Tesi sul tema "Écoulements de densité pyroclastiques"
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Penlou, Baptiste. "Étude expérimentale des écoulements gaz-particules en contexte de fontaine pyroclastique". Electronic Thesis or Diss., Université Clermont Auvergne (2021-...), 2023. http://www.theses.fr/2023UCFA0159.
Testo completoPyroclastic columns form during explosive volcanic eruptions in which a mixture of gases and particles is ejected at high speed from a vent and can lead to the formation of convective plumes. The stability of these columns depends on various parameters that can vary over time and cause partial or total collapse of the pyroclastic mixture. These collapses give rise to eruptive fountains, forming density currents called pyroclastic density currents (PDCs). The objective of this thesis is twofold: to study (1) the mechanisms of particle sedimentation in the plume and the dilute part of PDCs, and (2) the mechanisms of PDC emergence in the impact zones of the fountains. The chosen method is the experimental approach.A first series of experiments involves suspending particles ranging in size from 49 to 467.5 µm in a cylindrical device and measuring the local particle concentration for each mixture. For this purpose, two independent approaches were used and provided similar results: an acoustic method and the use of pressure sensors. These experiments highlight two mechanisms of particle sedimentation: enhanced sedimentation and delayed sedimentation. In suspensions of small particles (78 µm), the sedimentation rate increases with the local particle concentration due to the formation of « clusters » that fall at a speed four times higher than the terminal settling velocity of individual particles (enhanced sedimentation). However, in suspensions of larger particles (467.5 µm), the sedimentation rate decreases with increasing particle concentration, despite the presence of « clusters » and it is 30 % lower than the settling speed of individual particles (delayed sedimentation). These results suggest that the sedimentation mechanisms in the presence of « clusters » occurring in plumes or the dilute part of PDC should be considered in models used to simulate these volcanic phenomena to better predict deposit characteristics.A second series of experiments simulates a pyroclastic fountain by releasing particles of sizes ranging from 29 and 269 µm into a channel at a height of 3.27 meters. The results show that dilute mixtures (1.6 - 4.4 vol.%) in free fall accumulate in the impact zone to form concentrated granular flows (~ 45 - 48 vol.%) whose interstitial fluid pressure nearly compensates for the weight of particles for sizes < 76 µm. Furthermore, the maximum fluid pressure measured at the impact, the flow travel distance, and the horizontal stretching of deposits increase with decreasing particle size. Considering the experiment dimensions, these results indicate that a high interstitial fluid pressure can be generated in the impact zone of collapsing pyroclastic fountains. The small particle size, causing low permeability and a long pressure diffusion time, may be one of the main factors leading to the long runout distances covered by the flows
Mathé, Jordane. "Modélisation d'écoulements gravitaires fluidisés et applciation à la volcanologie". Thesis, Clermont-Ferrand 2, 2015. http://www.theses.fr/2015CLF22646/document.
Testo completoDuring 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
Chédeville-Monzo, Corentin. "Mécanismes d'auto-fluidisation des écoulements pyroclastiques : approche expérimentale". Thesis, Clermont-Ferrand 2, 2016. http://www.theses.fr/2016CLF22684/document.
Testo completoPyroclastic flows are hot mixtures of gas and particles that can propagate over large distances. This high “mobility” is often attributed to their ability to be fluidized, that is, to generate and retain high gas pore pressure that reduces internal friction forces. The main objective of this thesis is to understand how irregularities of substrates on which pyroclastic flows propagate can enhance their fluidization. A first set of laboratory experiments consisted of the generation of fine-grained flows (diameter of 45-90 μm) on substrate of various roughness. Results show that the flow runout distance increases with the substrate roughness, and is up to twice the runout on a smooth substrate. High speed video analyses and air pore pressure measurements at the flow base show that the flow head propagating over a rough substrate can auto-fluidize because of particles sedimentation into the substrate interstices, which forces the air to escape upward and percolate through the flow. This auto-fluidization mechanism is efficient at all inclinations investigated (0-30°), suggesting that it could occur during the whole emplacement of a pyroclastic flow. A second study consisted of the vertical release of beds of particles in a static column. Results show that the granular mixture can be fully fluidized, even when collapsing from a relatively low height (20 cm). When particles are fine enough (<100 μm), pore pressure in the deposit diffuses for several seconds, the diffusion duration increasing with increasing bed thickness and decreasing particle size. The longest diffusion durations are observed for pyroclastic flow deposit materials (~30 s for 28.5 cm thick beds). These results suggest that pyroclastic flows propagating on irregular terrains can auto-fluidize and preserve low internal friction during their emplacement
Girolami, Laurence. "Dynamique et sédimentation des écoulements pyroclastiques reproduits en laboratoire". Clermont-Ferrand 2, 2008. http://www.theses.fr/2008CLF21836.
Testo completoFormenti, Yvan. "Etude de la production et de la mobilité des écoulements pyroclastiques à Montserrat (Antilles)". Clermont-Ferrand 2, 2002. http://www.theses.fr/2002CLF21350.
Testo completoBernard, Julien. "Capacité érosive des écoulements pyroclastiques : impact sur les budgets éruptifs et implications pour l'aléa". Thesis, Clermont-Ferrand 2, 2015. http://www.theses.fr/2015CLF22553/document.
Testo completoPyroclastic flows (PFs) are hot mixtures of gas and particles that represent the most deadly volcanic hazard. To protect the populations, it is necessary to work on precise risk maps, which require having a deep knowledge of these phenomena. However, the causes and consequences of erosion and incorporation of non-juvenile material during PFs emplacement remain poorly known. This thesis aims at characterizing the erosive capacity of pyroclastic flows, defining erosion mechanisms and quantifying their impact on eruptive budgets and associated hazards. Here, we focus on small-volume PFs and use an approach based on field and textural investigations coupled with numerical modeling of PFs emplacement. The August 2006 PF-forming eruption of Tungurahua volcano (Ecuador) is used as a case-study for this work.An original method, based on high-resolution, stereologically-corrected image analyses, detailed textural analyses of PFs deposits and mass budget, enables determining the grain size distribution and the componentry of PFs products along their entire clast size range. Volume calculation and mass budgets show that about 50 wt. % of the whole deposit consists of non-juvenile materials incorporated during PFs emplacement, and mostly coming from the upper part of the volcano. The slope is a prevailing parameter that controls PFs erosive power. Eruptive budgets support a VEI 3 event (0.09 km 3 ) for the 2006 eruption of Tungurahua and highlight the importance of separating juvenile from non-juvenile material. Detailed analyses of deposits’ componentry suggest a strong dynamic density-driven segregation of the clasts during PFs emplacement, associated with sedimentation rates of ≈10 cm.s -1 . Lateral variations of lithological, grain size, and morphological data demonstrate the occurrence of componentry-driven clast fragmentation and abrasion processes. Massive components (e.g. old lavas) are the main grinding agents of scoriaceous components (e.g. bombs). During emplacement, these processes continuously create fine grained populations, which are transferred from the main dense flow to pyroclastic surge or Co-PF cloud. Numerical models of erosive PFs based on a new erosion law integrated into VolcFlow code show the ability of plastic rheology to reproduce natural erosion patterns of PFs. The erosion is produced by dynamic variations of normal stress / shear stress ratio during emplacement, due to thickness unsteadiness during flow deceleration. The thin, highly frictional and erosive front of PFs pulses is pushed by the thicker and non-erosive head and flow body. Incorporation implies longer PFs runouts of about 10-30%, depending on the amount of incorporated material, which is related to the quantity of erodible material available on the volcano’s flanks before the eruption. These results show that erosion has a significant role on PFs runouts, and thus in hazard assessment, which should be closely taken into account in future works
Fontane, Jérôme. "Transition des écoulements cisaillés libres à densité variable". Phd thesis, Toulouse, INPT, 2005. http://oatao.univ-toulouse.fr/7338/1/fontane.pdf.
Testo completoGuyez, Estelle. "Mélange d'interfaces de densité en écoulements de Taylor - Couette". Phd thesis, Université Joseph Fourier (Grenoble), 2006. http://tel.archives-ouvertes.fr/tel-00081798.
Testo completoGueugneau, Valentin. "Etude de la formation et de la mise en place des déferlantes pyroclastiques par modélisations numérique et expérimentale". Thesis, Université Clermont Auvergne (2017-2020), 2018. http://www.theses.fr/2018CLFAC050/document.
Testo completoSmall volume pyroclastic density currents are complex volcanic flows, whose physical behaviour is still debated. They comprise two parts: the pyroclastic flow, rich in particles and blocks, overridden by the ash-cloud surge, a turbulent and dilute flow. The interactions between these two parts are not fully understood, as well as their exchanges of mass and momentum. Therefore, the thesis focuses on the investigation of ash-cloud surge formation mechanisms from the pyroclastic flow. The experiments reveal a mechanism of dilute flow formation by alternation of air incorporation into and elutriation of fine particles from a dense granular bed subjected to vibrations. The air is aspirated into the granular bed during dilatations, and expulsed during the contraction phases. A part of the particles are then sustained by the turbulent expulsed air and form a mixture of gas and particles that transforms into a gravity current. Extrapolated to a volcanic edifice, this mechanism of air incorporation and elutriation can be reproduced by a rough topography, where each obstacle generates a compaction followed by a dilatation of the pyroclastic flow. The quantification of the mechanism has been accomplished and the mass flux from the dense flow to the ash-cloud surge has been deduced.The numerical model is first used to study the pyroclastic flow rheology, which controls the velocity of the flow, and then the mass flux previously mentioned. One chapter is dedicated to the fluidization effect on the pyroclastic flow rheology. Results show that this mechanism can explain the long runout of these flows, and also the formation of levées and channel morphologies. The air ingestion in the flow during its movement could explain a part of the pyroclastic flows dynamic. Simple rheologies has also been analyzed: a Coulomb rheology, a plastic rheology, and a variable friction coefficient rheology. Results show that the plastic rheology seems to be the most adapted rheology to simulate the pyroclastic flow dynamic. Then, the numerical model has been used to test the mass flow law obtained through experiments. Applied to the 25 June 1997 dome collapse at Soufrière Hills Volcano at Montserrat, results show that the simulations reproduce accurately the extension and the thickness of the surge deposits. The simulations are also able to reproduce the surge derived pyroclastic flow, generated by remobilisation of surge deposits. The cycles of ingestion/expulsion of air in the pyroclastic flow by interactions with the topography could explain both the great fluidity of these flows and the formation of ash-cloud surge. These results highlight a new mechanism that could be a key process in pyroclastic flow dynamic, which could improve significantly the hazard and risk assessment using numerical model
Chalayer, Rénald. "Méthodes de projection pour des écoulements à seuil, incompressibles et à densité variable". Thesis, Université Clermont Auvergne (2017-2020), 2019. http://www.theses.fr/2019CLFAC089.
Testo completoThis thesis deal with the use of projection methods for incompressible viscoplastic flows with a variable density. This manuscript is organized following three main lines. The first part is devoted to the mathematical model, the time-discretization of the model and the analysis of the numerical scheme. To construct the numerical scheme, on one side we adapt a time-stepping method already used for newtonian flows to viscoplastic flows and on the other side, we use a projection formulation of the stress tensor to solve the coupling between the velocity and the plastic part of the stress tensor in the momentum equation with an Uzawa-like method. Stability and error analysis of the numerical scheme are provided and a first-order estimate of the time error is derived for the velocity field, the density, the viscosity and the yield stress. The second part is devoted to the space discretization and the implementation of the scheme. A second-order cell-centred finite volume scheme on staggered grids is applied for the spatial discretization. The implementation of the numerical scheme has been performed using a Fortran 90 code and using the PetsC and MPI library The last part of the manuscript is devoted to numerical simulations. In the Rayleigh-Taylor instability configuration, we perform simulations by varying the yield stress, and describing the evolution of the interface and the localization of the rigid zones. In the dambreak configuration, we use Stick-Slip boundary conditions and we compare our results to the existing literature
Hérard, Jean-Marc. "Modélisation et calcul d'écoulements turbulents isothermes de gaz à densité variable". Toulouse, INPT, 1986. http://www.theses.fr/1986INPT080H.
Testo completoBechereau, Marie. "Élaboration de méthodes Lattice Boltzmann pour les écoulements bifluides à ratio de densité arbitraire". Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLN059/document.
Testo completoTwo-fluid extensions of Lattice Boltzmann methods with free boundaries usually consider ``microscopic'' pseudopotential interface models. In this paper, we rather propose an interface-capturing Lattice Boltzmann approach where the mass fraction variable is considered as an unknown and is advected. Several works have reported the difficulties of LBM methods to deal with such two-fluid systems especially for high-density ratio configurations. This is due to the mixing nature of LBM, as with Flux vector splitting approaches for Finite Volume methods. We here give another explanation of the lack of numerical diffusion of Lattice Boltzmann approaches to accurately capture contact discontinuities. To fix the problem, we propose an arbitrary Lagrangian-Eulerian (ALE) formulation of Lattice-Boltzmann methods. In the Lagrangian limit, it allows for a proper separated treatment of pressure waves and advection phenomenon. After the ALE solution, a remapping (advection) procedure is necessary to project the variables onto the Eulerian Lattice-Boltzmann grid.We explain how to derive this remapping procedure in order to get second-order accuracy and achieve sharp stable oscillation-free interfaces. It has been shown that mass fractions variables satisfy a local discrete maximum principle and thus stay in the range $[0,1]$. The theory is supported by numerical computations of rising bubbles (without taking into account surface tension at this current state of development).Even if our methods are currently used for inviscid flows (Euler equations) by projecting the discrete distributions onto equilibrium ones at each time step, we believe that it is possible to extend the framework formulation for multifluid viscous problems. This will be at the aim of a next work
Arratia, Cristobal. "Mécanismes d'instabilité non-modaux dans les écoulements cisaillés avec et sans stratification en densité". Phd thesis, Palaiseau, Ecole polytechnique, 2011. http://www.theses.fr/2011EPXX0084.
Testo completoArratia, Cristobal. "Mécanismes d'instabilité non-modaux dans les écoulements cisaillés avec et sans stratification en densité". Phd thesis, Ecole Polytechnique X, 2011. http://pastel.archives-ouvertes.fr/pastel-00672072.
Testo completoGrégoire, Olivier. "Modèle multiéchelle pour les écoulements turbulents et compressibles en présence de forts gradients de densité". Aix-Marseille 2, 1997. http://www.theses.fr/1997AIX22079.
Testo completoLaroche, Hugues. "Simulations numériques directes des écoulements d'un fluide tournant, ou stratifié en densité de façon stable, autour d'un obstacle". Grenoble 1, 1991. http://www.theses.fr/1991GRE10044.
Testo completoPolania, Oscar. "Polydispersity in Granular Flows : Exploring Effects in Dry and Submerged Environments". Electronic Thesis or Diss., Université de Montpellier (2022-....), 2023. http://www.theses.fr/2023UMONS061.
Testo completoGranular flows are complex and evolving systems where grains interact with each other and, if immersed, interact with an ambient fluid. These flows occur at different velocities and state variables, and could behave like solids, liquids or even gases. Granular flows are involved in many circumstances and scales, from geophysical mass flows such as landslides, debris flows, pyroclastic flows, and snow avalanches, to industrial processes like pharmaceuticals, food production, and construction. For simplicity, granular flows are commonly studied with a monodisperse distribution of grains (e.i., grains with nearly the same size); however, among these flows, the grains involved in these processes have different sizes, a property termed as polydispersity.This thesis focuses on the study of granular flows and, specifically, on the influence that polydispersity has on granular flows. We explore the effect that polydispersity has on steady flows with low inertia, where granular materials can be considered as solids, and high inertia, where granular materials can be considered as fluids. Additionally, we study dry and immersed granular flows in the granular column collapse configuration, that is a benchmark geometry for studying granular flows with phases of acceleration and deceleration.We study granular flows by means of experimental and numerical methods. The numerical simulations of granular flows are done with discrete element methods (DEM) and, for immersed cases, we use a coupled finite element method (FEM) with DEM. We also conduct a controlled experimental campaign in the triaxial test apparatus where we systematically vary the polydispersity level, aiming to study the strength of polydisperse granular materials in quasi-static conditions. Furthermore, we do the physical modelling of immersed and dry gravity-driven flows in the granular column collapse configuration. Our goal is to explore the influence of polydispersity on granular flows and to identify the influence of the basal fluid pressure on the mobility of granular flows. For the experiments, we use spherical beads, exclusively focusing on the effect that size polydispersity has on granular flows.Our results allow us to conclude that the shear strength of granular materials is independent of the size polydispersity from a quasistatic condition to a condition of high inertia. For very large inertial conditions, the shear strength of polydisperse materials is smaller compared to that of monodisperse materials. We found that this difference arises from distinct variations in geometric and force parameters belonging to the contact and force network. Additionally, we provide evidence that immersed granular flows are strongly influenced by an increase in polydispersity levels. We show that the difference between monodisperse and polydisperse materials essentially arises from different evolutions of the basal fluid pressure. The initiation of polydisperse flows is delayed compared to monodisperse flows, due to a sustained negative fluid pressure change with large amplitude. Then, as the flow deposits, polydisperse systems reach longer runout distances due to the generation of exceeding pore pressure that lasts longer than the exceeding pore pressure provoked by monodisperse systems. Finally, we propose a model that links flow kinetic energy with the mobility of granular flows, which applies to different polydispersity levels, and has been successfully validated through simulations and experiments. The results of this thesis provide new insights into the role of polydispersity in both dry and immersed granular flows
Talbot, Benoit. "Mélange et dynamique de la turbulence en écoulements libres à viscosité variable". Phd thesis, INSA de Rouen, 2009. http://tel.archives-ouvertes.fr/tel-00557891.
Testo completoKelfoun, Karim. "Processus de croissance et de déstabilisation des dômes de lave du volcan Merapi (Java centrale, Indonésie) : Modélisations numériques des dômes, dynamique des écoulements pyroclastiques associés et surveillance par stéréo-photogrammétrie". Clermont-Ferrand 2, 1999. http://www.theses.fr/1999CLF21132.
Testo completoTalbot, Benoît. "Mélange et dynamique de la turbulence en écoulements libres à viscosité variable". Thesis, Rouen, INSA, 2009. http://www.theses.fr/2009ISAM0006/document.
Testo completoThis work is devoted to the study of the undeveloped turbulence in heterogeneus gaseus mixtures, using experimental tools (Hot-wire Anemometry, Rayleigh Light Scattering, Laser Doppler Velocimetry) and analytical methods (variable-viscosity Navier Stokes equations). A new technique combining HWA and RLS is first adapted to reliabily measure the fluctuating velocity and concentration fields in variable-viscosity flows (herein, a propane-air mixture). A variable-viscosity round jet (propane emerging into an air-neon mixture) is characterized and compared with a turbulent air jet discharging into still air, at the same initial jet momentum. An analytical work is further performed with a particular focus on the jet axis, based on the Navier-Stokes equations including variable viscosity to support the experiments. It is shown that the kinetic energy dissipation rate is enhanced by several additional terms, particularly involving 'viscosity-velocity' correlations
Touazi, Ouardia. "Étude numérique d'un schéma de volumes finis sur des maillages non structurés pour des écoulements de fluide visqueux à densité constante ou faiblement variable". Université de Marne-la-Vallée, 2006. http://www.theses.fr/2006MARN0324.
Testo completoWeit, Anne. "Etude expérimentale de la concentration de particules solides dans les écoulements volcaniques biphasés turbulents". Thesis, Université Clermont Auvergne (2017-2020), 2018. http://www.theses.fr/2018CLFAC060.
Testo completoMixtures consisting of gas and particles can be found in various geophysical environments. Hot mixtures are generated by explosive volcanic eruptions and include conduit flows, jets and buoyant plumes, and pyroclastic density currents. The particle concentration within these volcanic mixtures can vary highly, from high concentrations (>50 vol. %) in dense fluidized flows to very low concentrations in dilute suspensions in which the particles are suspended by the turbulent gas phase. A concentration limit of less than ~1 vol. % in dilute suspensions was suggested by recent studies, as higher concentrations would require excessive turbulent kinetic energy. The main objective of this thesis was to investigate experimentally the behavior of a turbulent air flow in a pipe with increasing particle concentrations, for different Reynolds numbers and using different types of particles. The Reynolds numbers of the gas-particle mixtures in the experiments were up to ~106. A first set of experiments was conducted with glass beads of varying sizes from 75-80 μm up to 2 mm, for eight particle size ranges in total. Above a bulk concentration threshold of ~0.5-3 vol. %, which increased with the Reynolds number, the flow behavior changed from a homogeneous suspension of particles (below the maximum concentration) to a separation into a dense basal part and an upper dilute part carrying the maximum concentration of particles. This concentration threshold was detected with pressure measurements and a method that involved a ball of a slightly lower density than the bulk density of the particles, which could thus float over the dense basal part, if present. High-speed videos revealed that the occurrence of the maximum particle concentration coincided with the emergence of particle clusters in the dilute turbulent part. In a second part of the thesis, the experiments were repeated for five ceramic particle size ranges and they yielded the same general behavior as for the glass beads. For both types of particles, a maximum concentration could be detected for almost all particle size ranges and showed an increase with the mixture Reynolds number to the power 1/5 (glass beads) or 0.4 (ceramic beads). Considering the particle Reynolds number the maximum particle concentration then increase to the power 1/6 for both ceramic and glass particles. These results give new insights about the structure of volcanic gas-particle mixtures and they also provide constraints for input and output data of numerical simulations and for geophysical observations
Bel, Hassan Moulay. "Application des modèles de turbulence aux équations de transport des tensions de Reynolds au calcul des écoulements à densité variable et à forte composante de rotation". Ecully, Ecole centrale de Lyon, 1993. http://www.theses.fr/1993ECDL0003.
Testo completoBoukellal, Ghalia. "Détermination des paramètres non linéaires constitutifs de lois de comportement viscoélastique par mesures de champ dans des écoulements complexes". Phd thesis, École Nationale Supérieure des Mines de Paris, 2010. http://pastel.archives-ouvertes.fr/pastel-00566767.
Testo completoRebei, Mohamed Amine. "Développement de méthode d'optimisation topologique adaptée aux écoulements en régime turbulent : application au cas des échangeurs de chaleur". Thesis, Paris Sciences et Lettres (ComUE), 2019. http://www.theses.fr/2019PSLEM061.
Testo completoTopology optimization is used in fluid mechanics in order to compute optimal geometry for fluid conduits with respect to objective functions and constraints while allowing more freedom than shape optimization methods. This work offers a solution for such an optimization problem while taking into account turbulent flow and heat transfer. To do so, the turbulence model is modified. The topology problem is implemented as follows. The density method is used to define the solid/fluid distribution. The turbulence and energy model is applied with modifications to take into account the density. The gradient is defined using the continuous adjoint method which creates a set of adjoint equations to solve. At the end, the optimized solution allows us to combine turbulence with convective and conductive heat transfer. We present and apply the method. An analysis of the results is then provided in order to explain the effects of the new formulation on the optimized geometries
Vu, Do Huy Cuong. "Méthodes numériques pour les écoulements et le transport en milieu poreux". Thesis, Paris 11, 2014. http://www.theses.fr/2014PA112348/document.
Testo completoThis thesis bears on the modelling of groundwater flow and transport in porous media; we perform numerical simulations by means of finite volume methods and prove convergence results. In Chapter 1, we first apply a semi-implicit standard finite volume method and then the generalized finite volume method SUSHI for the numerical simulation of density driven flows in porous media; we solve a nonlinear convection-diffusion parabolic equation for the concentration coupled with an elliptic equation for the pressure. We apply the standard finite volume method to compute the solutions of a problem involving a rotating interface between salt and fresh water and of Henry's problem. We then apply the SUSHI scheme to the same problems as well as to a three dimensional saltpool problem. We use adaptive meshes, based upon square volume elements in space dimension two and cubic volume elements in space dimension three. In Chapter 2, we apply the generalized finite volume method SUSHI to the discretization of Richards equation, an elliptic-parabolic equation modeling groundwater flow, where the diffusion term can be anisotropic and heterogeneous. This class of locally conservative methods can be applied to a wide range of unstructured possibly non-matching polyhedral meshes in arbitrary space dimension. As is needed for Richards equation, the time discretization is fully implicit. We obtain a convergence result based upon a priori estimates and the application of the Fréchet-Kolmogorov compactness theorem. We implement the scheme and present numerical tests. In Chapter 3, we study a gradient scheme for the Signorini problem. Gradient schemes are nonconforming methods written in discrete variational formulation which are based on independent approximations of the functions and the gradients. We prove the existence and uniqueness of the discrete solution as well as its convergence to the weak solution of the Signorini problem. Finally we introduce a numerical scheme based upon the SUSHI discretization and present numerical results. In Chapter 4, we apply a semi-implicit scheme in time together with a generalized finite volume method for the numerical solution of density driven flows in porous media; it comes to solve nonlinear convection-diffusion parabolic equations for the solute and temperature transport as well as for the pressure. We compute the solutions for a specific problem which describes the advance of a warm fresh water front coupled to heat transfer in a confined aquifer which is initially charged with cold salt water. We use adaptive meshes, based upon square volume elements in space dimension two
Bertho, Yann. "Dynamique d'écoulements gaz-particules en conduite verticale". Palaiseau, Ecole polytechnique, 2003. https://pastel.archives-ouvertes.fr/tel-00171722.
Testo completoRodrigues, Luis Miguel. "Comportement en temps long des fluides visqueux bidimensionnels". Phd thesis, Grenoble 1, 2007. http://www.theses.fr/2007GRE10319.
Testo completoThis report investigates the long-time asymptotic behaviour of viscous bidimensional fluids, either homogeneous or weakly-inhomogeneous. Regarding homogeneous fluids, Thierry Gallay and C. Eugene Wayne have shown the major role of a family of self-similar solutions, the Oseen vortices, which attracts any solution of the Navier-Stokes equation with a finite measure as initial vorticity and non-zero circulation. Their result is non-explicit and the first task of this report is to make it explicit, getting this way a bound for the time-life of bidimensional turbulence. Then is shown the asymptotic stability of the Oseen vortices as density-dependent fluids, which also enables one to recover the result of Gallay and Wayne for slow weakly-inhomogeneous incompressible fluids. At last, it is proved that slow weakly-inhomogeneous compressible fluids, with zero circulation, behave asymptotically mainly as homogeneous fluids
Rodrigues, Luis Miguel. "Comportement en temps long des fluides visqueux bidimensionnels". Phd thesis, Université Joseph Fourier (Grenoble), 2007. http://tel.archives-ouvertes.fr/tel-00200818.
Testo completoFournier, Élise. "Modélisation et simulation des écoulements réactifs turbulents : validation et évaluation de la résolution de l'équation de transport de la fonction de densité de probalilité jointe de la composition (PDF) dans un réacteur de précipitation de type Taylor-Couette". Vandoeuvre-les-Nancy, INPL, 2002. http://www.theses.fr/2002INPL068N.
Testo completoNumerical simulation of industrial reactor promises to be a precious tool that could help in prospecting new mixing technologies, in scaling-up and optimising reactors. Fast reactions, such as polymerisation and precipitations, are difficult to handle when mixing processes and reactions strongly interacts. This work aims at evaluating and validating reactive flow simulation for chemical engineering reactors using a special method which is able to take into account mixing reaction interaction for fast reactions. This method is based on solving Navier-Stokes equations in turbulent flow and then on solving the transport equation of the jointtcomposition Probability Density Function (PDF) with a Monte-Carlo solver. The validation is carried-out on a semi-batch Taylor-Couette precipitator. Each calculation step is validated. Finally, comparison between experiment and simulation proved that the method we chose has to be handle with special cares. Calculation has to be validated and 2 model constants have to be fitted and chemical kinetics known quite precisely. Besides, Monte-Carlo algorithm produces numerical noise which can highly affect reliability of the results. The calculation of the specific studied reactor didn'nt give completely good agreement with experiments. Nevertheless, the discrepancy is assigned not to the numerical approach but to the fact that the studied reactive flow is rather tricky to be calculated
Viciconte, Giovanni. "Turbulent mixing driven by variable density and transport coefficients effects". Thesis, Lyon, 2019. http://www.theses.fr/2019LYSEC035.
Testo completoThis thesis is dedicated to the study of turbulent mixing in flows with variable density and non-uniform transport coefficients. We use a new direct numerical simulation (DNS) code based on a two-dimensional domain decomposition, capable of taking into account variable density and diffusive contributions. At first, we consider the case of turbulence in weakly-coupled plasmas under isotropic compression, which can experience a sudden dissipation of kinetic energy due to the growth of the viscosity coefficient due to temperature increase. In this case, in addition to DNS we use a spectral model based on the Eddy-Damped Quasi-Normal Markovian closure. We evidence the sensitivity of the flow dynamics to initial conditions for homogeneous isotropic turbulence and an inhomogeneous spherical turbulent layer. In the latter case, we find, also, the first hint of a sudden diffusion effect. The importance of initial conditions is also shown in the study of the variable density unstably stratified homogeneous turbulence. If the initial density contrasts are sufficiently strong, the large scales of the flow are modified with the consequent modification of the self-similar scaling laws. Finally, we consider an idealized configuration of inertial confinement fusion implosion, with both variable density and transport coefficients effects. During the compression, we evidence the competition between the plasma molecular diffusion, which is enhanced by the temperature increase, and the turbulent diffusion, which on the contrary decreases due to the increased viscous dissipation. In the last phase of the implosion, we highlight a sudden diffusion process, where compressed spherical mixing layers are quickly diffused
Boittin, Léa. "Modeling, analysis and simulation of two geophysical flows. Sediment transport and variable density flows". Thesis, Sorbonne université, 2019. http://www.theses.fr/2019SORUS033.
Testo completoThe present thesis deals with the modeling and numerical simulation of complex geophysical flows. Two processes are studied: sediment transport, and variable density flows. For both flows, the approach is the same. In each case, a reduced vertically-averaged model is derived from the 3D Navier-Stokes equations by making a specific asymptotic analysis. The models verify stability properties. Attention is paid to preserving these properties at the discrete level, in particular the entropy stability. The behavior of both models is illustrated numerically. Concerning the sediment transport model, the sediment layer is first studied alone. Then, a coupled sediment-water model is presented and simulated. The influence of a viscosity term in the model for the sediment layer is investigated. Due to this viscosity term, the sediment flux is non-local. A transport threshold is added to the model. The water layer is modeled by the Shallow Water equations. Adding some non-locality to the model allows to simulate dune growth and propagation. In the variable density flow model, the density is a function of one or several tracers such as temperature and salinity. The model derivation consists in removing the dependence of the density on the pressure. A layer-averaged formulation of the model is proposed, which is subsequently used to propose a numerical discretization. The numerical simulations emphasize the differences between this model and a model relying on the classical Boussinesq approximation
Gisclon, Marguerite. "Effet des conditions aux limites et analyse multi-échelles en mécanique des fluides, chromatographie et électromagnétisme". Habilitation à diriger des recherches, 2007. http://tel.archives-ouvertes.fr/tel-00259592.
Testo completoIl s'inscrit dans l'étude des problèmes hyperboliques, des problème mixtes et des équations cinétiques. Les domaines d'application sont la mécanique des fluides ou du solide, la propagation de composants chimiques, l'électromagnétisme, l'optique.
Mon activité concerne d'abord la modélisation de phénomènes physiques ou chimiques sous forme d'équations aux dérivées partielles non linéaires telles que les équations de Bloch, Korteweg, Navier-Stokes, Saint-Venant, puis vient l'étude mathématique de ces équations à travers les
problèmes d'existence, d'unicité, de régularité avec éventuellement la mise au point de méthodes numériques de résolution.
Ce document est divisé en une introduction générale et trois chapitres qui concernent respectivement les systèmes hyperboliques avec conditions aux limites et la chromatographie, les problèmes d'analyse asymptotique et enfin les méthodes cinétiques.
Dans chaque partie, un historique et une présentation des différents résultats mathématiques sont faits et quelques problèmes ouverts sont donnés.