Implementation of a new discrete Immersed Boundary Method in OpenFOAM

International audienceThe immersed boundary method proposed by Pinelli et al. (2010) has been implemented as a new object oriented library within the open source CFD solver OpenFOAM for incompressible bluff body fluid flows. The method encompasses the presence of fixed and moving solid obstacles in a computational mesh, without conforming to their boundaries. Standard Cartesian meshes are employed(uniform or stretched), which allows to use efficient and accurate flow solvers. The immersed obstacles are defined using a body force added on the conservation equations, and evaluated on Lagrangian markers that can move over the Eulerian mesh to capture the motion or the deformation of the body. The integration of the method in the finite-volume formalism and the succesfull integration of the method into the PISO algorithm will be detailed and a careful verification will be provided using a manufactured solution. The efficiency and the accuracy of the algorithm has been studied on various 2D and 3D simulations of flows around fixed and moving cylinder , including careful comparisons with available numerical and experimental results of the literature. Analysis of the computational cost, numerical behavior and accuracy of the numerical method show that the global properties of the OpenFOAM solver are not alterated. A quasi-linear scalability with the number of processors (up to 96) is obtained, with a slope slightly lower than the ideal scalability a feature that has been reported already in existing OpenFOAM studies. Work has been validated at Reynolds numbers in the range Re=30-500 and is in good agreement with reference data reported in the literature. Work is already in progress to extend the algorithm to the simulation of fluid structure interaction with induced oscillation and turbulent flows around bluff bodies for which preliminary results are in good agreement with reference data reported in the literature

\'Etude du cas rationnel de la th\'eorie des formes lin\'eaires de logarithmes. (French) [Study of the rational case of the theory of linear forms in logarithms]

We establish new measures of linear independence of logarithms on commutative algebraic groups in the so-called \emph{rational case}. More precisely, let k be a number field and v_{0} be an arbitrary place of k. Let G be a commutative algebraic group defined over k and H be a connected algebraic subgroup of G. Denote by Lie(H) its Lie algebra at the origin. Let u\in Lie(G(C_{v_{0}})) a logarithm of a point p\in G(k). Assuming (essentially) that p is not a torsion point modulo proper connected algebraic subgroups of G, we obtain lower bounds for the distance from u to Lie(H)\otimes_{k} C_{v_{0}}. For the most part, they generalize the measures already known when G is a linear group. The main feature of these results is to provide a better dependence in the height Log a of p, removing a polynomial term in LogLog a. The proof relies on sharp estimates of sizes of formal subschemes associated to H (in the sense of J.-B. Bost) obtained from a lemma by M. Raynaud as well as an absolute Siegel lemma and, in the ultrametric case, a recent interpolation lemma by D. Roy.Comment: Version d\'efinitiv

EFFECTS OF AXIAL MAGNETIC FIELD AND THERMAL CONVECTION ON A COUNTERROTATING VON KARMAN FLOW

International audienceThe effects of thermal convection and of a constant axial magnetic field on a von Karman flow driven by the exact counter-rotation of two lids are investigated in a vertical cylinder of aspect ratio Gamma(= height/radius) = 2 at a fixed Reynolds number Re(= Omega R-2/v) = 300. Direct numerical simulations are performed when varying separately the Rayleigh and Hartmann numbers in the range [0, 1800] and [0, 20], respectively, in the limit of the Boussinesq approximation and of a small magnetic Reynolds numbers, Re-m << 1. Without a magnetic field, the base flow symmetries of the von Karman flow are broken by thermal convection that becomes dominant in the range of Ra [500, 1000]. Three-dimensional solutions are characterized by the occurrence of a steady, m = 1, azimuthal mode exhibiting a cat's eye vortex in the circumferential plane. When increasing the Rayleigh number in the range [500, 1000], the vortex pulsates in an oscillatory manner, due to variations of the flow intensity. Otherwise, increasing the axial magnetic field intensity stabilizes the flow, and the oscillatory motion can be inhibited. Numerical solutions show that the critical Rayleigh number for transition increases linearly with the Hartmann number. Finally, results show that when varying the Rayleigh number, the structure of the electric potential can be strongly modified by thermal convection. Such an observation suggests new induction mechanisms in the case of small nonzero values of the magnetic Reynolds number

Stable isotopes reveal intrapopulation heterogeneity of porbeagle shark (Lamna nasus)

International audiencePorbeagle (Lamna nasus) is an ubiquist, highly mobile and long-living shark species with spatial and temporal sex and size segregation observed in catches. Porbeagle sharks were targeted by commercial fisheries in the European waters until the closure of the fishery in 2010. Most of the French fleet catches were located in Bay of Biscay, Celtic Sea and the English Channel. The aim of the study was to find out occurrences of spatial segregation within the shark population in the Northeast Atlantic Ocean based on the stable isotope method. Carbon and nitrogen stable isotopes (delta 13C, delta 15N) were measured in muscle of porbeagle sharks fished in the Bay of Biscay and the Celtic Sea between April and September 2008, and April and June 2009. Neither influence of sex nor ontogenic isotopic shift was detected in the sampled sharks, but muscle delta 13C and delta 15N values increased significantly from offshore towards coastal area, which reflected inter-areas variability. Realized isotopic niches were similar amongst offshore areas whilst in St Georges' Channel muscle isotopic composition exhibited higher interindividual variability. This underpins the lack of information on life history of porbeagle sharks and underlines the need of more information on the species movements to support population management in European waters

L'énergie à découvert

L'énergie est devenue une question vitale pour les sociétés, le citoyen, l'humanité tout entière. Sujet scientifique, économique, politique et écologique majeur, elle suscite des débats, parfois violents, sur les choix à faire aujourd'hui et leurs conséquences pour l'avenir des hommes et de la planète. Mais, alors que se tient le grand débat national sur la transition énergétique, comment se forger une opinion objective sans connaître les données scientifiques les plus complètes sur les potentiels et les limites de chaque source d'énergie ? Ce livre les met enfin à la disposition du public. L'énergie, qu'est-ce que c'est ? Quelles sont les grandes lois physiques qui la gouvernent ? Comment la produire, la transporter, la stocker ? Le solaire, la biomasse, l'éolien, l'hydraulique sont-ils des solutions alternatives suffisantes ? Et quelle part leur réserver à l'avenir ? Les nombreux articles de ce livre (près de 130) proposent au citoyen des outils pour se faire une opinion face à ces questions. Physiciens, chimistes, biologistes, géophysiciens, environnementalistes, géographes, économistes, y précisent, chiffres et schémas à l'appui, la place respective des énergies fossiles, du nucléaire et des énergies renouvelables. Au-delà, ils expliquent quelles sont les perspectives offertes par la science sur le mix énergétique, le problème du stockage, l'amélioration de nos usages de l'énergie, ses impacts environnementaux et sanitaires

Proceedings of the fourth Resilience Engineering Symposium

These proceedings document the various presentations at the Fourth Resilience Engineering Symposium held on June 8-10, 2011, in Sophia-Antipolis, France. The Symposium gathered participants from five continents and provided them with a forum to exchange experiences and problems, and to learn about Resilience Engineering from the latest scientific achievements to recent practical applications. The First Resilience Engineering Symposium was held in Söderköping, Sweden, on October 25-29 2004. The Second Resilience Engineering Symposium was held in Juan-les-Pins, France, on November 8-10 2006, The Third Resilience Engineering Symposium was held in Juan-les-Pins, France, on October 28-30 2008. Since the first Symposium, resilience engineering has fast become recognised as a valuable complement to the established approaches to safety. Both industry and academia have recognised that resilience engineering offers valuable conceptual and practical basis that can be used to attack the problems of interconnectedness and intractability of complex socio-technical systems. The concepts and principles of resilience engineering have been tested and refined by applications in such fields as air traffic management, offshore production, patient safety, and commercial fishing. Continued work has also made it clear that resilience is neither limited to handling threats and disturbances, nor confined to situations where something can go wrong. Today, resilience is understood as the intrinsic ability of a system to adjust its functioning prior to, during, or following changes and disturbances, so that it can sustain required operations under both expected and unexpected conditions. This definition emphasizes the ability to continue functioning, rather than simply to react and recover from disturbances and the ability to deal with diverse conditions of functioning, expected as well as unexpected. For anyone who is interested in learning more about Resilience Engineering, the books published in the Ashgate Studies in Resilience Engineering provide an excellent starting point. Another sign that Resilience Engineering is coming of age is the establishment of the Resilience Engineering Association. The goal of this association is to provide a forum for coordination and exchange of experiences, by bringing together researchers and professionals working in the Resilience Engineering domain and organisations applying or willing to apply Resilience Engineering principles in their operations. The Resilience Engineering Association held its first General Assembly during the Fourth Symposium, and will in the future play an active role in the organisation of symposia and other activities related to Resilience Engineering