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Drying of thin porous disks from pore network simulations

Drying of thin porous disks from pore network simulations

with a greater number of pore layers. Comparison of drying rate curves from pore networks with increasing disk height evidenced a progressive transition from the TPM (without distinct drying periods) to drying of traditional porous media with distinct first drying periods. Furthermore, illustration of the in-plane gas– liquid phase distributions clearly evidenced formation of two different drying fronts: a front penetrating the TMP from the surface (as in a classical invasion process) and in-plane drying fronts, penetrating the sublayers of the TMP from the periphery. This drying behavior was explained by the combined effect of the earlier and more significant fragmentation of the liquid phase in small clusters and the imposed evaporation profile at the top of the TPM. By increasing the disk height, it could be shown that the coupling of the external vapor transfer and the distribution of gas and liquid phase diminishes in thicker disks. Instead, it could be shown (by appli- cation of in-plane asymmetric pore structure) that with the transition from a TPM to a thick porous medium the structural parameter control the drying process. These findings imply to develop efficient coupling strat- egy between the evolution of the phase distribution within the TPM and the modeling of the external mass transfer. In this respect, the presented rather simple approach of coupling pore network simulations and an analytical formula corresponding to the evaporation from a wetted disk in a half-space, needs to be improved. It is proposed to use instead the concept of the equivalent wetted disk so as to take into account the shrinking of the wetted zone within the PN model. Still more challenging but more accurate would be to numerically recompute the external mass transfer after each pore/throat invasion. Another aspect that has been disregarded in our simulations is the role of wetting liquid films in TPM. However, it is expected that due to the small expansion of the TPM, liquid films are likely to cover the TPM in the through-plane direction over most of the drying process. It is thus proposed to investigate the dependence of the extent of the film region and the impact of the film transport on the height of the TPM.
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Drying of thin porous disks from pore network simulations

Drying of thin porous disks from pore network simulations

with a greater number of pore layers. Comparison of drying rate curves from pore networks with increasing disk height evidenced a progressive transition from the TPM (without distinct drying periods) to drying of traditional porous media with distinct first drying periods. Furthermore, illustration of the in-plane gas– liquid phase distributions clearly evidenced formation of two different drying fronts: a front penetrating the TMP from the surface (as in a classical invasion process) and in-plane drying fronts, penetrating the sublayers of the TMP from the periphery. This drying behavior was explained by the combined effect of the earlier and more significant fragmentation of the liquid phase in small clusters and the imposed evaporation profile at the top of the TPM. By increasing the disk height, it could be shown that the coupling of the external vapor transfer and the distribution of gas and liquid phase diminishes in thicker disks. Instead, it could be shown (by appli- cation of in-plane asymmetric pore structure) that with the transition from a TPM to a thick porous medium the structural parameter control the drying process. These findings imply to develop efficient coupling strat- egy between the evolution of the phase distribution within the TPM and the modeling of the external mass transfer. In this respect, the presented rather simple approach of coupling pore network simulations and an analytical formula corresponding to the evaporation from a wetted disk in a half-space, needs to be improved. It is proposed to use instead the concept of the equivalent wetted disk so as to take into account the shrinking of the wetted zone within the PN model. Still more challenging but more accurate would be to numerically recompute the external mass transfer after each pore/throat invasion. Another aspect that has been disregarded in our simulations is the role of wetting liquid films in TPM. However, it is expected that due to the small expansion of the TPM, liquid films are likely to cover the TPM in the through-plane direction over most of the drying process. It is thus proposed to investigate the dependence of the extent of the film region and the impact of the film transport on the height of the TPM.
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Kinematics in a slowly drying porous medium: Reconciliation of pore network simulations and continuum modeling

Kinematics in a slowly drying porous medium: Reconciliation of pore network simulations and continuum modeling

We study the velocity field in the liquid phase during the drying of a porous medium in the capillarity-dominated regime with evaporation from the top surface. A simple mass balance in the continuum framework leads to a linear variation of the filtration velocity across the sample. By contrast, the instan-taneous slice-averaged velocity field determined from pore network simulations leads to step velocity profiles. The vertical velocity profile is almost constant near the evaporative top surface and zero close to the bottom of the sample. The relative extent of the two regions with constant velocity is dictated by the position of the most unstable meniscus. It is shown that the continuum and pore network results can be reconciled by averaging the velocity field obtained from the pore network simulations over time. This opens up interesting prospects regarding the transport of dissolved species during drying. Also, the study reveals the existence of an edge effect, which is not taken into account in the classical continuum models of drying.
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Kinematics in a slowly drying porous medium: Reconciliation of pore network simulations and continuum modeling

Kinematics in a slowly drying porous medium: Reconciliation of pore network simulations and continuum modeling

This very first phase is further illustrated by looking at the average saturation of the top vertical throats (S tvt ) versus network saturation (Fig. 5 ). As can be seen, the sharp decrease in the average saturation of top vertical throats is followed by a drop in the evaporation rate during this phase. This also holds for the saturation of top horizontal throats (S tht ). One can refer to Fig. 2 for the definition of top vertical throats and top horizontal throats. As can be seen, the decrease in drying rate occurring in this very first phase affects the dry- ing rate during the CRP, which starts at the end of this initial phase. The evaporation rate is actually not perfectly constant but varies weakly during the “constant” rate period. This has also been observed in previous pore network simulations, e.g., Refs. 28 and 29 . Compared to classical experimental results, the CRP obtained from the simulations is quite short (the satu- ration range corresponding to the CRP is much narrower than in the experiments). Several factors explain this difference: First, the irreducible saturation, i.e., the saturation for which the liquid phase is no longer connected over a distance compa- rable to the system size, is quite high in our system (S irr ≈ 0.65) compared to a random packing of monodisperse spherical par- ticles (S irr ≈ 0.1, Ref. 31 ) or the fired clay brick considered in Ref. 12 (S irr < 0.1). This has to do with the coordination num- ber of our network (each pore is connected to 6 neighbor pores) which is low compared to the average coordination number in real systems. Second, the fact that the pores are volume- less and thus that all the pore space volume is in the channels (throats) leads to a greater irreducible saturation (S irr ≈ 0.65 vs
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Non-local equilibrium continuum modeling of partially saturated drying porous media: comparison with pore network simulations

Non-local equilibrium continuum modeling of partially saturated drying porous media: comparison with pore network simulations

Non-equilibrium effect Specific interfacial area Pore network simulations A two equation continuum model is developed to simulate the mass transfer in drying porous media. The main goal is to c apture the so c alled non equilibrium effe c t To this end, we opera te in a regime where the liquid phase is immobile so that non equilibrium mass ex c hange between liquid and vapor phase dom inates. The formulation of the model relies on an upscaling technique. This notably permits to formula te the non local equilibrium phase change term on a firmer basis. The upscaling also indicates that there is no reason to consider an enhan c ement factor in the vapor diffusion model. The ma c roscopi c model parameters are determined from pore network drying simulations. The same simulations are also used as a reference to compare with the predi c tions of the non local equilibrium continuum model. The solu tion of the two equation continuum model proves that this model simulates the non local equilibrium effe c t with reasonable a cc uracy. Also, the simulations indi c ate that the non local equilibrium effect is espe c ially significant at the porous medium surface.
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Brian2GeNN: accelerating spiking neural network simulations with graphics hardware

Brian2GeNN: accelerating spiking neural network simulations with graphics hardware

GPU acceleration emerged when creative academics discovered that modern graphics processing units (GPUs) could be used to execute general purpose algorithms, e.g. for neural network simulations 1 , 2 . The real revolution occurred when NVIDIA corporation embraced the idea of GPUs as general purpose computing accelerators and developed the CUDA application programming interface 3 in 2006. Since then, GPU acceleration has become a major factor in high performance computing and has fueled much of the recent renaissance in artificial intel- ligence. One of the remaining challenges when using GPU acceleration is the high degree of insight into GPU computing architecture and careful optimizations needed in order to achieve good acceleration, in spite of the abstractions that CUDA offers. A number of simulators have used GPUs to accelerate spiking neural network simulations, but the majority do not allow for easily defining new models, relying instead on a fixed set of existing models 4 – 8 . Since 2010 we have been developing the GPU enhanced neuronal networks (GeNN) framework 9 that uses code generation techniques 10 , 11 to simplify the use of GPU accelerators for the simulation of spiking neural networks. GPUs, and in particular GeNN, have been shown to enable efficient simulations compared to CPUs and even compared to dedicated neuromorphic hardware 12 . Other simulators that have taken this code generation approach are Brian2CUDA 13 (currently under development) and ANNarchy 14 (Linux only).
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Analysis of fluid-fluid porometry from pore network simulations

Analysis of fluid-fluid porometry from pore network simulations

2. Methods and results 2.1. Methods In the fluid fluid porometry [1], a wetting fluid initially saturating the thin porous medium of interest is displaced by injection of a non-wetting fluid through a series of pressure steps at the inlet while the pressure is kept constant at the outlet. In addition to the pressures, the volumetric flow-rate at the outlet is measured for each step after stabilization of the inlet pressure. Then a PSD is obtained from the parallel tubes model by combining the Young-Laplace and Poiseuille equations [1],[2]. The Young- Laplace equation allows determining the critical tube radius corresponding to each pressure step (only the tubes greater than the critical radius are invaded), whereas the Poiseuille equation allows computing the volumetric flow-rate as a function of the pressure difference in the tubes whose size is greater than the critical radius. The idea is then to test this method from pore network simulations.
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Liquid water in cathode gas diffusion layers of PEM fuel cells:Identification of various pore filling regimes from pore network simulations

Liquid water in cathode gas diffusion layers of PEM fuel cells:Identification of various pore filling regimes from pore network simulations

1. Introduction Water management, i.e. [1], and degradation mechanisms [2,3] are two related major issues for the development of PEM-FC (Poly- mer Electrolyte Membrane Fuel Cell) technology. In this context, a better understanding of the water formation and transfer mecha- nisms within the fuel cell can be highly helpful. Surprisingly, in spite of the considerable worldwide research effort on this technol- ogy, these mechanisms are not yet fully understood. This consti- tutes an obstacle for the design of efficient and durable fuel cells whose improvements still largely rely on costly trial and error tests. As in other technological branches, numerical simulations can be expected to help reduce the number of tests and to serve as design tool but this, of course, implies that the key mechanisms are well modelled. In this context, the present paper is an attempt to clarify the mechanisms leading to the occurrence of liquid water in the cathode gas diffusion layer (GDL) from numerical simula- tions. As pointed out for instance in [4], GDLs are integral part of PEM-FCs and play a key role for the distribution of the reactant from the channel to the catalyst layer (CL) as well as for removing the water produced in the CL. Due to its significance, the water
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Liquid water in cathode gas diffusion layers of PEM fuel cells:Identification of various pore filling regimes from pore network simulations

Liquid water in cathode gas diffusion layers of PEM fuel cells:Identification of various pore filling regimes from pore network simulations

network is used and a = 20% unless otherwise mentioned. 4. Comparison between the different models and experimental results As indicated in the introduction, two main kinds of PNM have been used so far to simulate the filling of the cathode GDL pore space by liquid water. The first one, referred to as LIPNM, is based on the assumption that water enters the cathode GDL in liquid form under isothermal conditions without consideration of liquid-vapour phase change phenomena. The second one, referred to as CPNM, assumes that the water generated by electrochemical reaction enters the GDL in vapour phase and diffuses within the pore network as the result of the vapour molar fraction difference between the GDL inlet and the bipolar plate channels. The pore fill- ing by liquid water, if any, is by condensation of the vapour. The third option is the mixed scenario presented in the present article (MIPNM). In the latter, water can enter the GDL in vapour phase but also in liquid phase in a finite number of injection points depending on the conditions at the GDL inlet.
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On the Validity of Flow-level TCP Network Models for Grid and Cloud Simulations

On the Validity of Flow-level TCP Network Models for Grid and Cloud Simulations

HENRI CASANOVA , Dept. of Computer and Information Sciences, University of Hawai‘i at Manoa, U.S.A ARNAUD LEGRAND , CNRS, Grenoble University, France Researchers in the area of grid/cloud computing perform many of their experiments using simulations that must capture network behavior. In this context, packet-level simulations, which are widely used to study network protocols, are too costly given the typical large scales of simulated systems and applications. An alternative is to implement network simulations with less costly flow-level models. Several flow-level models have been proposed and implemented in grid/cloud simulators. Surprisingly, published validations of these models, if any, consist of verifications for only a few simple cases. Consequently, even when they have been used to obtain published results, the ability of these simulators to produce scientifically meaningful results is in doubt. This work evaluates these state-of-the-art flow-level network models of TCP communication via comparison to packet-level simulation. While it is straightforward to show cases in which previously proposed models lead to good results, instead we follow the critical method, which places model refutation at the center of the scientific activity, and we systematically seek cases that lead to invalid results. Careful analysis of these cases reveal fundamental flaws and also suggest improvements. One contribution of this work is that these improvements lead to a new model that, while far from being perfect, improves upon all previously proposed models in the context of simulation of grids or clouds. A more important contribution, perhaps, is provided by the pitfalls and unexpected behaviors encountered in this work, leading to a number of enlightening lessons. In particular, this work shows that model validation cannot be achieved solely by exhibiting (possibly many) “good cases.” Confidence in the quality of a model can only be strengthened through an invalidation approach that attempts to prove the model wrong.
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Les simulations computationnelles dans les sciences sociales

Les simulations computationnelles dans les sciences sociales

Mots-clés : computationnel, sciences sociales, sciences computationnelles, modèle, simulation, épistémologie, réalisme, iconicité, fonction épistémique des modèles Abstract : Since the 1990’s, social sciences are living their computational turn. This paper aims at clarifying the epistemological meaning of this turn. To do this, it first shows that we have to discriminate between different epistemic functions of computation among the diverse uses of computers for modeling and simulation in the social sciences. Because of this introduction of a new – and often more user-friendly - way of formalizing and computing, the question of the realism of formalisms and of the proof value of computational treatments reemerges. Facing the spreading of computational simulations in all disciplines, some enthusiastic observers are led to the claim that we are entering a new era of unity for social sciences. Second, and finally, this article shows that the conceptual and epistemological distinctions presented in the first sections lead to a more mitigated position: the transdisciplinary computational turn is a great one, but of a methodological nature.
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Simulations magnétohydrodynamiques en régime idéal

Simulations magnétohydrodynamiques en régime idéal

DISCUSSION ET CONCLUSION Cette th` ese s’est concentr´ ee sur la mod´ elisation num´ erique des ´ ecoulements magn´ eto- hydrodynamiques multi-´ echelles de la physique solaire. Les nombres de Reynolds gi- gantesques caract´ erisant le plasma du Soleil compliquent la solution aux ´ equations MHD, de sorte que l’influence des termes nonlin´ eaires domine largement celle des termes dissipatifs associ´ es ` a la diffusion microscopique. En raison des limitations informatiques, les simulations num´ eriques doivent avoir recours ` a des diffusivit´ es ef- fectives explicites beaucoup plus grandes que les valeurs microscopiques caract´ erisant le plasma solaire. L’approche ILES a pour principe d’abandonner le traitement explicite de la dissipation pour d´ el´ eguer son action aux termes de troncature du sch´ ema num´ erique, permettant ainsi l’atteinte d’un r´ egime maximalement turbu- lent ` a r´ esolution fixe. Seules certaines classes d’algorithmes se prˆ etent ` a l’approche ILES, comme les m´ ethodes NFT, qui ont d´ emontr´ e pouvoir repr´ esenter l’effet des ´ echelles non r´ esolues de l’´ ecoulement sur les ´ echelles r´ esolues sans l’utilisation d’un mod` ele sous-maille explicite. Les exp´ eriences pr´ esent´ ees dans cette th` ese ont toutes ´ et´ e r´ ealis´ ees avec le mod` ele EULAG, qui est bas´ e sur des op´ erateurs de transport Eul´ erien et semi-Lagrangien NFT.
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ALE simulations using Metafor

ALE simulations using Metafor

Stationary wire extrusion using ALE formalism Stationary wire extrusion using ALE formalism Stationary wire extrusion using ALE formalism Stationary wire extrusion using ALE formalism. •[r]

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Reduced Vlasov-Maxwell simulations

Reduced Vlasov-Maxwell simulations

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignemen[r]

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Projects in Fracture Simulations

Projects in Fracture Simulations

2011 1 st GMSH Workshop: solid mechanics project 5 GMSH (elasticity) Solver Linear System Dof Manager (Bi)linear terms Function Space NonLinear MechSolver NLD[r]

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Modèles et simulations en design et architecture

Modèles et simulations en design et architecture

De telles simulations s’apparentent aussi à un ciment, mais un ciment dynamique. Ainsi, pour évoquer le réel, Claudine Tiercelin parle-t-elle du ciment des choses (2011) en insistant par là sur l’entrelacs des dispositions. On peut ainsi rappeler, comme le fait Christian Girard (2012), que le ciment, même s’il ne se trouve pas à l’état naturel, est sans doute le construit humain le plus révélateur de la matérialité au sens fort, dans la mesure où il est « ce matériau singulier qui joue le rôle d’assembleur d’autres matériaux, cette matière artificielle qui lie les choses, articule la brique, le bloc d’aggloméré, la pierre, le bloc de béton, et qui, lorsqu’elle est armée de la matière acier, sous forme de barres, de tiges, d’épingles, donne naissance au béton armé, ce matériau-matière qui de liquide passe au solide ». Une matérialité dynamique se dévoilerait donc dans ces dispositifs de simulation computationnelle multi-aspects et multi-échelles, imitant jusqu’à l’entrelacs des dispositions dont ferait preuve la matière première même.
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Une méthode pour naviguer dans l'espace des simulations

Une méthode pour naviguer dans l'espace des simulations

– des outils d’évaluations génériques ou spécifiques au domaine, afin d’identifier les phénomènes remarquables et d’en obtenir un retour quantifié. Au sein de l’équipe SMAC du LIFL, nous avons développé de tels moyens. Nous disposons en premier lieu d’une méthodologie de conception de simulations, dénommée IODA 1 (Mathieu et al., 2007), qui repose sur une séparation claire des agents, de leur comportement et du processus de sélection des actions. Dans cette méthodologie, les interactions sont réifiées indépendamment des agents qui les utilisent. De ce fait, nous pouvons établir des bibliothèques d’interactions pour un domaine particulier et adaptables à différentes familles d’agents, augmentant la généricité du travail de modélisation. Une implémentation réalisée au moyen du moteur JEDI (Kubera et al., 2008a) offre un support générique à l’utilisation de cette méthodologie.
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Flyweight Network Functions for Network Slicing in IoT

Flyweight Network Functions for Network Slicing in IoT

problematic for some IoT deployment targets (e.g. RPi used as IoT gateway) with very limited resources. Moreover for some NF, by their size, utility, to be instantiated in the form of VNF can be counterproductive. This type of NF is very similar to the anti pattern SOA known as Nanoservice [12]. Also, this method of instantiation of NF does not cover heterogeneity of the future 5G networks, i.e. the underlying networks will be both classical and cloud-enabled. The concept of network slicing, as it is conceived currently, is based on cloud-type infrastructure (to allow the deployment of VNFs) and will be hardly usable to achieve end-to-end slices, i.e. connecting data producers and consumers. Our proposal aims to extend the network slicing to environments that do not support virtualization, through the concept of Flyweight Network Functions.
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Construction d'un algorithme d'accélération de la méthode des «simulations dans les simulations» pour le calcul du capital économique Solvabilité II

Construction d'un algorithme d'accélération de la méthode des «simulations dans les simulations» pour le calcul du capital économique Solvabilité II

3. Calcul du capital économique par approche SdS 3.1. Les calculs stochastiques au sein des modèles ALM « vie » Les outils de projections des portefeuilles d’assurance-vie sont alimentés dans la pratique par des tables de scénarios (scénarios économiques, scénarios de mortalité,…) générées en amont au moyen d’applicatifs dédiés. Une table de scénarios économiques contient par exemple pour chaque simulation, l’évolution des courbes des taux, de l’indice « actions » de l’économie, de l’inflation, etc. Soulignons qu’une simulation dans un modèle ALM « vie » correspond à une projection déterministe de l’ensemble des éléments actuariels (éléments de bilan et de compte de résultat) effectuée sur la base d’un scénario particulier. Par conséquent, dans la suite de l’article, le terme « simulations » sera indifféremment utilisé pour désigner les tables de scénarios ou les calculs des éléments actuariels qui en découlent.
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Maritime Aerosol Network as a component of Aerosol Robotic Network

Maritime Aerosol Network as a component of Aerosol Robotic Network

3. AERONET Data [ 10 ] The Aerosol Robotic Network (AERONET) is a world-wide network of Sun/sky radiometers that operates over 300 sites worldwide [Holben et al., 1998]. Starting in 1996 AERONET has a number of operational island sites. Several publications based on the AERONET data [Holben et al., 2001; Eck et al., 2001, 2005; Smirnov et al., 2000, 2002, 2003a, 2003b] have analyzed aerosol optical proper- ties over the oceans, including pollution and dust aerosol transport affecting and modifying those properties. Accu- mulated statistics on aerosol optical depth and Angstrom parameter afforded estimation of the globally averaged parameters over the oceans (from island-based measure- ments). Yearly mean aerosol optical depth and Angstrom parameter for various island sites are shown in Figures 2a and 2b. Horizontal bars represent plus or minus one standard deviation. Locations of these AERONET sites are shown in Figure 3, while Table 1 presents coordinates, measurement period, and number of measurement days used in the statistics. Data quality level 2.0 was used for all sites except for Reunion.
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