Simulation numérique de la formation d'un dépôt de particules sur une surface poreuse. Application à la filtration d'arrêt

L'écoulement de Stokes à travers un dépôt constitué par l'empilement de particules sur une surface poreuse, rencontré en filtration, est classiquement modélisé par la loi de Darcy. Cependant, l'effet de singularités géométriques sur l'interface dépôt-surface poreuse met en défaut le modèle de Darcy lorsque la taille caractéristique des pores de la surface, est de l'ordre de la taille des grains ou des pores du dépôt (pas de séparation des échelles). Tout d'abord, lorsque la séparation des échelles est assurée, la perméabilité apparente de la couche de particules est calculée numériquement par la résolution semi-analytique de l'équation de Darcy en faisant l'hypothèse que la couche de particules a une épaisseur uniforme. Des corrélations simples sont établies entre la perméabilité apparente, la hauteur de la couche de particules et la porosité de la paroi filtrante. Ensuite, pour s'affranchir de l'hypothèse d'épaisseur uniforme, l'accumulation progressive des particules sur la surface poreuse a été simulée. Aux temps courts, les résultats mettent en évidence, pour un même volume déposé et une même porosité de paroi, une perméabilité apparente plus faible que dans le cas d'une couche de particules d'épaisseur uniforme. Aux temps longs, les perméabilités apparentes des dépôts uniforme et non uniforme sont identiques. Puis, une étude comparative des modèles de Brinkman et de Stokes pour calculer la perméabilité apparente d'un empilement régulier de cylindres a été effectuée. La résolution des équations de Stokes a été réalisée par une méthode originale d'éléments frontières spectraux. Contrairement à la perméabilité apparente évaluée par le modèle de Darcy, la perméabilité apparente évaluée avec le modèle de Brinkman est sensible à la compacité du dépôt. Elle rend compte notamment de la compétition entre la résistance hydraulique due aux particules et celle due au frottement sur la paroi. Finalement, lorsqu'il n'y a pas séparation des échelles, la modélisation de la formation d'un dépôt de particules sphériques a été mise en oeuvre par la simulation de la formation de la formation d'un dépôt en trois dimensions par l'équation de Brinkman. Cette méthode consiste à lâcher une par une des particules dans le domaine. Les particules suivent les lignes de courant jusqu'à leur immobilisation lorsqu'elles entrent en contact avec la paroi ou une autre particule déjà déposée. L'écoulement, dans le domaine fluide faiblement concentré en particules et au sein de la couche de particules déposées, est recalculé par l'équation de Brinkman après chaque dépôt de particule. Cette approche semble être un bon compromis entre les approches simplistes du point de vue hydrodynamique et les approches plus rigoureuses de type simulation numérique directe car elle permet de prendre en compte l'influence des particules sur l'écoulement. L'étude préliminaire a permis d'évaluer l'influence de la taille des particules et des pores de la paroi sur des paramètres macroscopiques telle que l'évolution du débit en fonction du volume déposé, de la compacité moyenne du dépôt et de l'efficacité de filtration. ABSTRACT : Stokes flow through a porous layer backed by a perforated plate, met in filtration, is classically modeled by the Darcy's law. However, the effect of geometrical singularities on the interface porous layer-perforated plate puts the Darcy's model in wrong when the characteristic size of the perforations, is about the size of the grains or the of the pores of the deposit (no separation of the scales). When there is separation of the scales, the apparent permeability of the layer of particles is numerically calculated by the semi-analytical resolution of the Darcy's equation by making the assumption that the layer of particles has a uniform thickness. Simple correlations are established between the apparent permeability, the height of the particles layer and the porosity of the perforated plate. Then, to free ourselves from the assumption of uniform thickness, the progressive accumulation of the particles on porous surface was simulated. For the same deposited volume and the same porosity of the perforated plate, the apparent permeability is weaker than in the case of a porous layer of uniform thickness. The apparent permeabilities of the uniform and nonuniform deposits tend to be identical when the porous layer is sufficiently thick. When there is not separation of the scales, a comparative study of the models of Brinkman and Stokes to compute the apparent permeability of an array of cylinders was carried out. The Stokes equations was solved by an original method of circular boundary elements. It was shown that the apparent permeability evaluated with the model of Brinkman is sensitive to the compactness and the radius of the particles contrary to the apparent permeability evaluated by the Darcy's model. The competition between hydraulic resistance due to the particles and friction of the wall is taken into account. The modeling of the formation of a deposit of spherical particles was modeled by the Brinkman's model. Particles enter one by one in th computation domain. The particles follow the stream lines until they stopped when they come into contact with the wall or another already deposited particle. The flow field slightly concentrated in particles the and within the layer of deposited particles, is recomputed by the equation of Brinkman after every deposition of particle. This approach seems to be a good compromise between the simplistic approaches from the hydrodynamic point of view and the more rigorous approaches of direct numerical simulation type because it makes it possible to take into account the influence of the particles on the flow. It was possible to evaluate the influence of the size of the particles and the pores of the perforated plate on macroscopic parameters such as the evolution of the flow according to deposited volume, of the average compactness of the deposit and the effectiveness of filtration

Internal friction and absence of dilatancy of packings of frictionless polygons

By means of numerical simulations, we show that assemblies of frictionless rigid pentagons in slow shear flow possess an internal friction coefficient (equal to 0.183 ± 0.008 with our choice of moderately polydisperse grains) but no macroscopic dilatancy. In other words, despite side-side contacts tending to hinder relative particle rotations, the solid fraction under quasistatic shear coincides with that of isotropic random close packings of pentagonal particles. Properties of polygonal grains are thus similar to those of disks in that respect. We argue that continuous reshuffling of the force-bearing network leads to frequent collapsing events at the microscale, thereby causing the macroscopic dilatancy to vanish. Despite such rearrangements, the shear flow favors an anisotropic structure that is at the origin of the ability of the system to sustain shear stress

Bose-Einstein transition temperature in a dilute repulsive gas

We discuss certain specific features of the calculation of the critical temperature of a dilute repulsive Bose gas. Interactions modify the critical temperature in two different ways. First, for gases in traps, temperature shifts are introduced by a change of the density profile, arising itself from a modification of the equation of state of the gas (reduced compressibility); these shifts can be calculated simply within mean field theory. Second, even in the absence of a trapping potential (homogeneous gas in a box), temperature shifts are introduced by the interactions; they arise from the correlations introduced in the gas, and thus lie inherently beyond mean field theory - in fact, their evaluation requires more elaborate, non-perturbative, calculations. One illustration of this non-perturbative character is provided by the solution of self-consistent equations, which relate together non-linearly the various energy shifts of the single particle levels k. These equations predict that repulsive interactions shift the critical temperature (at constant density) by an amount which is positive, and simply proportional to the scattering length a; nevertheless, the numerical coefficient is difficult to compute. Physically, the increase of the temperature can be interpreted in terms of the reduced density fluctuations introduced by the repulsive interactions, which facilitate the propagation of large exchange cycles across the sample.Comment: two minor corrections, two refs adde

Semantic smart contracts for blockchain-based services in the Internet of Things

International audienceThe emerging Blockchain (BC) and Distributed Ledger technologies have come to impact a variety of domains, from capital market sectors to digital asset management in the Internet of Things (IoT). As a result, more and more BC-based decentralized applications for numerous cross-domain services have been developed. These applications implement specialized decentralized computer programs called Smart Contracts (SCs) which are deployed into BC frameworks. Although these SCs are open ato public, it is challenging to discover and utilize such SCs for a wide range of usages from both systems and end-users because such SCs are already compiled in form of byte-codes without any associated meta-data. This motivates us to propose a solution called Semantic SC (SSC) which integrates RESTful semantic web technologies in SCs, deployed on the Ethereum Blockchain platform, for indexing, browsing and annotating such SCs. The solution also exposes the relevant distributed ledgers as Linked Data for enhancing the discovery capability. To achieve this goal, the OWLS service ontology is extended by incorporating some domain specific terminologies, which are used in the development of the proposed SSCs. As a result, SSC can be utilized to enrich queries for a domain-specific terms across multiple distributed ledgers, which greatly increases the discovery capability of decentralized IoT applications and services. Contribution in standardization is also discussed. We believe that our research work takes the first steps towards connecting BC-based decentralized services with semantic web services in order to provide better IoT ecosystems

Modélisation de la formation d'une couche de particules sur une paroi poreuse

En filtration frontale, le colmatage du filtre par accumulation de particules en surface s'accompagne d'une augmentation de la résistance hydraulique du système couche de particules + paroi filtrante. L'approximation classique consistant à additionner la résistance de la paroi sans particule et celle de la couche de particules ne tient compte ni de la modification de l'écoulement par les pores de la paroi, qui augmente la résistance hydraulique par l'allongement des lignes de courant, ni du caractère discret des particules lorsque leur taille est comparable à celle du pore. La détermination exacte de la résistance hydraulique de l'ensemble du système nécessite la connaissance détaillée de la structure de l'écoulement au niveau du pore de la paroi. Une étude de la morphologie du dépôt, de la variation du débit en fonction de son volume, de la taille des pores de la paroi et des particules est proposée

Where do children learn about pain? The role of caregiver responses to preschoolers' pain experience within natural settings

The preschool period is a critical stage of emotional and social development for young children (0–5 years), and socialisation methods are instrumental learning tools for the child's development during this stage. Pain experiences are common during the preschool period, including minor medical procedures (eg, immunisations), unintentional childhood injuries (eg, burns, shocks, and poisoning), and “everyday” incidents which lead to minor bumps, scrapes, and cuts. Consequently, the biopsychosocial model of pain, derived partly from social learning theory, is relevant to paediatric pain, as children's coping strategies may be inadequate, relying primarily on caregivers to interpret their distress. Furthermore, childhood pain experiences are influenced both by who is present and the responses exhibited by those present. Social learning and imitation provide a valuable source of information and children may learn how to respond to pain by observing others; eg, watching their parents respond to pain. However, this developmental period presents a unique context for pain: children have less refined motor skills, the risk of everyday pains and injuries is high, and the presence of caregivers provides opportunities to observe social influences during everyday painful moments. Given the frequency of minor pain events and their potential role in teaching children about pain management, the aims of this topical review are to (1) provide a critical reflection on the limited literature on “everyday” child pain experiences and the role of caregiver behaviour within natural contexts and (2) set out a research agenda, calling for innovative, and multimethod approaches to foster further research in this area

The Eukaryote Genome Annotation Platform at Genoscope

The Genoscope annotation workflow for eukaryote genomes relies on evidence from ab initio gene models predictions combined with homology searches, using collections of expressed sequences - full length cDNAs, ESTs or massive-scale mRNA sequences from the same or closely related organisms – proteins or other genomic sequences. Global analysis of these drafts or complete sequences are then combining both approaches in the form of gene prediction data integration using GAZE, capable to identify a majority of the existing gene features. Although of very good quality, gene-modelling remains still tentative at the end of the process. Even though computational predictors are useful on large scale annotation for global genomics analysis, there is no complete genome for which all gene structures, in terms of exons, introns and coding regions, have been experimentally confirmed.

Finished genomes can provide exciting insights into the genome organization and evolution. Additional experimental data generated by genome sequencing projects give assistance to genome annotation aiming to a better understanding of the biology of the organism. Therefore, gene models and annotation can be improved by human curation to find errors or to resolve incongruous evidence on the automatic annotation of the genome. 

We now provide to collaborators carrying sequencing projects with a distributed annotation platform allowing expert evaluation of the annotation, in addition to our automated gene prediction pipeline.

To ensure at most the participation of the scientific community, an annotation tool for revising annotations has been set up using components of the Generic Model Organism Database toolkit, which provides tools for managing organism databases. A CHADO database, linked to an Apollo graphical interface, permit users to correct gene structures and store them in a dedicated organism database, as we will show on a few examples. Such a tool would facilitate connecting and comparing predicted annotations with existing biological data, becoming the repository of complete annotated finished genome sequence

Conspicuous consumption of the elite: Social and self-congruity in tourism choices

This paper relies on social and economic psychology to explore how the travel choices of Portuguese citizens, with different status levels in their daily lives, perceive and adopt different conspicuous travel patterns because of public exposure. To account for the moderated role of public exposure on conspicuous travel patterns, 36 Portuguese citizens were interviewed. Q-methods were applied to explore the varying senses of conspicuous travel choices among citizens with different levels of public exposure, both individually and relative to each other. Complementary qualitative methods were applied, in order to explore how the interviewees construct tourism conspicuous meanings that match their social or self-representations. The results suggest that social contexts moderate the ways in which individuals perceive and experience conspicuous travel. Further, the results show that public groups with higher exposure tend to prefer subtle signals of conspicuousness, in order to differentiate themselves from the mainstream