized using unstructured tetrahedral meshes. We provide numerical results aiming at assessing its numerical convergence properties by considering a model problem on one hand, and its performance when applied to more realistic problems. We also include some performance comparisons with a centered flux time-implicit DGTD method
Productivity and wage premiums: Evidence from Vietnamese ordinary and processing exporters
No full textInternational audienc
Computer-Aided Formal Proofs about Dendritic Integration within a Neuron
No full textInternational audienceThis article is threefold: (i) we define the first formal framework able to model dendritic integration within biological neurons, (ii) we show how we can turn continuous time into discrete time consistently and (iii) we show how a Lustre model checker can automatically perform proofs about neuron input/output behavioursowing to our framework.Our innovative formal framework is a carefully defined trade-off between abstraction and biological relevance in order to facilitate proofs. This framework is hybrid: inputs entering the synapses as well as the soma output are discrete signals made of spikes but, inside the dendrites, we combine signals quantitatively using real numbers. The soma potential is inevitably specified as a differential equation to keep a biologically accurate modelling of signal accumulation. This prevents from performing simple formal proofs. This has been our motivation to discretize time. Owing to this discretization, we are able to encode our neuron models in Lustre. Lustre is a particularly well suited flow-based language for our purpose. We also encode in Lustre a property of input/output equivalence between neurons in such a way that the model checker Kind2 is able to automatically handle the proof
How Well Do Owner–Managers Forecast Potential Future Bankruptcy? Assessing Fear of Failure Using Employment Insurance for SMEs
No full textInternational audienc
A Model-checking Approach to Reduce Spiking Neural Networks
No full textInternational audienceIn this paper we formalize Boolean Probabilistic Leaky Integrate and Fire Neural Networks as Discrete-Time Markov Chains using the language PRISM. In our models, the probability for neurons to emit spikes is driven by the difference between their membrane potential and their firing threshold. The potential value of each neuron is computed taking into account both the current input signals and the past potential value. Taking advantage of this modeling, we propose a novel algorithm which aims at reducing the number of neurons and synaptical connections of a given network. The reduction preserves the desired dynamical behavior of the network, which is formalized by means of temporal logic formulas and verified thanks to the PRISM model checker