Discrete-time port-Hamiltonian systems: A definition based on symplectic integration

We introduce a new definition of discrete-time port-Hamiltonian systems (PHS), which results from structure-preserving discretization of explicit PHS in time. We discretize the underlying continuous-time Dirac structure with the collocation method and add discrete-time dynamics by the use of symplectic numerical integration schemes. The conservation of a discrete-time energy balance - expressed in terms of the discrete-time Dirac structure - extends the notion of symplecticity of geometric integration schemes to open systems. We discuss the energy approximation errors in the context of the presented definition and show that their order is consistent with the order of the numerical integration scheme. Implicit Gauss-Legendre methods and Lobatto IIIA/IIIB pairs for partitioned systems are examples for integration schemes that are covered by our definition. The statements on the numerical energy errors are illustrated by elementary numerical experiments.Comment: 12 pages. Preprint submitted to Systems & Control Letter

Weak Form of Stokes-Dirac Structures and Geometric Discretization of Port-Hamiltonian Systems

We present the mixed Galerkin discretization of distributed parameter port-Hamiltonian systems. On the prototypical example of hyperbolic systems of two conservation laws in arbitrary spatial dimension, we derive the main contributions: (i) A weak formulation of the underlying geometric (Stokes-Dirac) structure with a segmented boundary according to the causality of the boundary ports. (ii) The geometric approximation of the Stokes-Dirac structure by a finite-dimensional Dirac structure is realized using a mixed Galerkin approach and power-preserving linear maps, which define minimal discrete power variables. (iii) With a consistent approximation of the Hamiltonian, we obtain finite-dimensional port-Hamiltonian state space models. By the degrees of freedom in the power-preserving maps, the resulting family of structure-preserving schemes allows for trade-offs between centered approximations and upwinding. We illustrate the method on the example of Whitney finite elements on a 2D simplicial triangulation and compare the eigenvalue approximation in 1D with a related approach.Comment: Copyright 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0

L'émergence du Green-IT, pour une informatique plus verte

National audienceLe " Green IT " a pour but d'améliorer l'efficacité énergétique des systèmes distribués à grande échelle, comme les data-centers, les clouds et les réseaux de télécommunications. Ce domaine a pu se développer grâce aux modèles plus sophistiqués de l'usage énergétique des équipements informatiques. Et les enjeux écologiques et financiers sont devenus importants ces dernières années. Les plus gros data-centers au monde consomment énormément d'électricité : plus de 10 mégawatts. Et ce n'est que le début, car cette consommation pourrait atteindre 25 mégawatts avec la construction des futures machines exascale

Percolation of single-walled carbon nanotubes in ceramic matrix nanocomposites

The percolation of carbon nanotubes (CNT) in an electrical insulating ceramic is studied for the first time. The in situ synthesis of the CNT (0.2–25 vol%) by a CCVD route allows to achieve their homogeneous distribution in the spinel matrix. Up to 11 vol% CNT, the DC electrical conductivity (σ) is well fitted by the scaling law of the percolation theory σ=k(p−pc)t with a low percolation threshold pc=0.64 vol%. At the threshold, σ jumps over seven order of magnitude (from 10−10 to 0.0040 S cm−1) and then reaches a maximum at 8.5 S cm−1. The results are discussed in relation with the characteristics of the CNT, their damaging during the hot-pressing at 1300 °C and the microstructure of the composites. CNT-ceramic composites become attractive materials not only for their enhanced mechanical properties, but also for the possibility to tailor the electrical conductivity through the CNT content

Can we trace very cold dust from its emission alone ?

Context. Dust is a good tracer of cold dark clouds but its column density is difficult to quantify. Aims. We want to check whether the far-infrared and submillimeter high-resolution data from Herschel SPIRE and PACS cameras combined with ground-based telescope bolometers allow us to retrieve the whole dust content of cold dark clouds. Methods. We compare far-infrared and submillimeter emission across L183 to the 8 $\mu$m absorption map from Spitzer data and fit modified blackbody functions towards three different positions. Results. We find that none of the Herschel SPIRE channels follow the cold dust profile seen in absorption. Even the ground-based submillimeter telescope observations, although more closely following the absorption profile, cannot help to characterize the cold dust without external information such as the dust column density itself. The difference in dust opacity can reach up to a factor of 3 in prestellar cores of high extinction. Conclusions. In dark clouds, the amount of very cold dust cannot be measured from its emission alone. In particular, studies of dark clouds based only on Herschel data can miss a large fraction of the dust content. This has an impact on core and filament density profiles, masse and stability estimates.Comment: Letter to A&A (accepted for publication). must be viewed with ACROBAT READER for full enhancement. Otherwise, check images in Appendix

When Clouds become Green: the Green Open Cloud Architecture

Virtualization solutions appear as alternative approaches for companies to consolidate their operational services on a physical infrastructure, while preserving specific functionalities inside the Cloud perimeter (e.g., security, fault tolerance, reliability). These consolidation approaches are explored to propose some energy reduction while switching OFF unused computing nodes. We study the impact of virtual machines aggregation in terms of energy consumption. Some load-balancing strategies associated with the migration of virtual machines inside the Cloud infrastructures will be showed. We will present the design of a new original energy-efficient Cloud infrastructure called Green Open Cloud

Numérique : 1. Environnement : 0. Avec EcoInfo, changeons nos pratiques

National audience2015 : la France préside la 21e Conférence des Nations unies sur les changements climatiques 2015 : EcoInfo, récemment transformé en groupement de service, a 10 ans 2015 : les informaticiens se retrouvent aux JRES ! Une telle conjonction est une occasion unique pour imprimer dans ces JRES 2015 une empreinte Écoresponsable. Paris accueille la conférence sur le climat et nous, informaticiens, resterions en dehors des initiatives et des solutions pour lutter contre les bouleversements climatiques ? Non ! D'autant que les retours d'expérience se multiplient. Alors aujourd'hui, nous avons choisi de donner de la visibilité à vos bonnes pratiques, celles qui se mettent en place dans le monde de l'enseignement supérieur et de la recherche. Alors montez dans le train avec ceux qui y sont déjà et donnons ensemble un coup d'accélérateur à la réduction des émissions de gaz à effet de serre et autres polluants en tout genre ! Prise en compte de critères relatifs au développement durable dans nos achats, gestion de l'énergie de nos salles informatiques, éco-conception des data-centres quelle que soit leur taille, gestion des déchets : les frontières de nos métiers se déplacent. Le temps ou les informaticiens avaient les yeux rivés à l'écran et les mains collées au clavier est révolu, aujourd'hui, les bonnes pratiques émergent et commencent à se répandre. Aujourd'hui, des chercheurs n'ignorent plus les impacts des TICs sur l'environnement. Cette présentation se veut un catalyseur d'échanges autour d'un objectif commun : participer au mouvement COP21. JRES 2015 – Montpellier 1/

A year in the life of a large scale experimental distributed system: the Grid'5000 platform in 2008

This report presents the usage results of Grid'5000 over year 2008. Usage of the main operationnal Grid'5000 sites (Bordeaux, Lille, Lyon, Nancy, Orsay, Rennes, Sophia-Antipolis, Toulouse) is presented and analyzed