Modélisation et Simulation de suspensions concentrées de fibres courtes, rigides et flexibles

Suspensions involving nanoparticles such as nanofibers and nanotubesare widely used today in the development of functional materials.In order to optimize the usage of these materials and theirmanufacturing processes, a fine knowledge of the microstructure’sevolution in a flow is required. Thus, the study of such suspensionsis divided into two main categories: the dilute regime where theconcentration is low enough to describe each particle independentlyfrom its neighbors and the concentrated regime where the interactionbetween particles can no longer be neglected, nor the formationof aggregates (or clusters). The first type of suspensions is wellknown and treatable; the second one remains difficult to study. Fora more precise and fine description of the physics at the microscopicscale, a solution consists in performing a Direct numerical simulation(or DNS). DNS is based on the computation, in a representativevolume, of the motion of several hundreds of fibers and their interactions.It is a step by step process which derives kinematics as wellas macroscopic properties, while taking into account the forces appliedon each fiber at the microscopic scale. Thus the suspensionsare considered along with interaction forces acting on each fiber anda statistical description is built (number of interactions, magnitude offorces, elastic energy...). During the thesis an extensive 3D simulationcode based on DNS has been developed. It takes into accountthe kinematics of the concentrated fiber suspensions as well as theinteraction forces involved. Another more simple way to simulate concentratedfiber suspensions in a given flow is to use kinetic theoryapproaches. The kinetic theory incorporates a statistical orientationdistribution function, which represents the probability of having a particlein a given physical space, having a certain orientation, at a giventime. The simplicity of this theory is that it ignores the individuality ofthe entities (particles, fibers, nanotubes, ...), by introducing a probabilityfunction that acts on the mesoscopic scale. Thus, when theconcentration of the fibers is high enough, a cluster of fibers can beconsidered and the rheological properties can then be calculated.Les suspensions de nanoparticules - en particulier nanofibres et nanotubes- sont de plus en plus utilisées dans le cadre du développementde matériaux fonctionnels. Afin d’optimiser l’utilisation de cesmatériaux et leurs procédés de fabrication, une connaissance fine dela microstructure et de son évolution lors d’un écoulement est primordiale.Pour cela, l’étude des suspensions se divise en deux axes derecherche : le régime dilué où la concentration est faible et chaqueparticule peut être décrite seule, et le régime concentré où l’on nepeut plus négliger l’interaction entre les particules, ni la formationd’agrégats. Le premier type de suspensions est bien connu ; le secondreste encore problématique. Pour une description plus précisede la physique fine qui agit à l’échelle microscopique, des modèlesbasés sur la Simulation Numérique Directe (ou DNS) sont développés.Une DNS est basée sur le calcul dans un volume représentatif,du mouvement d’une centaine de fibres (particules) et de leurs interactions,à l’échelle microscopique lorsqu’un écoulement de cisaillementsimple est appliqué. Ainsi les suspensions sont considéréesavec des interactions entre les fibres et l’évolution statistique d’unepopulation de fibres (forces d’interaction et le nombre de contactsentre les fibres) est décrite. Un code de calcul intensif 3D basé surla DNS a été développé. Ce code calcule la cinématique associéeaux suspensions de fibres concentrées (contenues dans un volumeélémentaire) et prend en compte les forces d’interactions présentesà chaque pas de temps. Il existe une autre approche plus simplifiéeà l’échelle mésoscospique pour traiter le régime concentré : la théoriecinétique. Cela est possible grâce à une fonction de densité deprobabilité qui représente la probabilité de trouver une particule avecune orientation à un temps donné, dans l’espace. Lorsque la concentrationdu système devient très élevée, on considère un agrégat defibre (au lieu de considérer une fibre, on suit l’évolution d’un agrégatcomposé de fibres enchevêtrées)

Coupling multiphysics problems in transient regimes: application to liquid resin infusion process

Liquid resin infusion (LRI) process is widely considered in the aeronautics, due to its benefits (low void content and production of large parts), for high performance composite material forming. The main objective of the present work is to simulate nu- merically the LRI process, in a high performance computing framework, which consists in coupling fluid-solid mechanics. Hence, two fluid flow regimes are coupled with an ef- ficient ASGS stabilized monolithic finite element formulations: the resin flow in both a highly permeable distribution medium (Stokes) and low permeability fibrous orthotropic preforms (Darcy). Moreover, weak coupling algorithms are used along for coupling solid / fluid mechanics, solid / level-set problems and fluid / level-set problems; where the level-set method is used to capture the moving flow front and the Stokes-Darcy interface. To transfer the different physical variables between the above coupled problems, Message Passing Interface (MPI) library is chosen, to ensure the best data transfer performances

On the modelling of the aggregates' elasticity in a concentrated suspension of CNTs

International audienceSuspensions involving nanoparticules - in particular nano bers and nanotubes - are in much use in the development of functional materials. Thus in order to optimize the usage of these materials and their fabrication, it is essential to have a thorough knowledge of the microstructure and its evolution. In this work, the objective is to develop a two-scale kinetic theory description of concentrated suspensions including the modelling of nanotube aggregates and their evolution

Study of Concentrated Short Fiber Suspensions in Flows, Using Topological Data Analysis

The present study addresses the discrete simulation of the flow of concentrated suspensions encountered in the forming processes involving reinforced polymers, and more particularly the statistical characterization and description of the effects of the intense fiber interaction, occurring during the development of the flow induced orientation, on the fibers’ geometrical center trajectory. The number of interactions as well as the interaction intensity will depend on the fiber volume fraction and the applied shear, which should affect the stochastic trajectory. Topological data analysis (TDA) will be applied on the geometrical center trajectories of the simulated fiber to prove that a characteristic pattern can be extracted depending on the flow conditions (concentration and shear rate). This work proves that TDA allows capturing and extracting from the so-called persistence image, a pattern that characterizes the dependence of the fiber trajectory on the flow kinematics and the suspension concentration. Such a pattern could be used for classification and modeling purposes, in rheology or during processing monitoring

Kinetic Theory Microstructure Modeling in Concentrated Suspensions

When suspensions involving rigid rods become too concentrated, standard dilute theories fail to describe their behavior. Rich microstructures involving complex clusters are observed, and no model allows describing its kinematics and rheological effects. In previous works the authors propose a first attempt to describe such clusters from a micromechanical model, but neither its validity nor the rheological effects were addressed. Later, authors applied this model for fitting the rheological measurements in concentrated suspensions of carbon nanotubes (CNTs) by assuming a rheo-thinning behavior at the constitutive law level. However, three major issues were never addressed until now: (i) the validation of the micromechanical model by direct numerical simulation; (ii) the establishment of a general enough multi-scale kinetic theory description, taking into account interaction, diffusion and elastic effects; and (iii) proposing a numerical technique able to solve the kinetic theory description. This paper focuses on these three major issues, proving the validity of the micromechanical model, establishing a multi-scale kinetic theory description and, then, solving it by using an advanced and efficient separated representation of the cluster distribution function. These three aspects, never until now addressed in the past, constitute the main originality and the major contribution of the present paper

Numerical Simulation and Optimization of Highly Stable and Efficient Lead-Free Perovskite FA1−xCsxSnI3-Based Solar Cells Using SCAPS

Formamidinium tin iodide (FASnI3)-based perovskite solar cells (PSCs) have achieved significant progress in the past several years. However, these devices still suffer from low power conversion efficiency (PCE=6%) and poor stability. Recently, Cesium (Cs)-doped Formamidinium tin iodide (FA1−xCsxSnI3) showed enhanced air, thermal, and illumination stability of PSCs. Hence, in this work, FA1−xCsxSnI3 PSCs have been rigorously studied and compared to pure FASnI3 PSCs using a solar cell capacitance simulator (SCAPS) for the first time. The aim was to replace the conventional electron transport layer (ETL) TiO2 that reduces PSC stability under solar irradiation. Therefore, FA1−xCsxSnI3 PSCs with different Cs contents were analyzed with TiO2 and stable ZnOS as the ETLs. Perovskite light absorber parameters including Cs content, defect density, doping concentration and thickness, and the defect density at the interface were tuned to optimize the photovoltaic performance of the PSCs. The simulation results showed that the device efficiency was strongly governed by the ETL material, Cs content in the perovskite and its defect density. All the simulated devices with ZnOS ETL exhibited PCEs exceeding 20% when the defect density of the absorber layer was below 1015 cm−3, and deteriorated drastically at higher values. The optimized structure with FA75Cs25SnI3 as light absorber and ZnOS as ETL showed the highest PCE of 22% with an open circuit voltage Voc of 0.89 V, short-circuit current density Jsc of 31.4 mA·cm−2, and fill factor FF of 78.7%. Our results obtained from the first numerical simulation on Cs-doped FASnI3 could greatly increase its potential for practical production

Simulation and Investigation of 26% Efficient and Robust Inverted Planar Perovskite Solar Cells Based on GA0.2FA0.78SnI3-1%EDAI2 Films

A hybrid tin-based perovskite solar cell with p-i-n inverted structure is modeled and simulated using SCAPS. The inverted structure is composed of PEDOT:PSS (as hole transport layer—HTL)/GA0.2FA0.78SnI3-1% EDAI2 (as perovskite absorber layer)/C60-fullerene (as electron transport layer—ETL). Previous experimental studies showed that unlike conventional tin-based perovskite solar cells (PSC), the present hybrid tin-based PSC passes all harsh standard tests and generates a power conversion efficiency of only 8.3%. Despite the high stability that this material exhibits, emphasis on enhancing its power conversion efficiency (PCE) is crucial. To that end, various ETL and HTL materials have been rigorously investigated. The impact of energy level alignment between HTL/absorber and absorber/ETL interfaces have been elucidated. Moreover, the thickness and the doping concentration of all the previously mentioned layers have been varied to inspect their effect on the photovoltaic performance of the PSC. The optimized structure with CuI (copper iodide) as HTL and ZnOS (zinc oxysulphide) as ETL scored a PCE of 26%, which is more than three times greater than the efficiency of the initial structure. The current numerical simulation on GA0.2FA0.78SnI3-1% EDAI2 could greatly increase its chance for commercial development

Coupling multiphysics problems in transient regimes: application to liquid resin infusion process

Liquid resin infusion (LRI) process is widely considered in the aeronautics, due to its benefits (low void content and production of large parts), for high performance composite material forming. The main objective of the present work is to simulate nu- merically the LRI process, in a high performance computing framework, which consists in coupling fluid-solid mechanics. Hence, two fluid flow regimes are coupled with an ef- ficient ASGS stabilized monolithic finite element formulations: the resin flow in both a highly permeable distribution medium (Stokes) and low permeability fibrous orthotropic preforms (Darcy). Moreover, weak coupling algorithms are used along for coupling solid / fluid mechanics, solid / level-set problems and fluid / level-set problems; where the level-set method is used to capture the moving flow front and the Stokes-Darcy interface. To transfer the different physical variables between the above coupled problems, Message Passing Interface (MPI) library is chosen, to ensure the best data transfer performances

Mesoscopic modeling of high concentrated suspensions

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Kinetic theory based models for high concentrated suspensions in generic suspendant fluids

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