32 research outputs found
Settling of finite-size particles in isotropically forced, homogeneous turbulence: interface-resolved simulations
No full textWe have simulated the gravity-induced settling of finite-size particles in a turbulent background flow which is forced in a statistically-stationary fashion. The simulations are accurately resolving the solid-fluid interface with the aid of an immersed boundary technique [1]. The parameters of the simulation are (apart from background turbulence) identical to those of reference [2], where particle clustering was observed at a Galileo number of 178 and a solid volume fraction of 0.005. In the present case, it is found that a relative turbulence intensity of 0.24 leads to the disappearance of the clusters; as a consequence, the increase in average particle settling velocity found in [2] also vanishes. [1] M. Uhlmann. An immersed boundary method with direct forcing for the simulation of particulate flows. J. Comput. Phys., 209(2):448–476, 2005. [2] M. Uhlmann and T. Doychev. Sedimentation of a dilute suspension of rigid spheres at intermediate Galileo numbers: the effect of clustering upon the particle motion. J. Fluid Mech., 752:310–348, 2014
Étude numérique de la réduction de traînée par injection de bulles en écoulement de Taylor-Couette
No full textLa thèse porte sur l'étude de la réduction de traînée par injection de bulles. La réduction de traînée présente un intérêt pour les applications navales puisqu'elle est issue d'une modification des structures cohérentes qui contribuent le plus à la résistance à l'avancement. Le but de cette étude est d'analyser les mécanismes à l'origine de cette diminution du frottement pariétal. L'approche utilisée dans le cadre de cette étude est numérique, elle emploie le code JADIM par une approche Euler-Lagrange : la phase continue est simulée par Simulation Numérique Directe et la phase dispersée est simulée en suivant individuellement chaque bulle. La configuration retenue dans le cadre de cette étude est celle de l'écoulement de Taylor-Couette (écoulement compris entre deux cylindres en rotation). La première partie de la thèse vise à adapter l'outil numérique employé, afin de pouvoir prendre en compte le retour de la phase dispersée via des termes de forçage dans les équations bilan de matière et de quantité de mouvement. La deuxième partie de la thèse vise à étudier l'écoulement porteur en configuration monophasique, afin de disposer d'une référence sur l'écoulement non perturbé. La troisième partie de la thèse a pour objectif d'étudier la dispersion passive des bulles dans le système, afin d'analyser les mécanismes de migrations mis en jeu. Enfin la dernière partie de la thèse vise à étudier l'influence des bulles sur l'écoulement porteur en analysant l'effet de certains paramètres, notamment le taux de vide et la flottabilité
Numerical study of bubbly drag reduction in Taylor-Couette flow
No full textLa thèse porte sur l'étude de la réduction de traînée par injection de bulles. La réduction de traînée présente un intérêt pour les applications navales puisqu'elle est issue d'une modification des structures cohérentes qui contribuent le plus à la résistance à l'avancement. Le but de cette étude est d'analyser les mécanismes à l'origine de cette diminution du frottement pariétal. L'approche utilisée dans le cadre de cette étude est numérique, elle emploie le code JADIM par une approche Euler-Lagrange : la phase continue est simulée par Simulation Numérique Directe et la phase dispersée est simulée en suivant individuellement chaque bulle. La configuration retenue dans le cadre de cette étude est celle de l'écoulement de Taylor-Couette (écoulement compris entre deux cylindres en rotation). La première partie de la thèse vise à adapter l'outil numérique employé, afin de pouvoir prendre en compte le retour de la phase dispersée via des termes de forçage dans les équations bilan de matière et de quantité de mouvement. La deuxième partie de la thèse vise à étudier l'écoulement porteur en configuration monophasique, afin de disposer d'une référence sur l'écoulement non perturbé. La troisième partie de la thèse a pour objectif d'étudier la dispersion passive des bulles dans le système, afin d'analyser les mécanismes de migrations mis en jeu. Enfin la dernière partie de la thèse vise à étudier l'influence des bulles sur l'écoulement porteur en analysant l'effet de certains paramètres, notamment le taux de vide et la flottabilité.The study deals with drag reduction induced by bubble injection, its application concerns naval transport. The aim of the study is to shed more light on mechanisms that are involved in this wall friction reduction. The study is based on a numerical approach, and use the JADIM code with an Euler-Lagrange approach: the continuous phase is solved by Direct Numerical Simulation, and the disperse phase by a tracking of each bubble. Within the framework of this study we consider the Taylor-Couette flow configuration (flow between two concentric cylinders in rotation). The first part of the study deals with the modification of the numerical tool, in order to take into account the influence of the disperse phase on the continuous one through forcing terms in the mass and momentum balance equations. The aim of the second part is to study de Taylor-Couette flow in its monophasic configuration, for the purpose of providing a reference of the undisturbed flow. The third part deals with the passive dispersion of bubble in Taylor-Couette flow, in order to analyze the migration mechanisms involved. And the aim of the last part is to study the effects of the disperse phase on the continuous one, by analyzing the influence of bubbly phase parameters (like void fraction and buoyancy)
Multi-scale simulation of melting of a phase change material in a moving capsule
No full textNational audienceIn recent years, there has been growing interest in the development of reversible thermal storage systems for urban development.One of these systems consists of a fluid flow carrying microcapsules containing a phase-change material (mPCM) circulating in a porous layer integrated into the urban structure.To understand the physical mechanisms at work within this fluid and the solid that surrounds it, which is still incomplete to date, we need to study the various scales involved, particularly that of the pore, which is often difficult to measure experimentally and the subject of much numerical work.The present work aims to investigate the effect of the motion of the mPCM on the the melting process occurring in its inside, focuses on the rotational motion and deals with two scales: the capsule scale and the pore scale.In order to determine the order of magnitude of the angular velocity of small particles moving in a porous medium, a pore-scale numerical study is conducted, using a coupling between lattice Boltzmann method and discrete elements method.The resulting velocities are used in an other numerical study conducted at capsule scale in which an enthalpy-based model provides a realistic solution for the phase change.First the mathematical models at each one of the two scales is exposed in the first section. The numerical method are then validated through three cases.Subsequently, the results are examined in order to understand the impact of the Péclet number on the phase change kinetics. The effect of the angular velocity of the shell with respect to its content is evaluated in particular
Melting of a phase change material in a moving capsule: A parametric study
No full textInternational audienceIn recent years, there has been growing interest in the development of reversible thermal storage systems for urban development. The present work deals with one of these systems, which consists of a fluid flow carrying microcapsules containing a phase-change material (mPCM, cf. [1]) circulating in a porous layer integrated into the urban structure. It focuses on the the melting process occurring in the fluid-driven encapsulated phase changematerial.The fluid equations inside and outside the capsule are solved using the lattice-Boltzmann method on a two-dimensional simulation. A collision operator based on the partial saturated method ensures the boundary conditions at the interfaces between fluid and solid phases. A second set of distribution functions on the lattice is used to solve heat transfer, while an enthalpy-based model provides a realistic solution for the phase change, by means of an enhanced multi-relaxation-time collision operator.The results are detailed and investigated in order to understand the impact of several parameters (Reynolds number of the fluid, Stefan number of the phase-change material) on the phase change kinetics. The effect of the angular velocity of the shell with respect to its content is evaluated in particular
Wave Guide Imaging through Time Domain Topological Energy
No full textAbstractTime Domain Topological Energy (TDTE), uses a measure of the reflected ultrasonic field on an array of transducers placed on the boundary of the imaged medium. Two numerical determinations (direct and adjoint problems) of the acoustical field inside a reference medium are then necessary to obtain the image by computing the topological energy. This technique comes from the field of shape optimisation and mathematical developments for Non Destructive Testing and have shown close links with Time Reversal (TR) concepts. TR mirrors have been employed for various applications in a wide number of situations including wave guides (WG) where very good refocalisation performances have been obtained with a reduced number of transducers instead of an array. Moreover recent works have enlighten that the reverberation properties of a WG allow to re-focalise using TR with only one transducer. For TDTE imaging we choose to model a single transducer placed at one end of a wave guide. The boundaries of the WG create virtual sources that can be understood as a virtual array of transducers. Results obtained numerically for imaging using both TDTE and one transducer in a wave guide with increasing complexity : a hard spherical object and a set of three identical objects placed at the angles of an equilateral triangle are presented and preliminary experimental results are discussed
Pore-scale study of the dynamics of a suspension of solid particles in porous media
No full textInternational audienc
Numerical simulation of bubble dispersion in turbulent Taylor-Couette flow
No full textWe investigate bubble dispersion in turbulent Taylor-Couette flow. The aim of this study is to describe the main mechanisms yielding preferential bubble accumulation in near-wall structures of the flow. We first proceed to direct numerical simulation of Taylor-Couette flows for three different geometrical configurations (three radius ratios η = R 1/R 2: η = 0.5, η = 0.72, and η = 0.91 with the outer cylinder at rest) and Reynolds numbers corresponding to turbulent regime ranging from 3000 to 8000. The statistics of the flow are discussed using two different averaging procedures that permit to characterize the mean azimuthal velocity, the Taylor vortices contribution and the small-scale turbulent fluctuations. The simulations are compared and validated with experimental and numerical data from literature. The second part of this study is devoted to bubble dispersion. Bubble accumulation is analyzed by comparing the dispersion obtained with the full turbulent flow field to bubble dispersion occurring at lower Reynolds numbers in previous works. Several patterns of preferential accumulation of bubbles have been observed depending on bubble size and the effect of gravity. For the smaller size considered, bubbles disperse homogeneously throughout the gap, while for the larger size they accumulate along the inner wall for the large gap width (η = 0.5). Varying the intensity of buoyancy yields complex evolution of the bubble spatial distribution. For low gravity effect, bubble entrapment is strong leading to accumulation along the inner wall in outflow regions (streaks of low wall shear stress). When buoyancy effect dominates on vortex trapping, bubbles rise through the vortices, while spiral patterns stretched along the inner cylinder are clearly identified. Force balance is analyzed to identify dominating forces leading to this accumulation and accumulation patterns are compared with previous experiments
Melting of a phase change material in a moving capsule: A parametric study
No full textInternational audienceIn recent years, there has been growing interest in the development of reversible thermal storage systems for urban development. The present work deals with one of these systems, which consists of a fluid flow carrying microcapsules containing a phase-change material (mPCM, cf. [1]) circulating in a porous layer integrated into the urban structure. It focuses on the the melting process occurring in the fluid-driven encapsulated phase changematerial.The fluid equations inside and outside the capsule are solved using the lattice-Boltzmann method on a two-dimensional simulation. A collision operator based on the partial saturated method ensures the boundary conditions at the interfaces between fluid and solid phases. A second set of distribution functions on the lattice is used to solve heat transfer, while an enthalpy-based model provides a realistic solution for the phase change, by means of an enhanced multi-relaxation-time collision operator.The results are detailed and investigated in order to understand the impact of several parameters (Reynolds number of the fluid, Stefan number of the phase-change material) on the phase change kinetics. The effect of the angular velocity of the shell with respect to its content is evaluated in particular
Multi-scale simulation of melting of a phase change material in a moving capsule
No full textNational audienceIn recent years, there has been growing interest in the development of reversible thermal storage systems for urban development.One of these systems consists of a fluid flow carrying microcapsules containing a phase-change material (mPCM) circulating in a porous layer integrated into the urban structure.To understand the physical mechanisms at work within this fluid and the solid that surrounds it, which is still incomplete to date, we need to study the various scales involved, particularly that of the pore, which is often difficult to measure experimentally and the subject of much numerical work.The present work aims to investigate the effect of the motion of the mPCM on the the melting process occurring in its inside, focuses on the rotational motion and deals with two scales: the capsule scale and the pore scale.In order to determine the order of magnitude of the angular velocity of small particles moving in a porous medium, a pore-scale numerical study is conducted, using a coupling between lattice Boltzmann method and discrete elements method.The resulting velocities are used in an other numerical study conducted at capsule scale in which an enthalpy-based model provides a realistic solution for the phase change.First the mathematical models at each one of the two scales is exposed in the first section. The numerical method are then validated through three cases.Subsequently, the results are examined in order to understand the impact of the Péclet number on the phase change kinetics. The effect of the angular velocity of the shell with respect to its content is evaluated in particular
