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Numerical simulations utilizing the code SURFER++ with the incorporation of an insoluble surfactant in the VOF scheme were conducted to characterize the effects of surfactant on a drop in shear flow. The drop is suspended in a matrix liquid. A parameter called reduction, which specifically relates to a percentage decrease in effective surface tension, is used to measure the surfactant amount on the interface. In a model system where reduction = 0.1, viscosity ratio = 1 and density ratio = 1, it was found that stable drops tend to be more elongated and less inclined to the primary flow direction than drops unexposed to surfactant. This can be explained by the location of surfactant at the interface as the drop evolves. Breaking drops also show a flattened angle, but exhibit shorter necks and faster time to break than similar drops without surfactant. As reduction increases, various physical characteristics of the drops change across Reynolds number. / Ph. D.

The validation of the “interFoam” CFD model (part of the OpenFOAM) CFD library is described for a number of wave/structure interaction problems. The background to the research is described, including the reasons for the selection of a new, previously unvalidated CFD code for this purpose. The numerical aspects of the code are briefly reviewed as are some of its additional features including the simulation of porous media. The new wave-generating boundary condition, created as part of this project, is described. The model is validated for the propagation of waves, including violent, breaking waves, using the widely-known “Dingemans” test case as well as new data for wave and focussed wave group propagation over a bar. The model is validated for wave interaction with surface-piercing structures by examining a test case for focussed wave-group impact on a surface-piercing cylinder with one near-breaking wave and a second, breaking, wave. The model is shown to perform well in these cases without the need for calibration and can therefore be considered to be a valuable design tool. It is also shown that in these cases the model can run sufficiently fast to be practical and economic for use as a design tool. The model is validated for porous media with a case examining porepressure transmission through a porous breakwater. The model performs poorly without calibration, highlighting the high levels of uncertainty in the Darcy parameter, but once calibrated is found to produce accurate results in very reasonable time. A case study of a porous roundhead defence structure is also presented to further reinforce the practical usefulness of the model in design.

The transport of multi-phase flow in pipelines can be met in a wide range of industrial applications, including the oil and gas industry, showing great savings in developments. In addition, as the exploration of new fields in oil and gas expands to harsh environments, such as ocean or polar, the multi-phase flow transport sometimes becomes the only feasible option. The important features of such multi-phase flow applications include flow regimes, pressure drop and liquid holdup. The precise estimation of these parameters has significant technical and economical impacts on the design and operation of an oil and gas pipelines. Many prediction correlations and methods have been developed; computational fluid dynamics (CFD) being one of them. This type of modelling approach has many advantages over the conventional approaches such as its ability to solve 3D transient problems; offering access to a wealth of information which with conventional techniques is extremely difficult to obtain. Therefore, interest in applying CFD for multi-phase flow transport in pipelines has been on the rise. This thesis is aimed at presenting CFD simulations based on the use of the Volume of Fluid model (VOF) approach for various conditions of gas-liquid turbulent flow in a horizontal circular pipe. In the current VOF formulation in addition to the secondary phase transport equation, a geometric reconstruction technique based on a piecewise-linear interface construction approach is used for reconstructing the interface. A number of multi-phase studies using different turbulence models to the current one have recently appeared in the open literature for simple flow geometries such as rectangular channels. However, most of them assume specific boundary conditions (such as fully-separated phases for stratified flows, the use of square wave at the inlet to represent slug flow or imposing an interfacial disturbance to initiate slugging). These require case-by-case empirical information such as, interfacial roughness for stratified- or slug frequency for intermittent-flow. However, most of them have not presented any detailed validation of their results. The former two points are very crucial for the design of transport pipelines as a pre-knowledge of the operative flow regime and empirical information are not available at the design stage. The predictive accuracy of the present simulations is tested against most common mechanistic approaches and detailed measurements of stratified two-phase flow in a horizontal pipe of Strand (1993) and have been found to be in reasonable quantitative agreement. For the intermittent flow type cases, the numerical results are qualitatively compared against experiments in a horizontal pipe of Al-alweet (2008). The computed flow data of intermittent flow type are further tested against some empirical and mechanistic correlations; the numerical results are qualitatively in a reasonable agreement. Gas compressibility effects on the simulations of slug flow are also explored and are found to bring about some positive benefit. Overall, the predictive accuracy of the present approach is reasonable and promising, demonstrating the ability of the model to predict different types of flow regimes found in two-phase pipe flows. Furthermore, the proposed model shows potential for general applicability to the design of two-phase pipeline systems as it does not require pre-knowledge of the flow regime or any case-by-case empirical information.

This work presents an all-Mach method for two-phase inviscid flow in the presence of surface tension. A modified version of the Hartens, Lax, Leer and Contact (HLLC) approximate Riemann solver based on Garrick et al. [1] is developed and combined with the popular Volume of Fluid (VoF) method: Compressive Interface Capturing Scheme for Arbitrary Meshes (CICSAM). This novel combination yields a scheme with both HLLC shock capturing as well as accurate liquid-gas interface tracking characteristics. To ensure compatibility with VoF, the Monotone Upstream-centred Scheme for Conservation Laws (MUSCL) [2] is applied to non-conservative (primitive) variables, which yields both robustness and accuracy. Liquid-gas interface curvature is computed via both height functions [3, 4] and the convolution method [5]. This is in the interest of applicability to both cartesian and arbitrary meshes. The author emphasizes the use of VoF in the interest of surface tension modelling accuracy. The method is validated using a range of test-cases available in literature. The results show flow features that are in agreement with experimental and benchmark data. In particular, the use of the HLLC-VoF combination leads to a sharp volume fraction and energy field with improved accuracy (up to secondorder).

Yes / This paper presents the results of investigations carried out to study the effect of horizontal wall jets on a fixed rough bed in an open channel. The study used both numerical and experimental approaches. The numerical and experimental studies are compared for validation. The main objective of this study is to understand the effect of wall jets on a horizontal fixed rough bed in an open channel. The experimental study investigated the effect of wall jets on a fixed horizontal bed, with a known roughness in an open channel flume. A sid-looking Acoustic Doppler Velocimetry (ADV) was used to measure the velocity profile of the flow at different flow zones. The wave monitor was used to measure the free surface during the experiments. Computational fluid dynamics CFD simulations were conducted in a rectangular channel to compare with the laboratory tests using the volume of fluid VOF multiphase method and K- ࢿ model. The two phase (water and air) was used in this study. Computer simulations for the model were used to predict the fluid horizontal velocity (u) revealing the characteristics of the wall jet over different flow zones (developing, fully developed and recovering zones). The results showed that the velocity profiles distribution in the stream wise direction in the channel were reasonable. The reverse velocity was close to the wall jet and the maximum reverse velocity was observed near the water surface. Also the results showed that the depression was close to the wall jet. The agreement between the results obtained from the numerical and the experimental data were reasonable.

Au cours des dernières années, la culture de microalgues est largement étudiée pour produire des biocarburants et d’autres produits de valeur en fixant le dioxyde de carbone de l’atmosphère, afin d’atténuer simultanément les effets du changement climatique et de réduire la dépendance à l’égard des carburants fossiles. En comparaison avec les systèmes ouverts, les photobioréacteurs fermés sont davantage utilisés en laboratoire, car ils permettent de contrôler avec précision les facteurs environnementaux tels que le pH, la concentration en éléments nutritifs, etc. Le principe de fonctionnement d’un photobioréacteur repose sur l’injection de bulles dans le milieu de culture pour (i) apporter du dioxyde de carbone aux cellules (ii)agiter le liquide. Par l’apport d’énergie lumineuse les cellules transforment le carbone inorganique en carbone organique par photosynthèse. Ainsi, les phénomènes physiques - l’écoulement, transfert de matière, transfert radiatif - et les phénomènes biologiques - photosynthèse, croissance cellulaire et mort - coexistent dans un photobioréacteur. Plus important encore, tous les phénomènes de base ne sont pas complètement indépendants les uns des autres. Des recherches récentes ont révélé que le comportement des bulles avait également une incidence directe sur le processus biologique. En raison du comportement significatif des bulles sur la productivité d'un photobioréacteur, la génération de bulles a été étudiée dans cette thèse au moyen de méthodes expérimentales et numériques.Dans l'étude expérimentale, nous avons conçu puis fabriqué un nouveau photobioréacteur afin d'étudier le bullage in situ. L’emploi d’une technique d’ombroscopie couplée à une caméra vidéo a permis l’enregistrement de séries de bulles. Les images traitées ont permis de mesurer des caractéristiques de bulles (fréquence, volume, facteur de forme). Le volume moyen de bulle et la fréquence de formation de bulles augmentent avec le débit de gaz. De plus, la distribution volumique monodisperse à faible débit devient de plus en plus polydisperse par l’accroissement de celui-ci. L’évolution de la forme des bulles lors de leur remontée dans le liquide a été évaluée par l’emploi de facteurs de forme. Ces facteurs diminuent avec la remontée des bulles et traduisent une déformation horizontalement. A débit élevé, les formes des bulles oscillent et coalescent plus fréquemment.La simulation du bullage a été réalisé par l’emploi d’une méthode Volume of Fluid (VOF) et d’une bibliothèque open source de mécanique numérique des fluides OpenFOAM. Ces choix de méthodes sont motivés en raison de la robustesse d'OpenFOAM en matière de simulation d'écoulements diphasiques rapportée dans la littérature. Une première étude numérique de simulation 2D a permis de déterminer les valeurs appropriées des paramètres numériques (nombre de Courant et la taille du maillage) tout en minimisant le temps de calcul par rapport à une pré-étude 3D. Sans surprise, nous avons déterminé que la taille des mailles devait être inférieure au diamètre de la buse pour obtenir des résultats significatifs. De façon plus surprenante, nous avons observé que le nombre maximum de Courant n’a pas d’importance particulière pour ces simulations (dans une limite raisonnable : 0 à 1). Les simulations 3D ont été menées sur un supercalculateur. Elles ont montré que le volume des bulles et l’évolution de leur forme calculées numériquement étaient en accord avec les résultats expérimentaux. Cependant, les simulations 3D n’ont pas permis de représenter la polydispersité de la distribution volumique des bulles en raison d’un temps de calcul nécessaire trop important pour générer une population de bulles suffisamment nombreuse. Au final, l'outil numérique a aussi été utilisé avec succès pour explorer plusieurs caractéristiques hydrodynamiques de mélange dans le liquide. / The working principle of a typical photobioreactor is to inject gas bubbles into the culture medium, providing CO2 to the cells and also stirring the liquid. Subsequently, the cells convert inorganic carbon into organic carbon through photosynthesis under illumination. Therefore, physical phenomena, e.g. bubbly flow, mass transfer, radiative transfer, and biological phenomena, e.g. photosynthesis, cell growth and death, coexist in a photobioreactor. More importantly, all the basic phenomena are not completely independent to each other. For example, bubble volume and bubble shape can influence gas-liquid mass transfer according to Young-Laplace equation and Henry's law. Moreover, some recent research revealed that bubble behaviors also directly affect the biological process. In view of the important impact of bubble behaviors on productivity of a phototbioreactor, the bubbly flow was investigated in this thesis by both experimental and numerical methods.In the experimental study, we first manufactured a new photobioreactor in order to study the bubbles and other phenomena. Subsequently, the bubbles were captured by high speed camera by virtue of a shadowgraphy technique and bubble behaviors were obtained by processing and analyzing the images. From the experimental results, we found that both averaged bubble volume and bubbling frequency increased with gas flow rate. Furthermore, we also discovered that the distribution of bubble volume was almost monodisperse at low flow rate, and it became more and more polydisperse with increasing flow rate. Regarding bubble shape evolution, we used two shape factors, viz. aspect ration and circularity, to quantitatively study it. We found that both shape factors dropped rapidly during bubble rising (within the limit of the field of view of our video camera), which implied that bubbles were flattened in the course of rising. Nonetheless, bubbles became more vertically elongated at higher flow rate, partially due to the more frequent bubble coalescence at higher flow rate.In the numerical study, we adopted VOF method and OpenFOAM, an open source CFD library, as our numerical tool to represent bubbly flow. First of all, the robustness of OpenFOAM in simulating two-phase flow was validated by literature survey. Subsequently, 2D simulations were carried out for seeking the appropriate and not very time-consuming numerical parameters, i.e. maximum Courant number and mesh size. We found that mesh size should somehow be smaller than the nozzle diameter to have meaningful results. On the other hand, maximum Courant number had no particular importance in the simulations (as long as between 0 and 1). Furthermore, 3D simulations were in good agreement with the experiments in terms of bubble volume and bubble shape evolution. However, 3D simulations were not able to represent the polydispersity of bubble volume due to the limited computing power. In addition, several hydrodynamic characteristics were also explored by the proposed numerical tool, which gave reasonable results.To conclude, bubble behaviors were successfully captured by experimental methods and represented by numerical methods in this thesis, which will help us go further in understanding the complicated physical-biological phenomena of a photobioreactor.

Dans le cadre de la modélisation et de l’expérimentation multi-échelles (projet LabEx MMCD pour les matériaux pour la construction durable) de l’Université Paris-Est Marne-la-Vallée, cette thèse de doctorat vise à modéliser et caractériser les micro-matériaux conçus par impact de gouttelettes de céramique fondue. Les applications de ces matériaux revétus de couches minces sont des traitements de surface pour la construction durable tels que la protection anti-corrosion, les barrières thermiques, le traitement du verre ou les renforts mécaniques. En particulier, nous nous concentrons sur la physique associée à la dynamique des gouttelettes liquides (l'aire de contact et le temps de contact entre la gouttelette et la surface) en effectuant une série de simulations numériques pour les écoulements diphasiques à petite échelle avec le code maison Thetis. Nous avons considéré des variations des conditions d'impact initiales ainsi que l’influence des forces d'inertie, capillaire et visqueuses sur la dynamique des gouttelettes. Nous nous sommes intéressés en particulier au diamètre d'étalement maximal, au temps d’étalement maximal et au temps de contact, sur des surfaces solides de mouillabilité variable. Le code est basé sur l’utilisation d’une méthode Volume-Of-Fluid. Il introduit une fonction auxiliaire régularisée pour estimer la courbure locale et la normale à l'interface. Les principaux liquides de référence adoptés sont l'eau et la céramique fondue, l'eau est choisie pour valider notre code en comparant les simulations aux résultats expérimentaux. La céramique fondue est adoptée car elle est largement utilisée en projection thermique pour créer des barrières thermiques et chimiques (couches anti-oxydantes) ainsi que des renforts mécaniques sur des échantillons spécifiques. Nous nous concentrons sur les cas où les surfaces sont hydrophobes, même si les cas hydrophiles sont également considérés dans les configurations de validation pour des raisons de généralité. Egalement, en introduisant une partie de calcul de l'énergie dans la thèse, une analyse énergétique détaillée de la gouttelette après l'impact est effectuée dans les phases d'étalement et de rétraction pour bien comprendre la dynamique à l'intérieur de la gouttelette. Nous trouvons que le temps de projection est inversement proportionnel à la vitesse d’impact, indépendamment de l’angle de contact lors de l’étalement au temps courts. Une nouvelle mise à l'échelle entre l'étalement maximal et le temps d'étalement est proposée. Celle-ci s'accorde très bien avec les résultats expérimentaux. Par ailleurs, nous introduisons cette mise à l’échelle dans une classe de modèle basée sur la conservation de l’énergie pour prédire l’étalement maximal adimensionné, ce qui permet de mieux prévoir l’étalement maximal adimensionné. Pour finir, une mise à l'échelle du temps de contact est proposée en termes de nombre d'Ohnesorge et de Reynolds / Under the framework of the LabEx Multi-Scale Modelling and Experimentation of Materials for Sustainable Construction, of Université Paris-Est Marne-La-Vallée, the present PhD thesis aims at modelling and characterizing micro-material designed by impact of molten ceramic droplets. The applications of thin coating materials are surface treatments for sustainable construction such as anti-corrosion, heat barrier, glass treatment or mechanical reinforcement of specific structures.In particular, we focus on the physics behind the liquid droplets' dynamics (the contact area and the contact time between the droplet and surface) by conducting a series of small scale multiphase flow numerical simulations with home-made code Thetis. All simulations are axisymmetric. We have considered variations of initial impact conditions, and studied the influence of inertial, capillary and viscous forces on the droplets' dynamics, especially the maximum spreading diameter, spreading time and the contact time, on solid surfaces. The code is based on Volume-Of-Fluid techniques and introduces an auxiliary smooth function to estimate the local curvature and the normal to the interface. The major reference liquid adopted are the water and the molten ceramic, the water is chosen to validate our code against available experiments at the beginning. The molten ceramic is adopted as it is widely used in thermal spray to built thermal and chemical barriers (anti-oxidant layers) as well as mechanical reinforcements on specific samples. We focus on the cases in which the surfaces are hydrophobic, even if hydrophilic cases are also considered in validation configurations for the sake of generality. Meanwhile, by introducing an energy calculation part in the code, a detailed energetic analysis of the droplet after impact is performed in both the spreading and retraction stage to have a deep understanding of the dynamics inside the droplet.We find the jetting time is inversely proportional to the impact velocity, independent of the contact angle in the early spreading. A new scaling between maximum spreading and spreading time is observed, and agrees well with experimental results. Further, we introduce this scaling into the model based on energy conservation to predict the maximum spreading factor, which provides better prediction on maximum spreading factor than existing literature references. Also a scaling of contact time is proposed in terms of Ohnesorge number and Reynolds number

La présente étude se fonde sur la méthode du volume de fluide (en anglais VOF pour Volume-of-Fluid), proposée à l'origine par Hirt et Nicols. L'objectif de la première partie de ce travail est l'étude hydrodynamique de la cavitation isotherme dans les grands nuages de bulles. Cette étude s'inscrit plus généralement dans un large effort de recherche en micro--écaillage mené par le CEA. Une méthode capable de traiter la présence de cavités de vapeur de volume variable ou encore de pores a été formulée et implémentée dans un code existant, PARIS. L'écoulement est idéalisé en supposant un liquide parfait, des effets thermiques négligeables et une pression de vapeur nulle. Une étude innovante est présentée, traitant de l'interaction du nuage de bulles dans un liquide en expansion par simulation numérique directe. Les résultats des simulations révèlent l'existence d'un concours de cavités dans un certain régime caractérisé par le nombre de Weber.Dans la deuxième partie de l'étude, le système d'équations à résoudre est modifié et généralisé afin de décrire l'écoulement incompressible d'un fluide diphasique tout en incluant la possibilité d'un changement de phase à l'interface. Une nouvelle méthode VOF est proposée, dans laquelle une nouvelle technique d'advection de la fonction VOF permet de traiter à la fois la conservation de la masse et de l'énergie sous une forme conservative. Les expériences numériques démontrent la précision, la robustesse et la généralité de la méthode proposée, et témoignent d'un développement fondamental important pour l'application des méthodes VOF à la modélisation des changements de phase. / Direct numerical simulation of two-phase ow is used extensively for engineering research and fundamental fluid physics studies [54, 81]. This study is based on the Volume-Of-Fluid (VOF) method, originally created by Hirt and Nicols [30]. This method has gained increased popularity, especially when geometric advection techniques are used coupled with a planar reconstruction of the interface [14, 89]. The focus of the first part of this work is to investigate the hydrodynamics of isothermal cavitation in large bubble clouds, which originated from a larger study of micro-spalling [61], conducted by the French CEA. A method to deal with volume-changing vapour cavities, or pores, was formulated and implemented in an existing code, PARIS . The ow is idealized by assuming an inviscid liquid, negligible thermal effects and vanishing vapour pressure. A novel investigation of bubble cloud interaction in an expanding liquid using direct or detailed numerical simulation is presented. The simulation results reveal a pore competition, which is characterised by the Weber number in the ow. In the second part of the study the governing equations are extended to describe incompressible ow with phase change [79]. The description of the work commences with the derivation of the governing equations. Following this, a novel, geometric based, VOF solution method is proposed. In this method a novel way of advecting the VOF function is invented, which treats both mass and energy conservation in conservative form. New techniques include the advection of the interface in a discontinuous velocity field. The proposed algorithms are consistent and elegant, requiring minimal modifications to the existing code. Numerical experiments demonstrate accuracy, robustness and generality. This is viewed as a significant fundamental development in the use of VOF methods to model phase change.

[pt] Volume of Fluid (VOF) é um método amplamente empregado na predição de escoamentos multifásicos devido à sua simplicidade, boas características de conservação de massa e natural tratamento de interfaces topologicamente complexas. No entanto, para escoamentos dominados por tensão interfacial, a literatura tem mostrado que a precisão nas estimativas da tensão interfacial ainda é um problema em questão, que pode levar a correntes parasíticas e previsão imprecisa da condição de salto de pressão através das interfaces. Isto ocorre principalmente devido às variações abruptas do campo de fração volumétrica através das interfaces, que leva a imprecisão no cálculo das curvaturas interfaciais. Portanto, diferentes abordagens têm sido apresentadas para mitigar este problema, incluindo funções-altura, suavização da fração volumétrica, ajuste parabólico, entre outros. Este trabalho propõe uma nova abordagem para estimativa de curvatura em VOF, mas não limitado a este, que lança uma nova luz a este problema persistente. A ideia é amostrar a interface por nuvens de pontos e normais na isosuperfície de nível 0.5 do campo de fração volumétrica e calcular a curvatura para cada ponto da nuvem por uma técnica de Computação Gráfica (ajuste de normais). As curvaturas são, então, projetadas na malha Euleriana de maneira tal como no método Front-Tracking. O novo método foi implementado no código padrão de VOF do OpenFOAM (interFoam) resultando em melhorias nas estimativas de salto de pressão e em significativa redução das correntes espúrias. Simulações numéricas foram realizadas e resultados comparados a dados de referência demonstrando a viabilidade da ideia. / [en] Volume of Fluid (VOF) is a widely employed method for multiphase flows prediction for its simplicity, good mass conservation characteristics and natural handling of topologically complex interfaces. For surface tension dominated flows, however, literature has shown that accuracy in surface tension estimations is still an issue, what may cause parasitic currents and inaccurate prediction of pressure jump condition across interfaces. It occurs mainly due to abrupt changes in the volume fraction field across the interfaces, which takes to inaccurate estimates of interfacial curvatures. Therefore, different approaches have been proposed to mitigate this problem including height-functions, volume fraction smoothing, parabolic fittings, among others. This work proposes a novel approach for curvature estimation in VOF, but not limited to it, that sheds a new light on this persistent problem. The idea is to sample the interfaces with clouds of points and normals at the 0.5 level isosurface of the volume fraction field and to compute the curvature for each point of the cloud by a Computer Graphics technique (normal fitting). The curvatures are then projected onto the Eulerian grid in a Front-Tracking fashion. The new method was implemented in the standard OpenFOAM VOF solver (interFoam) resulting in improvements on the pressure jump estimations and in significant reduction of spurious currents. Numerical simulations were performed and results compared to benchmark data showing the feasibility of the idea.