A suitable parametrization to simulate slug flows with the Volume-Of-Fluid method

Diffuse–interface methods, such as the Volume-Of-Fluid method, are often used to simulate complex multiphase flows even if they require significant computation time. Moreover, it can be difficult to simulate some particular two-phase flows such as slug flows with thin liquid films. Suitable parametrization is necessary to provide accuracy and computation speed. Based on a numerical study of slug flows in capillary tubes, we show that it is not trivial to optimize the parametrization of these methods. Some simulation problems described in the literature are directly related to a poor model parametrization, such as an unsuitable discretization scheme or too large time steps. The weak influence of the mesh irregularity is also highlighted. It is shown how to capture accurately thin liquid films with reasonably low computation times

A penalization technique applied to the “Volume-Of-Fluid” method: wettability condition on immersed boundaries

A penalization approach is presented to simulate two-phase flow in the presence of immersed solid boundaries so as to consider highly complex objects such as porous media microstructures. Based on the standard Volume-Of-Fluid formulation, the method takes into account the wettability effects which may occur on the surface of immersed solid boundaries. A spatial shift between the no-slip and the wettability conditions is introduced to make the method stable, regardless of the simulation parameters. The penalized VOF model and the numerical choices are then validated by a series of tests on capillary-dominated flows, which represent the most challenging cases for VOF simulations

Morfotipos de ammonoideos del Albiense superior de la Cuenca Vasco-Cantábrica: paleoecología y relación con las facies sedimentarias

In this work we present a morphological study of 65 planispiral ammonoid specimens occurring in the stratigraphic succession of the lower upper Albian of the central region of the Basque-Cantabrian Basin, northern Spain. The specimens correspond to the Zones of the ammonoids Hysteroceras varicosum and Mortoniceras inflatum. They are grouped into different morphotypes following the method of “Westerman Morphospace” which suggests diverse habitats for ammonoids, based on shell architecture and shape. The objective of this work is to check the likely correspondence of the suggested habitats with the information given by sedimentology. Accordingly, it has been possible to establish a distribution of the ammonoid ecomorphotypes demersal, nektonic, planktonic and vertical migrant, from shallow (neritic) to deeper marine areas (epipelagic to mesopelagic). The distribution of these ecomorphotypes with different bathimetric affinities is in agreement with the environmental interpretation of sedimentological dataSe han estudiado 65 especímenes de ammonoideos planoespirales de las Zonas de Hysteroceras varicosum y de Mortoniceras inflatum obtenidos de la serie estratigráfica correspondiente a la parte inferior del Albiense superior de la región central de la Cuenca Vasco-Cantábrica, norte de España. Los especímenes se han agrupado en diferentes morfotipos siguiendo su análisis mediante el método de “Westerman Morphospace” , el cual sugiere diversos hábitats para ammonoideos, basado en la arquitectura y forma de la concha. El objetivo de este trabajo es contrastar la posible correspondencia de los hábitats con la información aportada por la sedimentología. De esta manera, se ha podido establecer una distribución de los ecomorfotipos de ammonoideos demersal, nectónico, planctónico y migrante vertical, en áreas marino someras (neríticas) a más profundas (epipelágicas a mesopelágicas). La distribución de estos ecomorfotipos con diferentes afinidades batimétricas se correlaciona de manera bastante precisa con la interpretación ambiental de los datos sedimentológico

A shallow-water cyrtocrinid crinoid (Articulata) from the upper Albian of the Western Pyrenees, North Spain

The cyrtocrinid crinoid Proholopus holopiformis (Remeš, 1902) is described from the upper Albian succession that may be included in the Albeniz unit to the east of Iruñea-Pamplona (Navarre, Western Pyrenees, Spain). Although based on partially disarticulated material, this taxon preserve calyx, stem, attachment structure, and brachial plates from the arms. Proholopus holopiformis was previously described from the Upper Jurassic-Lower Cretaceous of the Czech Republic and Lower Cretaceous of France and Crimea, thus the occurrence from Spain represents the youngest of the species. This allows a better characterization of the family Proholopodidae that was originally described based on calyx morphology only and expands its distribution to the upper Albian. Proholopus holopiformis inhabited the fore-reef areas of coral-sponge bioconstructions. Based on coral types, crinoids are thought to have dwelled near the euphotic-oligophotic zones transition and, thus, thrive in shallow depths. Most specimens have bite marks compatible with cidaroid predation. This represents one of the youngest occurrences of cyrtocrinids inhabiting shallow marine environments before their migration to the deep sea due to the ongoing Mesozoic Marine Revolution

A comparison of various methods for the numerical evaluation of porous media permeability tensor from pore-scale geometry

In this work, several boundary value problems used to numerically evaluate the absolute permeability tensors of porous media using core-scale images are compared and discussed. The various configurations differ by the type of boundary conditions used to compute the flow at the micro-scale. The issue is the ability of the method to capture anisotropy correctly and to avoid possible percolation artifacts. This study is carried on two-dimensional synthetic, isotropic or anisotropic, porous media, that are chosen to illustrate the various difficulties mentioned above. A new method is proposed which consists in embedding the porous medium in question in a homogenized one. Using an iterative optimization procedure on the surrounding permeability, the method determines the absolute permeability tensor of the original medium. The equivalent permeability tensor that minimizes the effect on the surrounding porous medium is, unlike that of classical methods, de facto symmetrical due to the use of periodic boundary conditions and exhibits significantly lower permeabilities. The way in which non-diagonal terms of the permeability tensor are obtainedwith the various methods are thoroughly discussed

Modélisation multi-échelle d'un écoulement gaz-liquide dans un lit fixe de particules

On s'intéresse dans ce travail à la modélisation d'un écoulement diphasique gaz-liquide co-courant descendant dans les réacteurs à lit fixe de particules, procédé largement utilisé dans le domaine industriel. En raison de la complexité de l'écoulement, induite par les nombreuses configurations multiphasiques pouvant coexister au sein du lit, les modèles développés directement à l'échelle du réacteur sont généralement issus d'approches semi-empiriques, en considérant l'écoulement homogène. Or, il a été observé que des hétérogénéités locales, géométrique et hydrodynamique, telle qu'une mal-distribution de la phase liquide, entrainaient une diminution du taux de réaction et conduisait les modèles existants à surestimer la productivité d'un réacteur. La nécessité de prendre en compte les phénomènes microscopiques dans un modèle macroscopique à l'échelle du réacteur rend l'utilisation d'approches multi-échelles indispensable. L'écoulement étant cependant d'une nature complexe, le changement d'échelle ne peut se faire de façon directe et nécessite donc la mise en place d'outils de modélisation adaptés à une échelle intermédiaire. Dans une première étape, la méthode de simulation numérique directe ``Volume-Of-Fluid'' (VOF) est validée dans le cas d'un film ruisselant dans un tube capillaire. Cette méthode est ensuite utilisée, à l'échelle microscopique, afin de proposer et de valider des relations de fermeture pour un modèle de type ``réseau de pores'' pouvant être utilisé à une échelle intermédiaire, celle du Volume Elémentaire Représentatif. Ce changement d'échelle est tout d'abord effectué dans le cas d'un lit fixe en deux dimensions, c'est-à-dire un empilement de cylindres entre deux plaques. Cette configuration permet la mise en place d'un dispositif expérimental qui, couplé à des simulations VOF 2D à plus grande échelle, valide l'approche de type "réseau de pores" adoptée. Le modèle réseau est ensuite étendu au cas d'un lit fixe réel, c'est-à-dire en trois dimensions, dont la géométrie est obtenue par micro-tomographie. Les lois de comportement locales sont redéfinies à l'aide de simulations numériques directes à l'échelle microscopique. Les résultats provenant de simulations de type « réseaux de pore » sont ensuite confrontés, dans le cas d'une répartition homogène des phases, aux modèles 1D habituellement utilisés pour les écoulements diphasiques en lit fixe. Enfin, une campagne expérimentale est menée afin d'observer, par imagerie scanner, l'étalement d'un jet de liquide sur un empilement de grains. Une comparaison qualitative est ensuite effectuée entre les observations expérimentales et les simulations numériques réseaux dans le cas spécifique de l'étalement d'un jet de liquide. ABSTRACT : We study in this work the modelling of two-phase cocurrent downflows in fixed bed reactors, a process widely used in industry. Due to the flow complexity, i.e., the presence of different interface configurations and, therefore, different phase interactions, most models have been developed using empirical approaches, with the assumption of a homogeneous flow in the reactor. However, several studies showed that local heterogeneities, geometric and hydrodynamic, such as the liquid distribution, could have a great influence on the flow at the reactor-scale and, therefore, on the reactor performance. Consider the microscopic phenomena in a macroscopic model require the use of multi-scale approaches. However, due to the flow complexity, the upscaling cannot be done directly and requires the development of modelling tools suitable for an intermediate scale. In a first step, the direct numerical method \ Volume-Of-Fluid" (VOF) is validated in the case of a two-phase flow in a capillary tube with the presence of a thin film. Then, this method is used, at a microscopic level to propose and validate closure laws for a pore-network model which will be used to simulate the flow at the intermediate scale. This upscaling approach is first tested in a two-dimensional case, i.e., an array of cylinders between two walls. This configuration allows the set up of an experimental approach, coupled with 2D VOF simulations at the intermediate scale, in order to validate the pore-network approach. The pore-network approach is then extended to a real fixed bed, i.e. in three dimensions, whose geometry is obtained by micro-tomography. Local laws of the pore-network model are redefined using direct numerical simulations at a microscopic scale. Pore-network simulations are then compared, for a homogenous phase distribution, with 1D models typically used for two-phase flow in fixed beds. Finally, an experimental campaign was set up to observe, by imaging scanner, the spreading of a liquid jet on a fixed bed pilot. A qualitative comparison is then performed between experimental observations and pore-network simulations in the specific case of the spreading of a liquid jet

Benchmark of different CFL conditions for IMPES

The IMplicit Pressure Explicit Saturation (IMPES) method is a prevalent way to simulate multiphase flows in porous media. The numerical stability of this sequential method implies limitations on the time step, which may depend on the flow regime studied. In this note, three stability criteria related to the IMPES method, that differ in their construction on the observed variables, are compared on homogeneous and heterogeneous configurations for different two-phase flow regimes (viscous/capillary/gravitational). This highlights that there is no single optimal criterion always ensuring stability and efficiency. For capillary dominated flows, the Todd’s condition is the most efficient one, while the standard Coat condition should be preferred for viscous flows. When gravity effects are present, Coat’s condition must be restricted, but remains more efficient than the Todd’s condition

L'andouille de Guéméné : de la matière première au produit fini, étude de rendement et investigations histologiques en fonction de la race de porc

L'andouille de Guéméné est constituée uniquement de gros intestins de porc. Une étude histologique et une étude de rendement en fonction de deux races de porcs, Duroc et Gallia,ont été mis en place. L'étude de la matière première conclue à une différence de longueur et de poids de gros intestins. En contre partie, d'un point de vue histologique, il n'est pas possible de distinguer Duroc et Gallia. Ensuite, en travaillant sur un process de fabrication industrielle, les rendements relevés aux différentes étapes ne sont statistiquement pas différents en fonction de la race. L'étude histologique le confirme mais souligne les caractéristiques de distinction entre andouille industrielle et andouille artisanale. Ces travaux illustrent avant tout la complexité de mise en oeuvre de tels protocoles expérimentaux