Geometrical statistics and vortex structures in helical and nonhelical turbulences

In this paper we conduct an analysis of the geometrical and vortical statistics in the small scales of helical and nonhelical turbulences generated with direct numerical simulations. Using a filtering approach, the helicity flux from large scales to small scales is represented by the subgrid-scale (SGS) helicity dissipation. The SGS helicity dissipation is proportional to the product between the SGS stress tensor and the symmetric part of the filtered vorticity gradient, a tensor we refer to as the vorticity strain rate. We document the statistics of the vorticity strain rate, the vorticity gradient, and the dual vector corresponding to the antisymmetric part of the vorticity gradient. These results provide new insights into the local structures of the vorticity field. We also study the relations between these quantities and vorticity, SGS helicity dissipation, SGS stress tensor, and other quantities. We observe the following in both helical and nonhelical turbulences: (1) there is a high probability to find the dual vector aligned with the intermediate eigenvector of the vorticity strain rate tensor; (2) vorticity tends to make an angle of 45 with both the most contractive and the most extensive eigendirections of the vorticity strain rate tensor; (3) the vorticity strain rate shows a preferred alignment configuration with the SGS stress tensor; (4) in regions with strong straining of the vortex lines, there is a negative correlation between the third order invariant of the vorticity gradient tensor and SGS helicity dissipation fluctuations. The correlation is qualitatively explained in terms of the self-induced motions of local vortex structures, which tend to wind up the vortex lines and generate SGS helicity dissipation. In helical turbulence, we observe that the joint probability density function of the second and third tensor invariants of the vorticity gradient displays skewed distributions, with the direction of skewness depending on the sign of helicity input. We also observe that the intermediate eigenvalue of the vorticity strain rate tensor is more probable to take negative values. These interesting observations, reported for the first time, call for further studies into their dynamical origins and implications. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3336012

Three-dimensionality in quasi-two dimensional flows: recirculations and barrel effects

A scenario is put forward for the appearance of three-dimensionality both in quasi-2D rotating flows and quasi-2D magnetohydrodynamic (MHD) flows. We show that 3D recirculating flows and currents originate in wall boundary layers and that, unlike in ordinary hydrodynamic flows, they cannot be ignited by confinement alone. They also induce a second form of three-dimensionality with quadratic variations of velocities and current across the channel. This scenario explains both the common tendency of these flows to two-dimensionality and the mechanisms of the recirculations through a single formal analogy covering a wide class of flow including rotating and MHD flows. These trans-disciplinary effects are thus active in atmospheres, oceans or the cooling blankets of nuclear fusion reactors.Comment: 6 pages, 1 Figur

On knotted streamtubes in incompressible hydrodynamical flow and a restricted conserved quantity

For certain families of fluid flow, a new conserved quantity -- stream-helicity -- has been established.Using examples of linked and knotted streamtubes, it has been shown that stream-helicity does, in certain cases, entertain itself with a very precise topological meaning viz, measure of the degree of knottedness or linkage of streamtubes.As a consequence, stream-helicity emerges as a robust topological invariant.Comment: This extended version is the basically a more clarified version of the previous submission physics/0611166v

Dynamic Provenance for SPARQL Update

While the Semantic Web currently can exhibit provenance information by using the W3C PROV standards, there is a "missing link" in connecting PROV to storing and querying for dynamic changes to RDF graphs using SPARQL. Solving this problem would be required for such clear use-cases as the creation of version control systems for RDF. While some provenance models and annotation techniques for storing and querying provenance data originally developed with databases or workflows in mind transfer readily to RDF and SPARQL, these techniques do not readily adapt to describing changes in dynamic RDF datasets over time. In this paper we explore how to adapt the dynamic copy-paste provenance model of Buneman et al. [2] to RDF datasets that change over time in response to SPARQL updates, how to represent the resulting provenance records themselves as RDF in a manner compatible with W3C PROV, and how the provenance information can be defined by reinterpreting SPARQL updates. The primary contribution of this paper is a semantic framework that enables the semantics of SPARQL Update to be used as the basis for a 'cut-and-paste' provenance model in a principled manner.Comment: Pre-publication version of ISWC 2014 pape

Force indeterminacy in the jammed state of hard disks

Granular packings of hard discs are investigated by means of contact dynamics which is an appropriate technique to explore the allowed force-realizations in the space of contact forces. Configurations are generated for given values of the friction coefficient, and then an ensemble of equilibrium forces is found for fixed contacts. We study the force fluctuations within this ensemble. In the limit of zero friction the fluctuations vanish in accordance with the isostaticity of the packing. The magnitude of the fluctuations has a non-monotonous friction dependence. The increase for small friction can be attributed to the opening of the angle of the Coulomb cone, while the decrease as friction increases is due to the reduction of connectivity of the contact-network, leading to local, independent clusters of indeterminacy. We discuss the relevance of indeterminacy to packings of deformable particles and to the mechanical response properties.Comment: 4 pages, 3 figures. Minor changes, journal reference adde

La production scientifique issue de la formation initiale à la faculté de médecine d’Angers entre 2002 et 2008 : de bonne qualité mais insuffisante

IntroductionL’activité de publication scientifique issue de la formation initiale est mal connue. Nous avons évalué la proportion des thèses et des mémoires de spécialité aboutissant à une publication, leur impact en termes de points SIGAPS et les principaux obstacles à la publication de ces travaux. Méthodes Les thèses de doctorat en médecine soutenues de 2002 à 2008 à la faculté de médecine d’Angers ont été recensées à partir du système universitaire de documentation (SUDOC), et les mémoires de spécialité directement auprès des diplômés. Les publications ont été recherchées dans Medline via Pubmed, ISI Web of Knowledge et dans les trois rapports SIGAPS de 2002 à 2008. Une enquête cherchant à connaître les obstacles à la publication a été lancée auprès de tous les directeurs de thèses et responsables de discipline impliqués. Résultats Cinq cent quatre-vingt-dix-huit thèses ont été soutenues, 311 (52 %) en médecine générale et 287 (48 %) en spécialités. Cent soixante-cinq thèses ont donné lieu à publication (28 %) dont 97 (16 %) indexés dans Medline via Pubmed (soit 11 % en médecine générale et 22 % en spécialité). Trente-trois de ces 97 articles (35 %) ont été publiés dans des journaux de haute qualité classés A, B ou C de la classification SIGAPS. Ces articles issus des thèses ont représenté 4,17 % du score SIGAPS du CHU calculé sur un total de 2088 articles au cours de cette période. Deux cent quatre mémoires de spécialités ont donné lieu à 69 articles (33,8 %), dont 50 (24,5 %) indexés dans Medline. Les taux de publication des mémoires de DES et de DESC ont été respectivement de 31 % (45/145) et de 40,7 % (24/59). Ils ont représenté 1,9 % (432 points) du score total SIGAPS. Les principaux obstacles ressentis pour la publication ont été le manque de temps des directeurs, l’éloignement des étudiants après la soutenance et le manque de moyens logistiques. Conclusion La production scientifique issue de la formation initiale à la faculté de médecine d’Angers a été de bonne qualité mais insuffisante quantitativement et peu contributive au financement du CHU malgré un nombre important de diplômés. Un soutien logistique mérite d’être envisagé afin de valoriser la production scientifique issue de la formation initiale