In this study, a comprehensive approach including mathematical, numerical and experimental study has been taken in order to develop new models for describing free surface flow behavior in porous media. The study suggested that modeling free-surface flow in porous media is possible using a single equation capable of showing proper transition between inertial and classical Darcian flow, based on the similarity distribution functions of depth and velocity. The developed integral model inherits both the flow regimes as depicted in the analysis. For both laminar and turbulent flows through porous media, the integral models give satisfactory results. Also the proposed algorithm for numerical simulation is capable of solving various problems of free-surface flow through porous media. This study adds a new dimension to fluid flow in porous media by replacing Darcy's equation with new models that are capable of representing both Darcy and non-Darcy flow behaviors. These are new nonlinear ordinary differential equations inherited both the flow regimes investigated. Integral formulations for unsteady depth distribution, velocity and front speed under constant water level and constant flux discharge inlet conditions have been developed based on similarity law. The formulations presented provide additional analytical insight about the intrusion dynamics. It is pointed out that, based on the self-similarity analysis, the temporal intrusion processes can be categorized into the inertia-pressure (IP) and the pressure-drag (PD) regimes. The early inertia-pressure regime is followed by the pressure-drag regime. In addition, the integral models proposed can be successfully used for the solution of a host of other nonlinear problems that admit self-similarity. The analytical and numerical solutions for constant inlet water level condition are verified with experimental observations. The unsteady distributions of flow depth, inflow velocity and front speeds are compared for various porous media characterized by its corresponding porosity and permeability. Analyses indicate that the integral models clearly represent the nonlinear flow behavior in porous media both in laminar and turbulent flow conditions. The integral model results are in agreement with those obtained by similarity solution for the temporal change of velocity, depth at inlet and front positions. The thesis also presents a computational fluid dynamics (CFD) model developed for the analysis of unsteady free-surface flows through porous media. Vertical two-dimensional numerical simulations are carried out for the free-surface flow inside the porous media governed by a set of Navier-Stokes equations extended for porous media flow. This model includes the convective and local inertia terms along with viscous diffusion term and resistance term comprising Darcy's linear resistance and Forchheimer's inertial resistance terms. The Finite volume method is applied using constrained interpolated propagation (CIP) method and highly simplified marker and cell (HSMAC) type pressure solver for the numerical solution. The evolution of moving free surface is governed by volume of fluid (VOF) method, adapted for the flow through porous media. To prevent the spurious oscillation and generate diffusion-free sharp interface, a third order monotone upstream-centered schemes for conservation laws (MUSCL) type total variation diminishing (TVD) schemes is used to solve the VOF convection equation. The power law derivation and validation for the general flux inflow condition are made for a channel having a backward facing step. The result of theoretical analysis is compared with that of the numerical simulation and it shows a good agreement. The model can be a tool for the proposition of some empirical flow relationships using multivariate correlation. In the case of rapid vertical infiltration of water through a vertical column filled with porous media, a number of experiments and analytical investigations are carried out to see the effect of acceleration in the intrusion process. It is concluded that the conventional infiltration models like Green-Ampts infiltration model cannot account for the acceleration effect in the case of high velocity flow. It is revealed that it takes certain time for intruding water to be accelerated to its peak velocity before decreasing to almost constant velocity. The investigations are made for two different cases: constant water level and variable water level above the porous media. For porous media having low permeability, the effect of acceleration was not so significant. In the case of dam break flow over horizontal porous strata, the model is applied to a complicated domain regarding both geometry and flow boundary conditions. Single set of governing equation is implemented to simulate the complex phenomenon. The model shows its capability in simulating the flow where interface between pressurized and open channel flow moves forward. The vertical acceleration has a significant effect on the rapid vertical infiltration which the shallow water equations cannot account for. In particular, it is shown that vertical two dimensional numerical solution that couples the fluid and solid systems simultaneously at macroscopic scale are feasible and extremely beneficial, shedding a new light into the phenomena unavailable otherwise. It is also found that the proposed numerical model can be used for the determination of storm water storage in porous sub-base in a typical road section. The capability of the model is assessed by using the unsteady inflow condition so as to simulate the condition during high precipitation. The model could be a promising tool for planners and decision makers for effective drainage calculations to mitigate urban flood. The model successfully simulates the free surface flow in the bulk fluid as well as in the porous region. The velocities and stresses are assumed to be continuous at the interface of free and porous media so that a single set of governing equations could be solved. The robustness of the model is demonstrated by the capability of the numerical approach proposed in this thesis. / Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第14916号 / 工博第3143号 / 新制||工||1471(附属図書館) / 27354 / UT51-2009-M830 / 京都大学大学院工学研究科都市社会工学専攻 / (主査)教授 細田 尚, 教授 戸田 圭一, 准教授 岸田 潔 / 学位規則第4条第1項該当

La formation de bouchon est un problème récurrent et presque inévitable lors de l'écoulement de solutions diluées dans des milieux poreux. Actuellement, on ne sait pas comment, à partir du processus initial de déposition de particules à la paroi, ces dernières s'accumulent dans le pore et finissent par le boucher. L'idée générale de ce travail est d'étudier la dynamique de formation de bouchon lors l'écoulement de particules colloïdales au sein de matériaux poreux modèles (canaux microfluidiques). Nous décrivons dans un premier temps, les différents phénomènes physiques impliqués dans la capture de particules et dans l'agrégation colloïdale. Nous faisons également une brève présentation des différentes techniques d'imagerie utilisées dans ce travail et des méthodes de préparation des solutions colloïdales ainsi que des dispositifs microfluidiques. Le troisième chapitre est consacré à l'étude du processus de colmatage en situation de fort confinement (2d). Nous avons identifié deux régimes de colmatage (régime de ''ligne'' et ''d'invasion''). Nous avons ensuite déterminé les processus de capture de particules à l'origine de ces deux régimes, à l'échelle de la particule. Nous avons montré que le processus de colmatage correspond à un phénomène d'auto-filtration. Alors que les premières particules sont capturées de manière « directe » par les parois du pore, la déposition de toutes les suivantes résulte systématiquement d'une interaction avec ces dernières. Finalement, nous avons abordé le colmatage de pore 3d, dont la hauteur est égale à la largeur du pore. Nous avons fourni une description détaillée de l'ensemble du processus de colmatage, à l'échelle du pore et de la particule. Nous avons déterminé les conditions d'adhésion des premières particules à la paroi du pore, les propriétés de croissance des agrégats, ainsi que la manière dont ils se connectent pour obstruer le pore. Nous avons montré que cette dynamique de formation conduit à une structure finale de bouchon très ténue. / Clog formation is a recurring and almost inevitable issue when dilute solution of particles flows in porous media. Currently, we do not know how, from the initial process of particle deposition on the pore wall, particles accumulate in the pore leading to its blocking. The main idea of this work is to study the dynamics of the clog formation, when colloidal particles flow through a single pore (microfluidics channels). In a first part, we describe the various physical phenomenon involved in the particle capture and the colloidal aggregation. We also describe briefly the imaging techniques used in this work as well as the colloidal solution and micro-fluidics chips preparation. The third chapter is devoted to the study of the clogging process in high confinement (2d). We identified two clogging regimes (“line” and “invasion”). We then studied the underlying capture mechanisms, at the particle scale, related to both clogging regimes. We showed that the blockage process corresponds to a self-filtration process. The first particles are captured “directly” by the pore walls, while the deposition of all the following ones systematically results from hydrodynamic interactions with those first still particles. Finally, we addressed the clogging of a 3d pore, in which the height of the pore is equal to its width. We gave a detailed description of the whole clogging process at the pore and at the particle scale. We provided the conditions for the adhesion of the first particles on the pore walls, the properties of subsequent aggregates growth, and how the aggregates eventually merge in order to block the pore. We showed that this dynamics of formation leads to a very loose clog structure.

La prévision du transport et de la rétention des colloïdes représente un enjeu environnemental majeur car ces particules peuvent entraîner des polluants adsorbés ou bien être des polluants eux-mêmes. Les modèles utilisés actuellement pour prédire le devenir des colloïdes dans les sols sont basés sur des mécanismes déduits des courbes de percée (évolution de la concentration en fonction du volume de pore ou du temps) après injection de particules dans une colonne de milieu poreux. Le but de cette thèse est de compléter cette compréhension avec des mesures internes grâce à l’Imagerie par Résonance Magnétique (IRM).L’IRM permet de mesurer la distribution 1D et/ou 2D de particules super-paramagnétiques le long d’un échantillon pendant une expérience de transport dans un milieu poreux. Le couplage de cette technique avec l'étude des courbes de percée donne une approche globale. Nous avons effectué plusieurs expériences de transport dans des colonnes de milieux poreux de complexité de porosité croissante : billes de verre, sable, agrégats de sol, et sol non perturbé.A partir des expériences de transport dans les milieux poreux modèles saturés, nous avons montré que la dispersion est moins importante que la théorie le prévoit et qu’elle est fortement dépendante des effets d’entrée dans la colonne. Cette dépendance est aussi observée pour les mécanismes d’adsorption. Les expériences dans les agrégats de sol ont montré une forte adsorption et un relargage constant, dépendants de la vitesse d’injection. Finalement, des expériences de pluie dans des colonnes de sol non perturbé insaturé ont permis de suivre l’évolution des teneurs en eau et en particules dans le temps / The ability to predict transport and retention of colloidal particles is a major environmental concern as such particles can carry adsorbed pollutants towards the groundwater or be pollutants themselves. The models currently used to predict the fate of colloids in soils are based on mechanisms inferred from breakthrough curves (evolution of concentration as a function of pore volume or time) after injection of particles into a column of porous media. In this thesis we aim to complement this comprehension with internal measurements by Magnetic Resonance Imaging (MRI).MRI provides 1D and/or 2D distribution of contrast agent particles in time along the sample axis during transport experiment through a porous medium. This technique, together with the study of breakthrough curves gives a global approach. We performed several transport experiment in columns of porous media of increasing complexity: glass beads, sand, soil aggregates, and undisturbed soil.From transport experiments in model porous media we show that dispersion is less important than expected and strongly dependent on entrance effects in the column. This dependence is also observed for adsorption. Experiments in soil aggregates showed a strong adsorption but also a constant release, dependent on the flow rate. Finally, rain experiments in undisturbed sol columns allowed following water content and particles as a function of time

No description available.

The mass and thermal transport in porous media play an important role in many engineering and geological processes. The hydrodynamic and thermal effects are two interesting aspects arising in the research of porous media. This thesis is concerned with numerical investigations of double-diffusive natural convective heat and mass transfer in saturated porous cavities with Soret and Dufour effects. An in-house FORTRAN code, named ALFARHANY, was developed for this study. The Darcy-Brinkman-Forchheimer (generalized) model with the Boussinesq approximation is used to solve the governing equations. In general, for high porosity (more than 0.6), Darcy law is not valid and the effects of inertia and viscosity force should be taken into account. Therefore, the generalized model is extremely suitable in describing all kinds of fluid flow in a porous medium. The numerical model adopted is based on the finite volume approach and the pressure velocity coupling is treated using the SIMPLE/SIMPLER algorithm as well as the alternating direction implicit (ADI) method was employed to solve the energy and species equations. Firstly, the model validation is accomplished through a comparison of the numerical solution with the reliable experimental, analytical/computational studies available in the literature. Additionally, transient conjugate natural convective heat transfer in two-dimensional porous square domain with finite wall thickness is investigated numerically. After that the effect of variable thermal conductivity and porosity investigated numerically for steady conjugate double-diffusive natural convective heat and mass transfer in two-dimensional variable porosity layer sandwiched between two walls. Then the work is extended to include the geometric effects. The results presented for two different studies (square and rectangular cavities) with the effect of inclination angle. Finally, the work is extended to include the Soret and Dufour effects on double-diffusive natural convection heat and mass transfer in a square porous cavity. In general, the results are presented over wide range of non-dimensional parameters including: the modified Rayleigh number (100 ≤ Ra* ≤ 1000), the Darcy number (10-6 ≤ Da ≤ 10-2), the Lewis number (0.1 ≤ Le ≤ 20), the buoyancy ratio (-5 ≤ N ≤ 5), the thermal conductivity ratio (0.1 ≤ Kr ≤ 10), the ratio of wall thickness to its height (0.1 ≤ D ≤ 0.4), the Soret parameter (-5 ≤ Sr ≤ 5), and the Dufour parameter (-2 ≤ Df ≤ 2).

The sample used for evaporative cooling system is Fabric defender 750 with Shelltite finish. From the experimental data and equations we have diffusion coefficient of 20.9 ± 3.71 x 10-6 m2/s for fabric with one layer with 17%-20% fluctuations from the theory, 27.8 ± 4.5 x 10-6 m2/s for fabric with two layers with 6%-14% fluctuations from the theory and 24.9 ± 4.1 x 10-6 m2/s for fabric with three layers with 13%-16% fluctuations from the theory. Since the thickness of the fabric increases so the mass transport rate decreases so the mass transport resistance should be increases. The intrinsic mass resistances of Fabri-1L, Fabri-2L and Fabri-3L are respectively 104 ± 10.2 s/m, 154 ± 23 s/m and 206 ± 26 s/m from the experiment.

Directional solidification has a wide interest due to its importance to the iron and steel industry. Examples of further application can be found in the aerospace industry regarding the manufacture of turbine blades and the semiconductor industry regarding single-crystal growth applications. Solute convection in the solidification process results in channel formation, which has a freckle-like appearance in cross-section and has a critical effect on the mechanical strength of a casting. For a solidification process that occurs via planar solidification from a solid boundary, one may consider the presence of three distinct regions often identified as horizontal layers, i.e. a fluid binary mixture (the melt), the solid layer and a two-phase (fluid-solid) mushy layer, separating the other two. The mushy layer is practically a porous medium consisting of an interconnected solid phase having its voids filled with the melt binary fluid. Channelling in the mushy layer and the creating of freckles are being considered the main reasons for non-homogeneous solidification and production of defects in the resulting solid product. The production of defects adversely affects the mechanical properties of the solid product leading to undesirable constraints on its industrial use. The purpose of this study is to evaluate the effect the vibrations have on the heat transfer during the solidification process as well as on the average density of the solid product and void formation. Experimental as well as theoretical investigations related to the solidification process were undertaken. Two effects that have been observed in previous experimental studies when metals and metal alloys are vibrated during solidification are a decrease in dendritic spacing, which directly affects density, and faster cooling rates and associated solidification times. Because these two effects happen simultaneously during solidification it is challenging to determine the one effect independently from the other. Most previous studies were on metals and metal alloys. In these studies, the one effect, i.e. the decrease in dendritic spacing, might influence the other, i.e. the faster cooling rates, and vice versa. The direct link between vibration and heat transfer has not yet been studied independently. The purpose of this study was to experimentally investigate the effect of vibration only on heat transfer and thus solidification rate. Experiments were conducted on paraffin wax, because it had a clearly defined macroscopic crystal structure consisting of mostly large straight-chain hydrocarbons. The advantage of the large straight-chain hydrocarbons was that the dendritic spacing was not affected by the cooling rate. Experiments were done with paraffin wax inside hollow plastic spheres of 40 mm diameter with 1 mm wall thickness. The paraffin wax was initially in a liquid state at a uniform temperature of 60°C and then submerged into a thermal bath at a uniform constant temperature of 15°C, which was approximately 20°C below the mean solidification temperature of the wax. Experiments were conducted in approximately 300 samples, with and without vibration at frequencies varying from 10 – 300 Hz. The first set of experiments were conducted to determine the solidification times. In the second set of experiments, the mass of wax solidified was determined at discrete time steps, with and without vibration. The results showed that paraffin wax had vibration independent of solid density contrary to other materials, eg. metals and metal alloys. Enhancement of heat transfer resulted in quicker solidification times and possible control over the heat transfer rate. The increase in heat transfer leading to faster solidifcation times was observed to first occur, as frequency increased and then to decrease. Experimental results showed that paraffin wax had vibration independent of solid density contrary to other materials, eg. metals and metal alloys. Enhancement of heat transfer resulted in quicker solidification times and possible control over the heat transfer rate. The increase in heat transfer leading to faster solidifcation times was observed to first occur, as frequency increased and then to decrease. Theoretical results of heat convection in a porous layer heated from below and subject to vibrations are presented by using a truncated spectral method in space. The partial differential equations governing the mass, momentum, heat, and solute transport were tranformed into a set of ordinary differential equations via a truncated modal expansion. Then the resutling equations were solved to identify the variety of regimes, and transitionbetween them, i.e. from steady convection, via periodic and quasi-periodic convection, towards chaotic or weak turbulent convection. The theoretcial results show that the heat convection subject to vibration is generally reduced when compared with the corresponding convection without vibrations. The exception for a certain frequency range shows about a 10% enhancement in the weak turbulent regime of convection, however, a 10% enhancement is still lower than the heat transfer prior to the transition to weak turbulence. Therefore, the heat transfer mechanism can be excluded as the main reason behind the improvement in solidification when vibrations are applied. Both experimental and theoretical results show an enhancement in heat transfer which correlate qualitativally. / Thesis (PhD)--University of Pretoria, 2014. / tm2015 / Mechanical and Aeronautical Engineering / PhD / Unrestricted

Fluid-structure interactions play a central role in an overwhelming number of physical phenomena. All fluid dynamic students are familiar with the common assumption of a "smooth boundary". While this assumption often is enough to provide a high level understanding of the dynamics and physics at hand in practice this is not true.  Much of the detail and the unique phenomena can be traced back to surface properties that deviate from this elementary assumption.  In this work we investigate three problems all motivated by the existence of non-smooth surfaces.   The first paper considers how inhomogeneous surfaces can generate a lift force for lubricated contacts. This work showcases how subtle changes to surface texture or chemistry modeled by a slip length can invoke non-trivial forces. These forces result in striking particle trajectories not possible in the presence of a smooth no-slip wall.    The next work focuses on porous surfaces. Often the geometry of surfaces in nature and industry are complex covering a wide range of length scales. Resolving all the scales of motion arising from fluid interaction with such surfaces are computationally expensive. Effective equations are often applied to reduce the cost of such simulations. The Brinkman equation is one common model choice for free-fluid and porous surface interface. Despite the common application of the Brinkman equation, fundamental questions about what the effective viscosity should be remain open. We compare pore-scale Stokes flow solutions to the Brinkman model for several porous surfaces. This study provides a scaling for the effective viscosity as well as error quantification of the Brinkman model.    Lastly, we investigate how porous surfaces modify a turbulent boundary layer. Streamwise preferential porous surfaces have recently been suggested as a surface modification that has the potential to reduce drag. We compare particle image velocimetry measurements with direct numerical simulations focusing on the near wall features that are modified from the canonical smooth wall case. We present some preliminary turbulent statistics and flow visualizations in the current report.

No description available.

It is evident that the sea ice cycle, from its formation to its melt, is governed by a complex interaction of the ocean, atmosphere and surrounding continents. Once sea water begins to freeze, physical, biological and chemical processes have implications on the evolution of the sea ice morphology [38]. The distinguishing factor between fresh and sea water ice is brine inclusions that get trapped within the ice pores during freezing. Salt inclusions within frozen ice influence the salinity as well as the physical properties of the sea ice [23]. These brine inclusions form part of a dynamic process within the ice characterized by the movement of brine and phase transition which are the foundation of many of its physical properties [23]. Brine removal subsequently begins to occur due to vertical gravity drainage into the underlying ocean water. This study introduces the application of a biphasic model based on the Theory of Porous Media (TPM) which considers a solid phase for the pore structure of the ice matrix as well as a liquid phase for the brine inclusions, respectively. This work explores the use of the TPM framework towards advancing the description and study of the various desalination mechanisms that are significant in aiding the salt flux into the Southern Ocean. This will foster understanding of brine rejection and how it is linked to the porous microstructure of Antarctic sea ice