Twenty years ago, Chang et al. (1998) introduced the three-yield stresses concept (dynamic, static and elastic limits) to describe yielding of waxy crude oils cooled below the wax appearance temperature (WAT). At the time, the limits in rheological instruments were such that they never actually measured the elastic-limit, a key fundamental property. Using modern instruments, this research succeeds in recording for the first time the entire yielding process down to stresses of 10-7 Pa and shear rate of 10-6 min-1 as a function of temperature, cooling rate and stress loading rate using two waxy oils of different origins and wax content. A four-yield stress model is established using derivative data (dynamic fluidity and failure acceleration). In addition, calorimetry (DSC) and microscopy (CPM) helped extract WAT, the gel and pour points and link gel crystal structure and its yielding and breakage to rheological properties. The yielding stresses measured rheologically were tested in laboratory pipelines at two diameter scales, 6.5mm and 13.5mm to compare stresses in uniform and non-uniform cooling. It is demonstrated that rheological instruments can only predict gel breaking pressure when the cooling rate is low, i.e. yielding at the pipe wall. A complementary heat transfer study was performed on a section of pipe statically cooled, both experimentally and theoretically to predict the gel front-liquid oil interface that develops in industrial pipeline where gel breaking occurs. This key information together with rheological data provide the means to predict accurately restart pressures of shut gelled pipelines that have eluded previous research. / Ministry of Higher Education of the Libyan Government

Thesis (M. Tech. (Engineering: Mechanical)) -- Central University of Technology, Free State, / The Centre for Rapid Prototyping and Manufacturing (CRPM) at the Central University of Technology, Free State (CUT) manufactures implants using Electro Optical Systems (EOS) titanium Ti-6Al-4V alloy powder (further referred to as EOS Ti64 powder) by means of Direct Metal Laser Sintering (DMLS) process on the EOSINT M 270 machine. For this reason, there is a need to characterise and acquire knowledge of the basic properties of direct metal laser sintered EOS titanium Ti-6Al-4V alloy samples (further referred to as DMLS Ti64 samples) under static tensile loading in order to provide the CRPM with engineering design data. The first objective of this Master’s study is to acquire the characteristics of EOS Ti64 powder in order to ascertain its suitability in the DMLS process. Secondly, the study aims to assess tensile properties and elastic constants of DMLS Ti64 samples produced from the set process parameters of EOSINT M 270 machine. Thirdly, it is to investigate microstructures of DMLS Ti64 samples subjected to different heat treatment techniques which will eventually assist in the determination of a suitable heat treatment technique that will yield higher ductility. Finally, the study aims to validate the static behaviour of DMLS Ti64 samples subjected to the static tensile loading up to a yield point in order to determine failure due to yielding. The samples were manufactured at CRPM Bloemfontein. The metallographic examinations, heat treatment and the determination of mechanical properties were done at the CSIR in Pretoria. Optical Microscope (OM) and Scanning Electron Microscope (SEM) were used to determine microstructures of DMLS Ti64 samples while Energy Dispersive X-Ray (EDX) analyses were performed using SEM. The samples were heat treated at temperatures of 700, 1000 and 1100°C respectively, and subsequently either cooled with the furnace, air or were water quenched. The mechanical property tests included tensile, hardness and determination of elastic constants. The static behaviour of DMLS Ti64 samples under static tensile load up to a yield point was predicted and verified using ABAQUSTM Finite Element Analysis (FEA). The stress-strain curves from ABAQUSTM were interpreted using MDSolid program. The point of interest was Von Mises yield stress at 0.2% offset, in order to determine failure due to yielding. EOS Ti64 powder particles were spherical in shape and the alpha and alpha+beta phases were identified. As-laser sintered samples possess a very fine and uniform alpha case with islands of martensitic plates; samples were brittle and showed low levels of ductility with an average elongation of 2.6% and an area reduction of 3.51%. Ultrasonic test results showed that DMLS Ti64 samples have Young’s modulus of 115 GPa, Shear modulus of 43 GP, a bulk modulus of 109 GPa and Poisson’s ratio of 0,323 while the density was 4.4 g/cm3. Slow cooling of DMLS Ti64 samples from 1000 and 1100oC resulted in a microstructure constituted more by the alpha phase of lower hardness than those from 700oC and as-laser sintered samples. High hardness was obtained by water quenching. The water quenched samples showed martensitic transformation and high hardness when compared to furnace cooled samples. Beta annealing tailored a microstructure of as-laser sintered samples into a lamellar structure with different lath sizes as per cooling rate. Beta annealing improved ductility levels up to 12.67% elongation for samples furnace cooled for 4 hours and even higher to 18.11% for samples furnace cooled for 34 hours, while area reduction increased to 25.94% and 33.39%, respectively. Beta annealing conversely reduced yield strength by 19.89% and ultimate tensile strength was reduced by 23.66%. The calculated maximum Von Mises stresses found were similar to the FEA interpreted results. The average percentage error, without the stress concentration factor, was approximately 8.29%; with the stress concentration factor included, it was 0.07%. The small reaction forces induced in both x-axis and z-axis contributed to this error of 0.07% between the calculations and ABAQUSTM FEA results. Samples that were not heat treated fell outside the Von Mises criterion and failed due to yielding. This justified the brittleness found in the tensile test results where elongation and area reduction were 2.6% and 3.51% respectively. However, all samples that were heat treated fell within the Von Mises criterion. The objectives of this study were achieved; the mechanical properties were similar to those of standard specification for wrought annealed Ti-6Al-4V alloy for surgical implant applications and EOS GmbH manufacturer’s material data sheet. DMLS Ti64 samples must be beta annealed in order to attain higher levels of ductility. A recommendation was made to further investigate the effect of heat treatment on the other mechanical properties. Furthermore, detailed results of basic properties of DMLS Ti64 samples are provided in the appendices in chart format and were written on a CD disc.

[pt] Uma preocupação constante referente às operações de reinício de escoamento em poços produtores de óleo é a previsibilidade da mínima pressão necessária para iniciar o fluxo após uma parada de produção. Tal pressão se refere àquela que promove tensões de cisalhamento junto à parede do duto que superam o valor da tensão limite de escoamento do referido fluido. A necessidade de desenvolvimento de diferentes tipos de campos de petróleo ao redor do mundo tem promovido discussões sobre importantes aspectos de garantia de escoamento dos fluidos, especialmente em cenários de produção em águas ultra-profundas. Nesses casos, quando o fluido apresenta valores elevados para a temperatura mínima de aparecimento de cristais (TIAC) ou ainda para a tensão limite de escoamento, o procedimento de repartida de poço após uma parada de produção pode representar um problema. Em geral, os modelos tixotrópicos utilizados na avaliação de reinício do escoamento são definidos por uma equação constitutiva baseada no modelo de Bingham, na qual a tensão de cisalhamento depende do grau de estruturação do fluido no interior do duto, representado por um parâmetro adimensional positivo (lambda). A evolução deste parâmetro no tempo é governada por uma equação constitutiva que considera um termo de construção e um termo de quebra do gel. Este trabalho tem como principal objetivo avaliar o comportamento do reinício da produção através da variação destes parâmetros/termos segundo modelos tixotrópicos específicos (Houska e SMT) e a representatividade dos resultados de simulação quando comparados aos dados experimentais. / [en] The main concern regarding to the restart operations for production wells in oil and gas systems is the forecast of minimum pressure needed to overcome the gel strength, i.e. the pressure which generates a wall stress higher than the yield stress of the gelled oil. The petroleum accumulations around the world have raised big issues regarding flow assurance aspects, especially in some ultra-deep water scenarios. In these cases, when the oil presents unusual high values for the Wax Appearance Temperature (WAT) and even for the yield stress, the flow restart procedure can be an issue, after shutdown times of the oil production. Usually, the thixotropic models applied for start-up flow are defined by a Bingham-like stress equation whose yield stress depends on a structure parameter (lambda), following Houska assumptions. This parameter is a non-negative scalar number that represents the structuring level of the material inside the pipeline. The structure parameter is governed by an evolution equation that considers a build-up term and a breakdown term. The main objective of this work is to evaluate the relationship between the parameters of the constitutive equations for Houska and SMT models (dimensionless coefficients of the evolution equations for the structure parameter) and the representativeness of the simulation results obtained by these models, regarding times for the production stabilization after a restart, delay times and yield stresses, when compared to experimental data.

L’objectif de cette thèse est de développer un modèle numérique 3D afin d’étudier le phénomène de laves torrentielles ; écoulements visqueux fortement chargés en matière solide, surgissant en montagne lors d’orages violents. Aujourd’hui, la prévision des zones de vulnérabilité s’appuie sur des outils de calcul 0D, 1D ou 2D. Or ces outils ne peuvent représenter pleinement le comportement à surface libre des écoulements du fait de nombreuses approximations et hypothèses. Ainsi cette thèse met en œuvre un code numérique 3D pour étudier ces écoulements. Ce travail aboutit à la construction d’un modèle 3D à partir de données réelles de terrain. Plusieurs scénarios ont été étudiés et comparés à des résultats issus d’un modèle 2D. Les résultats mettent en évidence les apports non négligeables de la modélisation 3D : zones d’étalement et de dépôt, phénomènes d’encombrement, modélisation fine des écoulements dans les zones chenalisées. Préalablement, le modèle 3D a été validé en comparant les résultats numériques à des données expérimentales issues de la littérature, pour des typologies d’écoulement représentatives de celles observées sur des sites grandeur nature. / The objective of this thesis is to develop a 3D numerical model to assess debris flow. These viscous flows, heavily loaded with solid matter, form when heavy rain occurs in mountains. Today, forecasts of potentially impacted areas are based on 0D, 1D and 2D numerical tools. However, these tools cannot fully represent the free surface behaviour of debris flows due to the approximations and assumptions on which they are based. Thus, this work utilises a 3D numerical code to study this phenomenon. A specific model is built with real field data. Several flow scenarios are studied and compared with a 2D numerical model. The results highlight the significant benefits of a 3D approach by providing information on the fine representation of flow dynamics over the catchment area. The model also predicts the impact of debris flow (overflowing on a road bridge) and the zones of deposition and spreading. It highlights possible congestion phenomena and reproduces flows in the channels by fully accounting for parietal friction, capabilities not provided by 2D models. Prior to this application, the 3D model was evaluated with five sets of experimental data to validate its ability to represent viscoplastic flows. Different types of flows are studied and are representative of those observed on real sites when debris flow occur.

On considère l'évolution adiabatique des états vitreux sous perturbations extérieures. Bien que le formalisme que nous utilisons soit très général, nous nous concentrons ici sur les sphères dures en dimension infinie où une analyse exacte est possible. Nous considérons perturbations de la frontière, notamment compression ou cisaillement simple et nous calculons la réponse des états vitreux à ces perturbations : pression et contrainte de cisaillement. Nous constatons un dépassement des deux quantités avant que l'état vitreux ne devienne instable à un point spinodal, où il fond dans le liquide (ou cède). Nous estimons également la limite d'élasticité du verre. Enfin, nous étudions la stabilité des bassins vitreux vers la rupture en sous-bassins, correspondant à une transition de Gardner. Nous constatons que près de la transition dynamique, les verres subissent une transition de Gardner après une perturbation infinitésimale. Nous étudions ensuite le régime de haute pression et haut cisaillement au-delà de la transition de Gardner. / We consider the adiabatic evolution of glassy states under external perturbations. Although the formalism we use is very general, we focus here on infinite-dimensional hard spheres where an exact analysis is possible. We consider perturbations of the boundary, i. e. compression or (volume preserving) shear-strain, and we compute the response of glassy states to such perturbations: pressure and shear-stress. We find that both quantities over shoot before the glass state becomes unstable at a spinodal point where it melts into a liquid (or yields). We also estimate the yield stress of the glass. Finally, we study the stability of the glass basins towards breaking into sub-basins, corresponding to a Gardner transition. We find that close to the dynamical transition, glasses undergo a Gardner transition after an infinitesimal perturbation. We then study the high-pressure and high-strain regime beyond the Gardner transition.

Cette thèse, composée de deux parties, porte sur l’étude d’instabilités au sein d’un milieugranulaire.La première partie de cette thèse est consacrée à l’instabilité de tôle ondulée, c’est-àdirel’apparition d’un motif de rides sur une piste soumise au passage répété de véhicules.Nos travaux expérimentaux ainsi que des simulations numériques de dynamique moléculairenous ont permis de progresser dans la compréhension de ce phénomène. Pour uneroue rigide tirée à vitesse constante, nous avons étudié quantitativement l’impact de lacohésion du milieu granulaire sur les caractéristiques de l’instabilité : vitesse critique, longueurd’onde et taux de croissance. Nous avons enfin entrepris des études préliminairessur un fluide à seuil simple : un microgel de carbopol.Dans la seconde partie, nous étudions la ségrégation granulaire au sein des astéroïdeslâches. Plusieurs indices montrent qu’un grand nombre d’entre eux sont des empilementsde grains, sans cohésion interne et liés par la gravité. Leur répartition n’est pas homogène,avec des zones lisses recouvertes de fines poussières et d’autres où s’accumulent les grosblocs. Nous avons simulé numériquement un empilement granulaire et nous l’avons soumisà des secousses répétées : nous avons réussi à reproduire la ségrégation. Nous avonsétudié l’influence des paramètres physiques et numériques sur le niveau de ségrégation etla dynamique du phénomène. Nous nous sommes aussi intéressés aux phénomènes physiquespouvant causer la ségrégation : convection granulaire, tamisage cinétique, pressionde déplétion, etc. Enfin, nous avons mis en évidence une instabilité azimutale qui peutexpliquer les observations faites pendant les missions spatiales. / This thesis, consisting of two parts, is focusing on instabilities into a granular assembly.The first part deals with washboard road instability, which is the growth of a ripplepattern on a track subjected to repeated passages of vehicles. Our experimental work aswell as soft spheres numerical simulations provide us a better understanding of this phenomenon.In the case of a rigid wheel dragged at constant velocity, we quantitatively studiedthe impact of the cohesion into the granular media on the main features of the instability :critical velocity, wavelength and growth rates. Finally, we have begun preliminary studieson a yield stress fluid, namely a carbopol microgel.The second part is devoted to the granular segregation in asteroids. Several keys showthat a significant part of them are rubble-piles, without internal cohesion and held togetherby their weak self-gravity. The distribution of grains is heterogeneous : while some regionsconsist in fine sand or powder, large boulders seem to accumulate in other parts. Wenumerically simulated a granular pile and subjected it to repeated quakes : we managedto reproduce segregation. We studied the influence of physical and numerical parameterson the segregation level and the dynamics of the phenomenon. We also investigate variouscauses of the segregation : granular convection, kinetic sieving, depletion pressure, etc.Finally, we highlighted an azimuthal segregation which can explain observations duringspatial missions.

La matière molle se consacre à l'étude des propriétés de fluides complexes. Ces fluides diffèrent des fluides simples à cause de l'existence d'une microstructure qui provient de l'arrangement particulier des éléments mésoscopiques constitutifs du matériau (agrégats de particules de noir de carbone, enchevêtrements de polymères, micelles de molécules tensioactives). C'est le couplage entre microstructure et déformation qui confère aux fluides complexes des comportements singuliers et qui engendre des écoulements hétérogènes. Comprendre ces états hors-équilibre et les dynamiques associées présente un intérêt à la fois industriel et fondamental. La rhéologie en cellule de Taylor-Couette est une technique très répandue pour l'étude de la déformation et de l'écoulement de fluides complexes. Cependant, cette méthode n'est pas adaptée à l'étude des écoulements hétérogènes car elle ne fournit qu'une description globale de l'écoulement. Pour pallier ce problème, une technique de vélocimétrie ultrasonore à deux dimensions a été couplée à la rhéologie classique. Cette visualisation locale nous a permis d'étudier l'instabilité inertielle de Taylor-Couette dans les fluides newtoniens, les instabilités élastiques de fluides viscoélastiques (polymères et solutions micellaires), la fluidification de fluides à seuil (gels de noir de carbone, microgels de carbopol et émulsions) et enfin la rupture de gels de protéine soumis à une contrainte de cisaillement. Tous ces exemples montrent des coexistences entre différents états induits par l'écoulement et permettent de revisiter les approches rhéologiques à partir de caractérisations locales des champs de déformation et de vitesse. / Soft matter scientists are dedicated to studying the properties of complex fluids. Complex fluids differ from simple fluids in that they possess a microstructure resulting from the particular arrangement of mesoscopic elements which constitute the material (aggregates of carbon black particles, entangled polymers, micelles of surfactant molecules, etc.). Peculiar flow behaviors in complex fluids, such as heterogeneous flows, arise from the coupling between microstructure and flow. Understanding these non-equilibrium states and the associated dynamics is both of industrial and fundamental interest. Rheology in a Taylor-Couette cell is a wide-spread technique for investigating the deformation and flow of complex fluids. However, this method is mostly blind to heterogeneous flows as it only provides a global description of the flow. To overcome this problem, an ultrasonic imaging technique has been combined with classical rheology. This local visualisation has allowed us to study the inertial Taylor-Couette instability in Newtonian fluids, elastic instabilities in viscoelastic fluids (polymers and micellar solutions), the fluidisation of yield stress fluids (carbon black gels, carbopol microgels and emulsions) and finally the failure of protein gels under stress. In all these cases we evidence a coexistence between different flow-induced states and revisit global rheological approaches through local characterizations of deformation and velocity fields.

Durant les trois années de la thèse, j’ai eu le plaisir de travailler en collaboration avec à la fois des volcanologues, des physiciens de laboratoire et des mathématiciens. Ce mémoire est l’occasion de présenter la démarche et les résultats de mes recherches dans le domaine de la modélisation d’écoulements granulaires denses fluidisés. Ces derniers consistent à développer un nouveau modèle mathématique et son étude théorique et numérique. Sur la base d’observations faites lors d’expériences de laboratoire, nous proposons une façon de modéliser le changement comportemental d’un écoulement granulaire initialement fluidisé au travers de la définition de sa rhéologie viscoplastique à seuil variable. Plus précisément, le seuil de plasticité est défini par la différence entre la pression lithostatique et la pression du fluide interstitiel. La nouveauté apportée par ce modèle ouvre de nouvelles perspectives à la fois pour le champ de recherche en mathématiques et pour la compréhension des lits granulaires fluidisés et leur application à la volcanologie. Du point de vue mathématique, une étude théorique du modèle a été menée. En proposant une preuve de l’existence de solutions faibles à un problème lié à la version homogène du modèle, nous apportons une extension au champ de connaissances autour des écoulements des fluides non-newtoniens. D’autre part, dans le but de reproduire numériquement des expériences de laboratoire de chute de colonne granulaire fluidisée, nous avons développé un code de simulation numérique incluant une nouvelle méthode de résolution des équations d’écoulement de fluides à seuil. Dans ce manuscrit, je décris et justifie les différents choix stratégiques pour le développement de ce code. Par ailleurs, je présente quelques tests académiques permettant de valider le code. Enfin, je donne les résultats de simulation de chute de colonne granulaire, qu’elle soit fluidisée ou non. Une comparaison avec les données de laboratoire est effectuée afin d’évaluer les points forts et les défauts du modèle par rapport à la réalité des expériences. En conclusion, dans la continuité du travail mené dans ce projet, des perspectives d’amélioration sont proposées. / During these three years, I enjoyed to work with collaborators from volcanology, laboratory physics and mathematics. This document presents the steps and results of my research in the field of modelling of fluidised granular flows. The last consists in the development of a new mathematical model and its theoretical and numerical study. Based on observations made on experimental studies, the model focuses on the change in the behaviour of an initially fluidised granular flow through the definition of its viscoplastic rheology with variable threshold. More precisely, the threshold (aslo called yield stress) is defined via the difference between the lithostatic pressure and the pressure of the interstitial fluid. The innovation of this model opens perspectives for the mathematical research as well as for the study of fluidised granular flows and their application to volcanology. From a mathematical point of view, a theoretical study has been conducted. Proving the existence of weak solution for the homogeneous version of the model, we offer an extension in the field of knowledges of non-newtonian fluid flows. Also, we have developped a numerical code to simulate dambreak experiments with fluidised granular media. This one includes a new method to solve the flow equations of viscoplastic fluids. In this thesis, I describe and justify the numerical strategy chosen. Moreover, I present some academic tests to validate the code. At the end, I give the numerical results in the case of the dambreak simulation for dry and fluidised fluids. By comparing with experimental data, we evaluate the validity of the model and its resolution, and highlight the advantages and inconvenients. To conclude the project, I propose some perspectives of improvement for later work.

Cette thèse présente une étude expérimentale de l'impact de gouttes de fluides à seuil. Au-delà des applications (impression à jet d’encre solide, modélisation d’impact solide à grandes vitesses), cette étude permet de sonder le rôle de l'élasticité sur le comportement à temps court de ces fluides complexes. D'abord, nous nous sommes intéressés aux impacts sur une surface rigide. L'utilisation de fluides à seuil modèles (solutions concentrées d’argiles, micro-gel de Carbopol) et de surfaces d'impact variées (partiellement mouillante ou super-hydrophobe), révèle une grande variété de comportements, allant de l'étalement viscoplastique irréversible jusqu'à des déformations élastiques géantes. Un modèle minimal d'étalement inertiel, incluant une rhéologie élasto/viscoplastique, permet de décrire dans un cadre unique les principaux régimes observés. Au cours de cette étude, nous avons mis en évidence un phénomène spécifique avec le Carbopol : pour des grandes vitesses d'impact, on observe un étalement beaucoup plus grand sur des surfaces rugueuses hydrophobes que sur des surfaces lisses. Cette réduction apparente du frottement basal est discutée en termes de longueur de glissement et d'instabilité de « splash ». Enfin, nous avons étudié l’impact d'une goutte de fluide sur un sol constitué du même fluide, en utilisant un fluide à seuil transparent (Carbopol). La combinaison de lois d'échelle, d’expériences en « micro-gravité » et de mesures locales du champ de déformation montre que la dynamique du cratère transitoire est dominée par l’élasticité, même au-delà du seuil d’écoulement. Ces résultats pourraient avoir des implications dans le contexte des impacts de météorites en astrophysique. / This thesis presents an experimental study on the drop impact of yield-stress fluids. Beyond applications (solid ink-jet printing, lab modelling of high-speed collision of solids), this study offers a mean to probe the role of the elasticity on the short-time behaviour of these complex fluids. We have first studied drop impacts on solid rigid surfaces. Using different model yield-stress fluids (clay suspensions, Carbopol micro-gel) and impacted surfaces (partially wettable, super-hydrophobic), we have observed a rich variety of behaviours ranging from irreversible viscoplastic coating to giant elastic spreading and recoil. A minimal model of inertial spreading, including an elasto/viscoplastic rheology, allows explaining in a single framework the different regimes and scaling laws. In this study, we identified a specific phenomenon with Carbopol: for large impact velocities, the drop spreads much more on rough hydrophobic surfaces than on smooth surfaces. This apparent reduction of the basal friction is discussed in terms of slip length and splash instability. Endly, we investigated the impact of a drop onto a pool of the same fluid, using a transparent yield-stress fluid (Carbopol). The combination of scaling laws, micro-gravity experiments and local deformation measurements shows that the transient crater is dominated by elasticity, even beyond the flow threshold. These results could have implications for impact cratering in Planetary Sciences.

This diploma thesis covers the topic of magnetic fluids and their utilisation in hydrodynamic dampers. The first part of the work consists of research on the current state of technology in hydraulic dampers with the focus on dampers using magnetorhelogical fluid. This chapter is followed by research on magnetic fluids with regard to their physical properties and mathematical description, which is used for CFD simulation of flow. The second part deals with the computational simulation of the flow of MR liquid in real MR damper in order to determine the damping characteristic and it's comparison with the experimental data.