Tesis sobre el tema "Volume of Fluid (VOF)"

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

Die in dieser Arbeit vorgestellten Simulationen aufsteigender fluider Partikel wurden mit dem CFD-Programm FS3D durchgeführt, welches auf der Volume-of-Fluid (VoF) Methode basiert. Die Validierung des Codes erfolgt durch Vergleich der numerischen Lösungen für schleichende Strömungen mit analytischen Lösungen, wobei eine gute Übereinstimmung festgestellt wird. Im ersten Teil der Dissertation werden Simulationen für den freien Aufstieg von Öltropfen in Wasser mit experimentellen Beobachtungen hinsichtlich der Aufstiegsgeschwindigkeit, der Tropfenform und der Bewegungsbahn verglichen. Die Aufstiegsgeschwindigkeiten und Widerstandsbeiwerte sind vergleichbar, die simulierten Tropfen sind jedoch deutlich flacher. Dieser Unterschied kann durch Verunreinigungen der Grenzfläche im Experiment verursacht sein. Der Übergang von einem gradlinigen Aufstieg zu zickzack-förmigen Aufstiegsbahnen kann mit Hilfe der Simulationen auf Instabilitäten im Nachlauf der Blasen zurückgeführt werden, die zu einer periodischen Wirbelablösung führen. Im zweiten Teil der Dissertation wird der Aufstieg von Blasen in linearen Scherströmungen untersucht. Steigen die Blasen in einer vertikalen Scherströmung auf, so beobachtet man eine seitliche Migration. Diese seitliche Migration der Blasen wird durch die sogenannte Liftkraft verursacht, deren Vorzeichen und Betrag von der Blasengröße und den Stoffeigenschaften der Flüssigkeit abhängt. Die Simulationen zeigen, daß das Vorzeichen der Liftkraft für eher sphärische Blasen durch den Bernoulli-Effekt erklärt werden kann. An stark deformierten Blasen hingegen wirkt die Liftkraft in umgekehrter Richtung. Dieses Phänomen tritt auch in den Simulationen auf. Verschiedene Hypothesen für die Ursache dieses Phänomens werden überprüft. Die bekannteste experimentelle Korrelation für die Liftkraft von Tomiyama u.a. (2002) wird durch Simulation von realen Flüssigkeiten mit bekannten Stoffeigenschaften wie auch von Modellfluiden mit willkürlichen Stoffeigenschaften validiert und weitgehend bestätigt. Die Lift-Korrelation hat demnach hinsichtlich der Stoffeigenschaften der Flüssigkeit einen größeren Geltungsbereich, als bisher experimentell überprüft wurde. The simulations presented in this thesis were performed with the CFD code FS3D which is based on the Volume of Fluid method. The code is validated using analytical solutions for creeping flows and a good agreement is observed between simulation and analytical solution. In the first part of the thesis, the free rise of oil drops in water is simulated and compared with experimental observations. The results show that the rising velocities and the drag coefficients are similar in both cases, but the simulated drops are flatter (more oblate). This difference may be caused by impurities of the particle surface (surfactants) in the experiments. The simulations show that the transition from rectilinear to periodic trajectories is caused by instabilities in the wake, which lead to a periodic vortex shedding. In the second part of the thesis, the rise of bubbles in linear shear flows is investigated. If bubbles rise in a vertical shear flow, a lateral migration can be observed. This migration is caused by the so called lift force. Sign and magnitude of the lift force depend on the size of the bubble and the material properties of the liquid. The simulation results show that the sign of the lift force on spherical bubbles can be explained by the Bernoulli effect. However, the lift force on more distorted bubbles acts in the opposite direction. This phenomenon can also be observed in the simulation. In this work several hypotheses for the reason of this phenomenon are checked. Furthermore, most common correlation for the lift force (developed by Tomiyama et al. in 2002) is validated for fluids of known material and model fluids with arbitrary material data. The correlation is valid in a wider range of fluid material properties than proved experimentally up to now.

Die in dieser Arbeit vorgestellten Simulationen aufsteigender fluider Partikel wurden mit dem CFD-Programm FS3D durchgeführt, welches auf der Volume-of-Fluid (VoF) Methode basiert. Die Validierung des Codes erfolgt durch Vergleich der numerischen Lösungen für schleichende Strömungen mit analytischen Lösungen, wobei eine gute Übereinstimmung festgestellt wird. Im ersten Teil der Dissertation werden Simulationen für den freien Aufstieg von Öltropfen in Wasser mit experimentellen Beobachtungen hinsichtlich der Aufstiegsgeschwindigkeit, der Tropfenform und der Bewegungsbahn verglichen. Die Aufstiegsgeschwindigkeiten und Widerstandsbeiwerte sind vergleichbar, die simulierten Tropfen sind jedoch deutlich flacher. Dieser Unterschied kann durch Verunreinigungen der Grenzfläche im Experiment verursacht sein. Der Übergang von einem gradlinigen Aufstieg zu zickzack-förmigen Aufstiegsbahnen kann mit Hilfe der Simulationen auf Instabilitäten im Nachlauf der Blasen zurückgeführt werden, die zu einer periodischen Wirbelablösung führen. Im zweiten Teil der Dissertation wird der Aufstieg von Blasen in linearen Scherströmungen untersucht. Steigen die Blasen in einer vertikalen Scherströmung auf, so beobachtet man eine seitliche Migration. Diese seitliche Migration der Blasen wird durch die sogenannte Liftkraft verursacht, deren Vorzeichen und Betrag von der Blasengröße und den Stoffeigenschaften der Flüssigkeit abhängt. Die Simulationen zeigen, daß das Vorzeichen der Liftkraft für eher sphärische Blasen durch den Bernoulli-Effekt erklärt werden kann. An stark deformierten Blasen hingegen wirkt die Liftkraft in umgekehrter Richtung. Dieses Phänomen tritt auch in den Simulationen auf. Verschiedene Hypothesen für die Ursache dieses Phänomens werden überprüft. Die bekannteste experimentelle Korrelation für die Liftkraft von Tomiyama u.a. (2002) wird durch Simulation von realen Flüssigkeiten mit bekannten Stoffeigenschaften wie auch von Modellfluiden mit willkürlichen Stoffeigenschaften validiert und weitgehend bestätigt. Die Lift-Korrelation hat demnach hinsichtlich der Stoffeigenschaften der Flüssigkeit einen größeren Geltungsbereich, als bisher experimentell überprüft wurde. The simulations presented in this thesis were performed with the CFD code FS3D which is based on the Volume of Fluid method. The code is validated using analytical solutions for creeping flows and a good agreement is observed between simulation and analytical solution. In the first part of the thesis, the free rise of oil drops in water is simulated and compared with experimental observations. The results show that the rising velocities and the drag coefficients are similar in both cases, but the simulated drops are flatter (more oblate). This difference may be caused by impurities of the particle surface (surfactants) in the experiments. The simulations show that the transition from rectilinear to periodic trajectories is caused by instabilities in the wake, which lead to a periodic vortex shedding. In the second part of the thesis, the rise of bubbles in linear shear flows is investigated. If bubbles rise in a vertical shear flow, a lateral migration can be observed. This migration is caused by the so called lift force. Sign and magnitude of the lift force depend on the size of the bubble and the material properties of the liquid. The simulation results show that the sign of the lift force on spherical bubbles can be explained by the Bernoulli effect. However, the lift force on more distorted bubbles acts in the opposite direction. This phenomenon can also be observed in the simulation. In this work several hypotheses for the reason of this phenomenon are checked. Furthermore, most common correlation for the lift force (developed by Tomiyama et al. in 2002) is validated for fluids of known material and model fluids with arbitrary material data. The correlation is valid in a wider range of fluid material properties than proved experimentally up to now.

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

A VOF based multiphase Lattice Boltzmann method that explicitly prescribes kinematic boundary conditions at the interface is developed. The advantage of the method is the direct control over the surface tension value. The details of the numerical method are presented. The Saffman instability, Taylor instability, and flow of deformable suspensions in a channel are used as example-problems to demonstrate the accuracy of the method. The method allows for relatively large viscosity and density ratios.

Abstract In ladle metallurgy, gas stirring and the behaviour of the slag layer are very important for alloying and the homogenization of the steel. When gas is injected through a nozzle located at the bottom of the ladle into the metal bath, the gas jet exiting the nozzle breaks up into gas bubbles. The rising bubbles break the slag layer and create an open-eye. The size of the open-eye is very important as the efficiency of the metal-slag reactions depend on the interaction between the slag and steel created during the stirring process, and information about the position and size of the open-eye is important for effective alloying practice. Moreover, the open-eye has an effect on the energy balance since it increases heat losses. In this study, experimental measurements and numerical simulations were performed to study the effect of different operating parameters on the formation of the open-eye and mixing time in a water model and industrial ladle. Experimental measurements were performed to study the effect of the gas flow rate, slag layer thickness, slag layer densities and number of porous plugs in a 1/5 scale water model and in a 150-ton steelmaking ladle. For numerical modelling, a multi-phase volume of fluid (VOF) model was used to simulate the system including the behaviour of the slag layer. The numerical simulation of the open-eye size and mixing time was found to be in good agreement with the experimental data obtained from the water model and data obtained from the industrial measurements
Tiivistelmä Senkkametallurgiassa kaasuhuuhtelu ja kuonakerroksen käyttäytyminen ovat tärkeitä teräksen seostamisen ja homogenisoinnin näkökulmasta. Senkan pohjalla sijaitsevasta suuttimesta puhallettava kaasu hajoaa kupliksi, jotka rikkovat kuonakerroksen ja muodostavat avoimen silmäkkeen. Avoimen silmäkkeen koko on yhteydessä voimakkaampaan kuonan emulgoitumiseen, joka tehostaa metallisulan ja kuonan välisiä reaktioita. Tietoa avoimen silmäkkeen paikasta ja koosta tarvitaan myös tehokkaaseen seostuspraktiikkaan. Avoin silmäke vaikuttaa lisäksi prosessin energiataseeseen lisäten sen lämpöhäviöitä. Tässä tutkimuksessa tutkittiin kokeellisesti ja laskennallisesti erilaisten operointiparametrien vaikutusta avoimen silmäkkeen muodostumiseen vesimallissa ja terässenkassateollisessa senkassa. Kokeellisia mittauksia tehtiin kaasuhuuhtelun, kuonakerroksen paksuuden, ja suuttimien määrän vaikutuksen tutkimiseksi 1/5-mittakaavan vesimallissa ja 150 tonnin terässenkassa. Numeerisessa mallinnuksessa systeemin ja siihen lukeutuvan kuonakerroksen käyttäytymisen simuloimiseen käytettiin volume of fluid (VOF) –monifaasimenetelmää. Avoimen silmäkkeen kokoon ja sekoittumisaikaan liittyvien numeeristen simulointien havaittiin vastaavan hyvin vesimallista ja teollisista mittauksista saatua kokeellista aineistoa

The development of a fuel filler pipe is based solely on experience and physical experiment. The challenge lies in designing the pipe to fulfill the customer needs. In other words designing the pipe such as the fuel flow does not splash back on the fuel dispenser causing a premature shut off. To improve this “trial-and-error” based development a computational fluid dynamics (CFD) model of the refueling process is investigated. In this thesis a CFD model has been developed that can predict the fuel flow in the filler pipe. Worst case scenario of the refueling process is during the first second when the tank is partially filled. The most critical fluid is diesel due to the commercially high volume flow of 55 l/min. Due to limitations of computational resources the simulations are focused on the first second of the refueling process. The challenge in this project is creating a CFD model that is time efficient, thus require the least amount of computational resources necessary to provide useful information. A multiphase model is required to simulate the refueling process. In this project the implicit volume of fluid (VOF) has been used which has previously proven to be a suitable choice for refueling simulations. The project is divided into two parts. Part one starts with experiments and simulations of a simplified fuel system with water as acting liquid with a Reynolds number of 90 000. A short comparison between three different turbulence models has been investigated (LES, DES and URANS) where the most promising turbulence model is URANS, specifically the SST k-ω model. A sensitivity analysis was performed on the chosen turbulence model. Between the chosen mesh and the densest mesh the difference of streamwise velocity in the boundary layer was 2.6 %. The chosen mesh with 1.9 M cells and a time step of 1e-4 s was found to be the best correlating model with respect to the experiments. In part two a real fuel filling system was investigated both with experiments and simulations with the same computational model as the chosen one from part one. The change of fluid and geometry resulted in a lower Reynolds number of 12 000. Two different versions of the fuel system was investigated; with a bypass pipe and without a bypass pipe. Because of a larger volumetric region the resulting mesh had 3.7 M cells. The finished model takes about 230 h on a local workstation with 11 cores. On a cluster with 200 cores the same simulation takes 30 h. The resulting model suffered from interpolation errors at the inlet which resulted in a volume flow of 50 l/min as opposed to 55 l/min in the experiments. Despite the difference the model could capture the key flow characteristics. With the developed model a new filler pipe can be easily implemented and provide results in shorter time than a prototype filler pipe can be ordered. This will increase the chances of ordering one single prototype that fulfills all requirements. While the simulation model cannot completely replace verification by experiments it provides information that transforms the development of the filler pipe to knowledge based development.

The study and modelling of two-phase flow, even the simplest ones such as the bubbly flow, remains a challenge that requires exploring the physical phenomena from different spatial and temporal resolution levels. CFD (Computational Fluid Dynamics) is a widespread and promising tool for modelling, but nowadays, there is no single approach or method to predict the dynamics of these systems at the different resolution levels providing enough precision of the results. The inherent difficulties of the events occurring in this flow, mainly those related with the interface between phases, makes that low or intermediate resolution level approaches as system codes (RELAP, TRACE, ...) or 3D TFM (Two-Fluid Model) have significant issues to reproduce acceptable results, unless well-known scenarios and global values are considered. Instead, methods based on high resolution level such as Interfacial Tracking Method (ITM) or Volume Of Fluid (VOF) require a high computational effort that makes unfeasible its use in complex systems. In this thesis, an open-source simulation framework has been designed and developed using the OpenFOAM library to analyze the cases from microescale to macroscale levels. The different approaches and the information that is required in each one of them have been studied for bubbly flow. In the first part, the dynamics of single bubbles at a high resolution level have been examined through VOF. This technique has allowed to obtain accurate results related to the bubble formation, terminal velocity, path, wake and instabilities produced by the wake. However, this approach has been impractical for real scenarios with more than dozens of bubbles. Alternatively, this thesis proposes a CFD Discrete Element Method (CFD-DEM) technique, where each bubble is represented discretely. A novel solver for bubbly flow has been developed in this thesis. This includes a large number of improvements necessary to reproduce the bubble-bubble and bubble-wall interactions, turbulence, velocity seen by the bubbles, momentum and mass exchange term over the cells or bubble expansion, among others. But also new implementations as an algorithm to seed the bubbles in the system have been incorporated. As a result, this new solver gives more accurate results as the provided up to date. Following the decrease on resolution level, and therefore the required computational resources, a 3D TFM have been developed with a population balance equation solved with an implementation of the Quadrature Method Of Moments (QMOM). The solver is implemented with the same closure models as the CFD-DEM to analyze the effects involved with the lost of information due to the averaging of the instantaneous Navier-Stokes equation. The analysis of the results with CFD-DEM reveals the discrepancies found by considering averaged values and homogeneous flow in the models of the classical TFM formulation. Finally, for the lowest resolution level approach, the system code RELAP5/MOD3 is used for modelling the bubbly flow regime. The code has been modified to reproduce properly the two-phase flow characteristics in vertical pipes, comparing the performance of the calculation of the drag term based on drift-velocity and drag coefficient approaches.
El estudio y modelado de flujos bifásicos, incluso los más simples como el bubbly flow, sigue siendo un reto que conlleva aproximarse a los fenómenos físicos que lo rigen desde diferentes niveles de resolución espacial y temporal. El uso de códigos CFD (Computational Fluid Dynamics) como herramienta de modelado está muy extendida y resulta prometedora, pero hoy por hoy, no existe una única aproximación o técnica de resolución que permita predecir la dinámica de estos sistemas en los diferentes niveles de resolución, y que ofrezca suficiente precisión en sus resultados. La dificultad intrínseca de los fenómenos que allí ocurren, sobre todo los ligados a la interfase entre ambas fases, hace que los códigos de bajo o medio nivel de resolución, como pueden ser los códigos de sistema (RELAP, TRACE, etc.) o los basados en aproximaciones 3D TFM (Two-Fluid Model) tengan serios problemas para ofrecer resultados aceptables, a no ser que se trate de escenarios muy conocidos y se busquen resultados globales. En cambio, códigos basados en alto nivel de resolución, como los que utilizan VOF (Volume Of Fluid), requirieren de un esfuerzo computacional tan elevado que no pueden ser aplicados a sistemas complejos. En esta tesis, mediante el uso de la librería OpenFOAM se ha creado un marco de simulación de código abierto para analizar los escenarios desde niveles de resolución de microescala a macroescala, analizando las diferentes aproximaciones, así como la información que es necesaria aportar en cada una de ellas, para el estudio del régimen de bubbly flow. En la primera parte se estudia la dinámica de burbujas individuales a un alto nivel de resolución mediante el uso del método VOF (Volume Of Fluid). Esta técnica ha permitido obtener resultados precisos como la formación de la burbuja, velocidad terminal, camino recorrido, estela producida por la burbuja e inestabilidades que produce en su camino. Pero esta aproximación resulta inviable para entornos reales con la participación de más de unas pocas decenas de burbujas. Como alternativa, se propone el uso de técnicas CFD-DEM (Discrete Element Methods) en la que se representa a las burbujas como partículas discretas. En esta tesis se ha desarrollado un nuevo solver para bubbly flow en el que se han añadido un gran número de nuevos modelos, como los necesarios para contemplar los choques entre burbujas o con las paredes, la turbulencia, la velocidad vista por las burbujas, la distribución del intercambio de momento y masas con el fluido en las diferentes celdas por cada una de las burbujas o la expansión de la fase gaseosa entre otros. Pero también se han tenido que incluir nuevos algoritmos como el necesario para inyectar de forma adecuada la fase gaseosa en el sistema. Este nuevo solver ofrece resultados con un nivel de resolución superior a los desarrollados hasta la fecha. Siguiendo con la reducción del nivel de resolución, y por tanto los recursos computacionales necesarios, se efectúa el desarrollo de un solver tridimensional de TFM en el que se ha implementado el método QMOM (Quadrature Method Of Moments) para resolver la ecuación de balance poblacional. El solver se desarrolla con los mismos modelos de cierre que el CFD-DEM para analizar los efectos relacionados con la pérdida de información debido al promediado de las ecuaciones instantáneas de Navier-Stokes. El análisis de resultados de CFD-DEM permite determinar las discrepancias encontradas por considerar los valores promediados y el flujo homogéneo de los modelos clásicos de TFM. Por último, como aproximación de nivel de resolución más bajo, se investiga el uso uso de códigos de sistema, utilizando el código RELAP5/MOD3 para analizar el modelado del flujo en condiciones de bubbly flow. El código es modificado para reproducir correctamente el flujo bifásico en tuberías verticales, comparando el comportamiento de aproximaciones para el cálculo del término d
L'estudi i modelatge de fluxos bifàsics, fins i tot els més simples com bubbly flow, segueix sent un repte que comporta aproximar-se als fenòmens físics que ho regeixen des de diferents nivells de resolució espacial i temporal. L'ús de codis CFD (Computational Fluid Dynamics) com a eina de modelatge està molt estesa i resulta prometedora, però ara per ara, no existeix una única aproximació o tècnica de resolució que permeta predir la dinàmica d'aquests sistemes en els diferents nivells de resolució, i que oferisca suficient precisió en els seus resultats. Les dificultat intrínseques dels fenòmens que allí ocorren, sobre tots els lligats a la interfase entre les dues fases, fa que els codis de baix o mig nivell de resolució, com poden ser els codis de sistema (RELAP,TRACE, etc.) o els basats en aproximacions 3D TFM (Two-Fluid Model) tinguen seriosos problemes per a oferir resultats acceptables , llevat que es tracte d'escenaris molt coneguts i se persegueixen resultats globals. En canvi, codis basats en alt nivell de resolució, com els que utilitzen VOF (Volume Of Fluid), requereixen d'un esforç computacional tan elevat que no poden ser aplicats a sistemes complexos. En aquesta tesi, mitjançant l'ús de la llibreria OpenFOAM s'ha creat un marc de simulació de codi obert per a analitzar els escenaris des de nivells de resolució de microescala a macroescala, analitzant les diferents aproximacions, així com la informació que és necessària aportar en cadascuna d'elles, per a l'estudi del règim de bubbly flow. En la primera part s'estudia la dinàmica de bambolles individuals a un alt nivell de resolució mitjançant l'ús del mètode VOF. Aquesta tècnica ha permès obtenir resultats precisos com la formació de la bambolla, velocitat terminal, camí recorregut, estela produida per la bambolla i inestabilitats que produeix en el seu camí. Però aquesta aproximació resulta inviable per a entorns reals amb la participació de més d'unes poques desenes de bambolles. Com a alternativa en aqueix cas es proposa l'ús de tècniques CFD-DEM (Discrete Element Methods) en la qual es representa a les bambolles com a partícules discretes. En aquesta tesi s'ha desenvolupat un nou solver per a bubbly flow en el qual s'han afegit un gran nombre de nous models, com els necessaris per a contemplar els xocs entre bambolles o amb les parets, la turbulència, la velocitat vista per les bambolles, la distribució de l'intercanvi de moment i masses amb el fluid en les diferents cel·les per cadascuna de les bambolles o els models d'expansió de la fase gasosa entre uns altres. Però també s'ha hagut d'incloure nous algoritmes com el necessari per a injectar de forma adequada la fase gasosa en el sistema. Aquest nou solver ofereix resultats amb un nivell de resolució superior als desenvolupat fins la data. Seguint amb la reducció del nivell de resolució, i per tant els recursos computacionals necessaris, s'efectua el desenvolupament d'un solver tridimensional de TFM en el qual s'ha implementat el mètode QMOM (Quadrature Method Of Moments) per a resoldre l'equació de balanç poblacional. El solver es desenvolupa amb els mateixos models de tancament que el CFD-DEM per a analitzar els efectes relacionats amb la pèrdua d'informació a causa del promitjat de les equacions instantànies de Navier-Stokes. L'anàlisi de resultats de CFD-DEM permet determinar les discrepàncies ocasionades per considerar els valors promitjats i el flux homogeni dels models clàssics de TFM. Finalment, com a aproximació de nivell de resolució més baix, s'analitza l'ús de codis de sistema, utilitzant el codi RELAP5/MOD3 per a analitzar el modelatge del fluxos en règim de bubbly flow. El codi és modificat per a reproduir correctament les característiques del flux bifàsic en canonades verticals, comparant el comportament d'aproximacions per al càlcul del terme de drag basades en velocitat de drift flux model i de les basades en coe
Peña Monferrer, C. (2017). Computational fluid dynamics multiscale modelling of bubbly flow. A critical study and new developments on volume of fluid, discrete element and two-fluid methods [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/90493
TESIS

Water management is a serious issue that affects the performance and durability of PEM fuel cells. It is known, from previous experimental investigations, that surface wettability has influence on water behaviour and fuel cell performance. This finding has lead researchers to develop numerical tools for further investigation of the liquid water behaviour in gas channels. The Volume-of-Fluid (VOF) method has been used in a wide range of studies for its advantage of showing the multi-phase interface in a Computational Fluid Dynamics (CFD) simulation to understand liquid water behaviour in gas channels. In this thesis, numerical study has been carried out to examine the behaviour of liquid water in gas channels. The dynamic movement of the liquid water in the channel and the associated pressure drop, water saturation and water coverage of the GDL have been investigated. Firstly, flow diffusion into the GDL was examined to determine its effect on liquid droplet behaviour in a small section of a gas channel. Furthermore, the effects of the percentage of flow diffusion, GDL wettability, pore size, and water inlet velocity were investigated. Fluid diffusion into GDL found to have insignificant impact on liquid water behaviour so further investigations has been carried with a solid GDL surface. Secondly, gas channel geometry effect on liquid water behaviour was studied. Square, semicircle, triangle, trapezoid with a long base and trapezoid with a short base were compared to find suitable cross section geometry to carry wall wettability investigations. Among the examined geometries, the square cross section showed reasonable results for both scenarios of geometry design, fixed Reynolds number and fixed GDL interface. The effect of wall wettability was assessed by comparing nine different wall/GDL wettability combinations for straight and bend channels. Wall wettability found to have an impact on liquid water behaviour but not as much as GDL wettability. It affects liquid water saturation in the channel by a great deal by accumulating water in the channel edges affecting water behaviour. This was also proven in the last test case of a long channel where water accumulation was investigated by running the calculation until the percentage of water saturation is stabilized. It is also concluded that changing wall wettability from hydrophobic to hydrophilic doubles the percentage of channel occupied by liquid water and increases the time to reach steady state.

In 1999, California Air Resources Board (CARB) implemented a regulation that required all gasoline cars sold in California be fitted with an Onboard Refueling Vapor Recovery System (ORVR). The ORVR system is designed to prevent Volatile Organic Compounds (VOCs) from escaping into the atmosphere during refuelling by storing the gas vapours in a carbon canister. Due to the complex nature of the fuel system, making design changes could have large implications on the ORVR performance of the vehicle. It is therefore desirable to develop a CFD model that can predict the effects of design changes, thereby reducing the need to perform physical tests on each design iteration. This master thesis project was performed at the Fuel Systems department at Volvo Cars in order to help reduce project lead times and product development costs by incorporating CFD as a part of the fuel system development cycle. The CFD results obtained were validated through experimental tests that were also performed as part of this project. In this master thesis project, a CFD model was developed to simulate the refuelling of gasoline for a California specification Volvo XC90 with an OPW-11B pump pistol. The model was set up in STAR-CCM+ using the Eulerian Volume of Fluid model for multiphase flow, the RANS realizable k − ε turbulence model and the two layer all y + wall treatment. The effects of the carbon canister were modelled as a porous baffle interface in the simulations where viscous and inertial resistances of the porous media were adjusted to obtain a desired pressure drop across the canister. This method proved to be a suitable simplification for this study. The effects of evaporation as well as a chemical adsorption model for the carbon canister have been excluded from the project due to time limitations. It was found that the CFD simulations were in good agreement with the experimental results, especially with respect to capturing the overall behaviour of the fuel system during refuelling. It was found that resolving the flow spatially (and temporally) in the filler pipe was a crucial part in ensuring solver stability. A pressure difference between experiment and simulation was also observed as a consequence of excluding evaporation from the CFD model. After the CFD model had been verified and validated, changes to different parts of the fuel system were investigated to observe their effects on ORVR performance. These included changing the recirculation line diameter, changing the carbon canister properties and changing the angle of how the pump pistol was inserted into the capless unit. It was found that the recirculation line diameter is a very sensitive design parameter and increasing the diameter would result in fuel vapour leaking back out into the atmosphere. Similarly, increasing the back pressure by swapping to a different carbon canister would result in the leakage of fuel vapour. On the other hand, insignificant changes in system behaviour were observed when the fuel pistol angle was changed.In 1999, California Air Resources Board (CARB) implemented a regulation that required all gasoline cars sold in California be fitted with an Onboard Refueling Vapor Recovery System (ORVR). The ORVR system is designed to prevent Volatile Organic Compounds (VOCs) from escaping into the atmosphere during refuelling by storing the gas vapours in a carbon canister. Due to the complex nature of the fuel system, making design changes could have large implications on the ORVR performance of the vehicle. It is therefore desirable to develop a CFD model that can predict the effects of design changes, thereby reducing the need to perform physical tests on each design iteration. This master thesis project was performed at the Fuel Systems department at Volvo Cars in order to help reduce project lead times and product development costs by incorporating CFD as a part of the fuel system development cycle. The CFD results obtained were validated through experimental tests that were also performed as part of this project. In this master thesis project, a CFD model was developed to simulate the refuelling of gasoline for a California specification Volvo XC90 with an OPW-11B pump pistol. The model was set up in STAR-CCM+ using the Eulerian Volume of Fluid model for multiphase flow, the RANS realizable k − ε turbulence model and the two layer all y + wall treatment. The effects of the carbon canister were modelled as a porous baffle interface in the simulations where viscous and inertial resistances of the porous media were adjusted to obtain a desired pressure drop across the canister. This method proved to be a suitable simplification for this study. The effects of evaporation as well as a chemical adsorption model for the carbon canister have been excluded from the project due to time limitations. It was found that the CFD simulations were in good agreement with the experimental results, especially with respect to capturing the overall behaviour of the fuel system during refuelling. It was found that resolving the flow spatially (and temporally) in the filler pipe was a crucial part in ensuring solver stability. A pressure difference between experiment and simulation was also observed as a consequence of excluding evaporation from the CFD model. After the CFD model had been verified and validated, changes to different parts of the fuel system were investigated to observe their effects on ORVR performance. These included changing the recirculation line diameter, changing the carbon canister properties and changing the angle of how the pump pistol was inserted into the capless unit. It was found that the recirculation line diameter is a very sensitive design parameter and increasing the diameter would result in fuel vapour leaking back out into the atmosphere. Similarly, increasing the back pressure by swapping to a different carbon canister would result in the leakage of fuel vapour. On the other hand, insignificant changes in system behaviour were observed when the fuel pistol angle was changed.

Orientador: Dirceu Noriler.
Coorientador: Henry França Meier.
Dissertação (Mestrado em Engenharia Química) - Programa de Pós-Graduação em Engenharia Química, Centro de Ciências Tecnológicas, Universidade Regional de Blumenau, Blumenau.

The development of oil and gas fields in offshore deep waters (more than 1000 m) will become more common in the future. Inevitably, production systems will operate under multiphase flow conditions. The two–phase flow of gas–liquid in pipes with different inclinations has been studied intensively for many years. The reliable prediction of flow pattern, pressure drop, and liquid holdup in a two–phase flow is thereby important.With the increase of computer power and development of modelling software, the investigation of two–phase flows of gas–liquid problems using computational fluid dynamics (CFD) approaches is gradually becoming attractive in the various engineering disciplines. The use of CFD as a modelling tool in multiphase flow simulation has enormously increased in the last decades and is the focus of this thesis. Two basic CFD techniques are utilized to simulate the gas–liquid flow, the Volume of Fluid (VOF) model, and the Eulerian–Eulerian (E–E) model.The purpose of this thesis is to investigate the risk of hydrate formation in a low–spot flowline by assessing the flow pattern and droplet hydrodynamics in gas– dominated restarts using the VOF method, and also to develop and validate a model for gas–liquid two–phase flow in horizontal pipelines using the Eulerian– Eulerian method; the purpose of this is to predict the pressure drop and liquid holdup encountered during two–phase (i.e. gas–oil, gas–water) production at different flow conditions, such as fluid properties, volume fractions of liquid, superficial velocities, and mass fluxes.In the first part of this thesis, the VOF approach was used to simulate the droplet formation and flow pattern at various levels of liquid patched and restart gas superficial velocities. The effect of restart gas superficial velocity on the liquid displacement from the low section of the pipe showed a decrease in the remaining liquid with an increase in gas superficial velocity, and the amount of liquid depends on the fluid properties, such as density and viscosity. Moreover, the flow pattern is also strongly dependent on the restart gas superficial velocity as well as the patched liquid in the low section. A low gas superficial velocity with different patched liquids illustrated no risk of hydrate formation due to the observed flow pattern that is often a stratified flow. However, as the restart gas superficial velocity is increased, regardless of initial liquid patching, hydrate formation is more likely to be observed due to the observed flow pattern, such as annular, churn or dispersed flow.In the second part, the E–E model was employed to establish a computational model to predict the pressure drop and liquid holdup in a horizontal pipeline. Due to the complicated process phenomena of two–phase flow, a new drag coefficient was implemented to model the pressure drop and liquid holdup in the 3D pipe. Different simulations were performed with various superficial velocities of two–phase and liquid volume fractions, and were carried out using RNG k-ε model to account for turbulence. Based on the results from the numerical model and previous experimental study, the currently used E–E model is improved to get more accurate prediction for the pressure drop and liquid holdup in horizontal pipes compared with the existing models of Hart et al. (1989) and Chen et al. (1997).The improved model is validated by previously reported experimental data (Badie et al., 2000). The deviation of pressure drop and liquid holdup obtained throughout the CFD simulation with regard to the experimental data was found to be relatively small at low superficial gas velocities. It was observed that the pressure gradient increased with the system parameters, such as the drop size, liquid and gas superficial velocity and the liquid volume fraction, where the liquid holdup decreased.The developed model provided a basis for studying the pressure drop and liquid holdup in a horizontal pipe. Different parameters have been examined, such as gas and liquid mass flux and liquid volume fraction. Two empirical correlations have been examined (Beggs and Brill (1973), and Mukherjee and Brill (1985)) against the CFD simulation results of pressure drop and liquid holdup, it was noted that they gave better agreement with the air–oil system rather than the air–water system, but shows reasonable agreement over the entire gas mass fluxIn the third part, the coupling of Eulerian–Eulerian multiphase model with the population balance equation (PBE), accounting for droplet coalescence and breakage, is considered. Strengths and weaknesses of each numerical approach for solving PBE have been given in details. The Quadrature Method of Moments (QMOM) is used and particular coalescence and breakup kernels were utilized to demonstrate the droplet size distribution behaviour. Numerical simulations on a two–phase flow in a horizontal pipe, including coalescence and breakage are performed. The QMOM is shown to give the solution of the PBE with reasonable agreement. The numerical data are compared with the experiment data of Simmons and Henratty (2001). The flow variables, such as liquid volume fractions, gas and liquid superficial velocities are employed to examine the droplet size distribution and the potential of the multiphase k–ε with population balance model for predicting the two–phase pressure drop and liquid holdup.The significance of this work is to assist in understanding the risk of hydrate formation in bend pipes at gas–dominated restarts with different patched liquid values. The knowledge gained from this work can be utilized to avoid the hydrate formation operating conditions. The developed of multiphase flow E–E model will provide an accurate prediction for two–phase pressure drop and liquid holdup in a horizontal pipe which will be of benefit to the design of tubing and surface facilities.

L’émergence de bruits auparavant inaudibles dans les réservoirs à carburants automobiles requiert des constructeurs une meilleure compréhension des phénomènes physiques intervenants au sein de leurs produits. Dans cette thèse, différents travaux ont été conduits autour de l’étude du ballottement de fluide dans une cuve rigide rectangulaire partiellement remplie de fluide et soumise à une excitation extérieure. La première partie présente un état de l’art sur le sloshing suivant trois approches complémentaires - approche analytique, approche numérique et approche expérimentale - permettant d’orienter les travaux. Dans une deuxième partie, une étude préliminaire sur le sloshing dans une cuve rectangulaire soumise à une excitation harmonique forcée est réalisée. La confrontation des résultats numériques entre une approche linéaire - basée sur la théorie d’écoulement potentiel tenant compte de la viscosité du fluide [Schotté et Ohayon, 2013] - et une approche non linéaire commerciale – basée sur la résolution des équations de Navier-Stokes - permet de définir un paramètre de linéarité. Ce dernier permet de déterminer les cas de sloshing qui nécessitent une résolution non linéaire et ceux pour lesquels la théorie linéaire suffit pour prédire le phénomène. La troisième partie de ce document présente une étude expérimentale du ballottement de fluide dans une cuve rectangulaire rigide soumise à un freinage automobile. Deux niveaux de remplissage créant deux types d’impacts contre les parois (avec et sans enfermement de poche d’air) ont été analysés. Les essais menés ont permis de mesurer les forces engendrées par le mouvement du fluide, les pressions d’impact en paroi ainsi que le champ de vitesse par méthode Particle Image Velocimetry (PIV). Ce chapitre constitue une importante base de données expérimentales ayant permis d’étudier précisément le phénomène physique. L’étude est complétée par une confrontation des résultats expérimentaux avec des résultats Computational Fluid Dynamics (CFD). Enfin, pour conclure ce mémoire, une étude du sloshing dans un réservoir en tenant compte de la Fluid-Structure Interaction (FSI) est présentée. Le choix du couplage a été porté sur un schéma partitionné itératif faible avec, dans un premier temps, une approche potentielle instationnaire, puis avec une approche Volume Of Fluid (VOF) pour la physique fluide. Les limites d’un tel couplage dans le cas d’étude d’un réservoir partiellement rempli de fluide et attaché de manière flexible en fonction du rapport de masse fluide-réservoir ont été mises en évidence. La correction du schéma de couplage par l’effet de masse ajoutée présentée dans [Song et al., 2013] permet la résolution d’un système couplé quel que soit le rapport de masse en jeu et améliore de manière significative la convergence en réduisant également fortement le temps de calcul
The present thesis focuses on an investigation of the sloshing phenomenon in a partially filled fuel tank submitted to a harmonic excitation motion. In the first part, the confrontation of numerical results between a linear approach - taking into account viscosity - and a nonlinear approach based on a commercial code leads to define a parameter of linearity. This parameter allows determining cases of sloshing who require non-linear resolution and those who need a linear theory to predict the phenomenon. An experimental study of fluid sloshing in a rectangular tank submitted to an automotive braking is conducted. Tests leaded allow measuring global forces engendered by the motion of the fluid, pressure of fluid impact and velocity field by PIV. This chapter provides an important data base and helps to investigate on the physical phenomenon. This study is completed by CFD results. To conclude, a numerical model for fluid-structure interactions is presented. Limits of this segregated partitioned coupling in case of sloshing in tank flexibly attached are highlighted, depending mostly on the mass ratio between fluid and tank structure. An added-mass term is integrated to the corrected staggered scheme ensuring systematically the convergence of the coupled solution and reducing significantly the iterations required

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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)
Denominam-se problemas tipo rompimento de barreira os fenômenos nos quais um fluido é liberado de maneira abrupta. Quando o fluido apresenta natureza hiperconcentrada, a relação entre a tensão de cisalhamento e a taxa de deformação pode se tornar não-linear, passando a apresentar reologia não-Newtoniana. Problemas deste tipo podem ser encontrados em muitos fenômenos tanto na natureza quanto em processos industriais. O estudo de tal problema é, geralmente, conduzido usando simplificações, como a aproximação de águas rasas e a separação do escoamento em regimes dominantemente inerciais ou viscosos. O presente trabalho é composto de duas partes, uma experimental e outra, numérica. No campo experimental, duas soluções controladas são usadas: soluções aquosas de açúcar e de Carbopol 940, esta última com várias concentrações volumétricas. O aparato experimental consiste em um canal retangular de acrílico, contendo uma comporta, a montante da qual, o fluido é retido e, pela ruptura (levantamento da comporta), começa a escoar. O escoamento é estudado através de técnicas avançadas de filmagem. No campo numérico, são realizadas simulações usando o programa CFX, no qual é usado um método de rastreamento de interface, o VOF e sem o emprego das simplificações citadas. Os resultados experimentais são comparados com os numéricos e com resultados da literatura que usam tais simplificações. Na comparação a aproximação de águas rasas, apesar de descrever bem a forma da interface, se distancia dos valores reais da posição da frente de onda.
The dam break problem describes a phenomenon in which there is an abrupt release of fluid. When the fluid is hiperconcentrated, the relation between the shear stress and the strain rate can become non-linear, and so present a non-Newtonian rheology. The non-Newtonian dam break problem may be found in many phenomena in nature and industrial process. The study of such a problem is, generally, conducted using simplified hypothesis such as the shallow water approximation and the separation of the flow in inertial and viscous dominated regimes. The present work is composed of two parts, one experimental and other, numerical. In the experimental field, two controlled solutions were used: water solutions of sugar and of Carbopol 940, the last one with a wide range of volume concentrations. These fluids have, respectively, Newtonian and non-Newtonian rheologies. The experimental setup consists of an acrylic rectangular channel, which has a dam and upstream of that the fluid is retained and, by the rupture, it begins to flow. The flow is studied by using advanced filming techniques. In the numerical field, simulations are conducted using the CFX software, which uses an interface tracking method, the VOF, and without the shallow water approximation and the division of the flow. So the experimental, numerical and literature results, that uses such simplifications, are compared and it is showed that the shallow water approximation, however describes very well the shape of the surface, is not accurate in calculate the wave front position.

Orientador: Geraldo de Freitas Maciel
Banca: Sérgio Said Mansur
Banca: Jean Paul Vila
Resumo: Denominam-se problemas tipo rompimento de barreira os fenômenos nos quais um fluido é liberado de maneira abrupta. Quando o fluido apresenta natureza hiperconcentrada, a relação entre a tensão de cisalhamento e a taxa de deformação pode se tornar não-linear, passando a apresentar reologia não-Newtoniana. Problemas deste tipo podem ser encontrados em muitos fenômenos tanto na natureza quanto em processos industriais. O estudo de tal problema é, geralmente, conduzido usando simplificações, como a aproximação de águas rasas e a separação do escoamento em regimes dominantemente inerciais ou viscosos. O presente trabalho é composto de duas partes, uma experimental e outra, numérica. No campo experimental, duas soluções controladas são usadas: soluções aquosas de açúcar e de Carbopol 940, esta última com várias concentrações volumétricas. O aparato experimental consiste em um canal retangular de acrílico, contendo uma comporta, a montante da qual, o fluido é retido e, pela ruptura (levantamento da comporta), começa a escoar. O escoamento é estudado através de técnicas avançadas de filmagem. No campo numérico, são realizadas simulações usando o programa CFX, no qual é usado um método de rastreamento de interface, o VOF e sem o emprego das simplificações citadas. Os resultados experimentais são comparados com os numéricos e com resultados da literatura que usam tais simplificações. Na comparação a aproximação de águas rasas, apesar de descrever bem a forma da interface, se distancia dos valores reais da posição da frente de onda.
Abstract: The dam break problem describes a phenomenon in which there is an abrupt release of fluid. When the fluid is hiperconcentrated, the relation between the shear stress and the strain rate can become non-linear, and so present a non-Newtonian rheology. The non-Newtonian dam break problem may be found in many phenomena in nature and industrial process. The study of such a problem is, generally, conducted using simplified hypothesis such as the shallow water approximation and the separation of the flow in inertial and viscous dominated regimes. The present work is composed of two parts, one experimental and other, numerical. In the experimental field, two controlled solutions were used: water solutions of sugar and of Carbopol 940, the last one with a wide range of volume concentrations. These fluids have, respectively, Newtonian and non-Newtonian rheologies. The experimental setup consists of an acrylic rectangular channel, which has a dam and upstream of that the fluid is retained and, by the rupture, it begins to flow. The flow is studied by using advanced filming techniques. In the numerical field, simulations are conducted using the CFX software, which uses an interface tracking method, the VOF, and without the shallow water approximation and the division of the flow. So the experimental, numerical and literature results, that uses such simplifications, are compared and it is showed that the shallow water approximation, however describes very well the shape of the surface, is not accurate in calculate the wave front position.
Mestre

High speed planing hulls are currently widely used for example in recreational and emergency vessel applications. However, very little CFD research has been done for planing vessels, especially for those with stepped hulls. A validated CFD method for planing stepped hulls could be a valuable improvement for the design phase of such hulls. In this thesis, a CFD method for stepped hulls, with a primary focus on two-step hulls, is developed using STAR-CCM+. As a secondary objective, porpoising instability of two-step hulls is investigated. The simulations are divided into two parts: In the first part a method is developed and validated with existing experimental and numerical data for a simple model scale planing hull with one step. In the second part the method is applied for two two-step hulls provided with Hydrolift AS. A maximum two degrees of freedom, trim and heave, are used, as well as RANS based k-w SST turbulence model and Volume of Fluid (VOF) as a free surface model. The results for the one-step hull mostly corresponded well with the validation data. For the two-step hulls, validation data did not exists and they were first simulated with a fixed trim and sinkage and compered between each other. In the simulations with free trim and heave both hulls experienced unstable porpoising behavior.

Early in the ship design process, naval architects must often evaluate and compare multiple hull forms for a specific set of requirements. Analytical tools are useful for quick comparisons, but they usually specialize in a specific hull type and are therefore not adequate for comparing dissimilar hull types. Scale model hydrodynamic testing is the traditional evaluation method, and is applicable to most hull forms. Scale model tests are usually performed on the largest model possible in order to achieve the most accurate performance predictions. However, such testing is very resource intensive, and is therefore not a cost effective method of evaluating multiple hull forms. This thesis explores the testing of small scale models. It is hypothesized that although the data acquired by these tests will not be accurate enough for performance predictions, they will be accurate enough to rank the performance of the multiple hull forms being evaluated.

The World Health Organization estimates that over two billion people did not have access to a toilet in 2017. The dejects produced often end up in bodies of water and, as one in every ten crops is watered with wastewater, this is estimated to cause half a billion deaths per year. There are currently many initiatives to reach this sanitation Global Goal, including those from people such as Bill Gates and projects such as The Solar Project. The Solar Project is based in South Africa and is based on cooperation between various companies which provides them with the different components for the installation of Resource Centers, that produce fertilizers of human waste. One of the components of these Resource centers are dry-toilets, which are provided by SUPERFUNKYFUTURE. This thesis is about an analysis of a toilet model and building a function tree, performing a 2D Volume of Fluid simulation on it to see even more critical geometries. Each one of those geometries had one alteration designed. These alterations were also combined into different models and they were all tested in both 2D and 3D simulations. The results from the simulations were then post-processed in CFD-Post and animated, to allow the qualitative analysis to be performed through a Pugh Matrix and decide between designs to recommend an improved internal geometry to SUPERFUNKYFUTURE.
Världshälsoorganisationen, WHO, beräknar att mer än två miljarder människor inte hade tillgång till toaletter 2017. Avföringen från dessa människor hamnade ofta i sjöar och floder och det vattnet användes sedan i sin tur för bevattning av grödor. Detta beräknas vara orsaken till en halv miljard dödsfall per år. Det finns för tillfället många initiativ att uppnå det globala målet för rent vatten och sanitet för alla, däribland arbetar Bill Gates och projekt så som “Solar Project”. “Solar Project” har sin bas i Sydafrika och är ett samarbete mellan flera företag som tillhandahåller olika komponenter som krävs för att skapa resurscenter, som kan användas för att producera gödsel av mänsklig avföring. En av komponenterna för dessa resurscenter är torrtoaletter som är försedda av SUPERFUNKYFUTURE. Denna tes handlar om att analysera en toalettmodell, bygga ett funktionsträd, utföra en 2D “Volume of Fluid”-simulering för att upptäcka kritiska geometrier. Varje sådan geometri fick en ändring designad. Dessa förändringar kombinerades till olika modeller som sedan blev testade med både 2D- och 3D-simuleringar. Resultaten från simuleringarna blev sedan bearbetade i CFD-Post och blev animerade, detta för att möjliggöra en kvalitativ analys genom en konceptviktningsmatris. Från Konceptviktningsmatrisen kunde man välja mellan designerna och rekommendera en förbättrad intern geometri till SUPERFUNKYFUTURE.

Intravenous fluid infusion during surgeries is based on clinical practice guidelines. Many factors impact the fluid distribution in the body, mainly the effect of anesthetic gases and surgical stress. Volume kinetics is a method to simulate the distribution and elimination of infusion fluids by considering the dilution of plasma over time. In this work, two volume kinetic models for fluid therapy are described – the single and two-fluid space model. The goal was to estimate five volume kinetic parameters for implementation in a population kinetic model. The method was based on data from an experiment at the University of Texas Medical Branch where the purpose was to examine the effect of the anesthetic gas isoflurane on fluid distribution after a controlled bleeding. In this project, measured hemoglobin concentrations from the experiment were used to determine the plasma dilution over time. Volume kinetic models were constructed by approximating terms in corresponding differential equations. As opposed to the single-fluid space model, the two-fluid space model gave a closer estimation to the experimental data. The two-fluid space model parameters were considered to be suitable for further population kinetic analysis.

Thesis: Ph. D. in Medical Engineering and Medical Physics, Harvard-MIT Program in Health Sciences and Technology, 2016.
"June 2016." Cataloged from PDF version of thesis.
Includes bibliographical references (pages 131-154).
Fluid volume status is a physiologic parameter that currently lacks a reliable diagnostic tool. Volume control becomes an issue during sickness and/or stress (physical and mental) in a wide range of populations. Unfortunately, current diagnostics suffer from being imprecise, invasive, and/or easily confounded and cannot unambiguously and practically inform volume status. There exists a need for a tool that can inform individuals and clinicians of fluid status in a noninvasive, rapid, and reliable manner. Drawing on the molecular sensitivity of IH nuclear magnetic resonance (NMR), we explored the ability of NMR methods to quantitate physiologic fluid volume changes. We first proved that NMR methods could detect volume changes in an animal model of dehydration. Correlation between NMR value changes in specific tissues and clinical tools used to assess dehydration validate NMR as a viable tool. We then proceeded to design and fabricate practical NMR sensors that could be easily integrated into the clinic. New methods of magnetic instrument design optimized for both field strength and spatial resolution were developed resulting in compact device prototypes with signal fidelity rivaling those of impractical commercial systems. Finally, we explored the ability of these devices to detect intravascular fluid changes during hemodialysis. Our methods and devices were able to detect intravascular blood property changes associated with blood dilution, in addition to overall fluid volume changes due to hemodialysis therapy. These results, methods, and devices provide the foundation and framework for the integration of NMR-based personalized fluid volume assessment into standard clinical practice.
by Matthew Li.
Ph. D. in Medical Engineering and Medical Physics

Numerical simulations of the complex flows of viscoelastic fluids are investigated. The viscoelastic fluids are modelled, primarily, via the Johnson-Segalman constitutive model. Our Numerical approach is based on finite volume method, based on the Johnson-Segalman constitutive model and implemented on the OpenFOAM® platform. The Johnson-Segalman model also easily reduces to the Oldroyd-B model under certain conditions of the material parameters. Since computations using the Oldroyd-B model have been extensively documented in the literature, we take advantage of the mathematical modelling connection between the Johnson-Segalman and Oldroyd-B models to validate the accuracy of our Johnson-Segalman solver via reduction to the Oldroyd-B model. Numerical validation of our results is conducted via the most commonly used benchmark problems. The final aim of our work is to assess the viability and efficiency of our numerical solver via an investigation into the complex fluid dynamical processes associated with shear banding.

Outlined is a new approach to the problem of surfacing particle-based fluid simulations. The key idea is to construct a surface that is as smooth as possible while remaining faithful to the particle locations. We describe a mesh-based algorithm that expresses the surface in terms of a constrained optimization problem. Our algorithm incorporates a secondary contribution in Marching Tiles, a generalization of the Marching Cubes isosurfacing algorithm. Marching Tiles provides guarantees on the minimum vertex valence, making the surface mesh more amenable to numerical operators such as the Bilaplacian.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2004.
Includes bibliographical references (leaves 160-166).
The thesis that follows consists of a collection of work supporting and extending a novel reformulation of fluid mechanics, wherein the linear momentum per unit mass in a fluid continuum, m, is supposed equal to the volume velocity v[sub]v. The latter differs from the barycentric velocity V[sub]m by the vector field j[sub]v, where j[sub]v = v[sub]v - v[sub]m represents the heretofore largely ignored diffusive transport of volume. We will begin by giving a motivating discussion containing example problems which point to the possible need for a change in the constitutive choice for in. This will be followed by a brief outline of the kinematic concepts necessary to understand and utilize volume transport, including a general expression for j[sub]v. We will conclude by presenting the modified governing equations that result from the constitutive choice m = v[sub]v. Upon completing the required overview of existing ideas, a detailed linear irreversible thermodynamic study of the modified governing equations which result from the choice m = v[sub]v is presented. This analysis yields, inter alia, an expression for the entropy production per unit volume in the fluid which requires that the deviatoric stress tensor be expressed in terms of the volume velocity. Furthermore, an expression for the diffusive flux of internal energy is derived that differs from classical results by a term proportional to the diffusive flux of volume. This change in the internal energy flux stems from the explicit recognition of a diffusive volume flux, and precedes any specific choice of constitutive expression for the molecular flux of heat or species.
(cont.) The remainder of the thesis, which constitutes the bulk of the work performed, focuses on testing the proposed equation set against known experimental data. Each of the physically measurable phenomena treated herein was previously believed outside the realm of classical continuum fluid dynamics. We begin by considering the thermophoretic and diffusiophoretic motion of particles suspended in gases or liquids. We continue by studying the thermo-molecular pressure drop which results from applying a temperature gradient across the ends of a closed capillary. We conclude by presenting a hydrodynamic/Brownian motion model of thermal diffusion in liquids, wherein theoretical predictions for the Soret coefficient in a binary liquid system are presented that may be evaluated from readily available physicochemical data. It is shown, in each case, that the predictions of our modified theory are in agreement with experimental data. The final chapter of this dissertation is dedicated to utilizing the results derived in the previous chapters to comment on the veracity of the claim that the specific linear momentum in a fluid is given by the volume, rather than the barycentric, velocity. General arguments supporting this claim are presented and then followed by a list of questions which remain to be answered. Finally, a list of proposed experiments are detailed which could further test the predictions made herein.
by James R. Bielenberg.
Ph.D.

Les cols composites des tuyères de propulseurs sont soumis à des conditions extrêmes qui entraînent une récession de la paroi ou ablation. Le but de l'étude est de calculer la réactivité effective du composite C/C et ainsi de mieux évaluer les épaisseurs consommées. Les cas des surfaces récessives et non-récessives ont été envisagés. Dans le second cas, le problème est résolu par prise de moyenne : la réactivité effective est alors inférieure à la moyenne arithmétique des réactivités. Dans le cas récessif, l'évolution de la rugosité est calculée par un outil original basé sur une méthode VOF. La réactivité effective est alors supérieure au cas non-récessif et égale au maximum des réactivités en régime contrôlé par la réaction. L'application aux matériaux C/C a permis d'identifier les réactivités élémentaires des composites par analyse inverse et de comparer le comportement de matériaux fortement non uniformes
Composite rocket throat parts suffer extreme conditions leading to a wall recession namedablation. The aim of the study is to compute the effective reactivity of the C/C composite in order to have a better evaluation of consumed thickness. The cases of recessive and non-recessive surfaces had been considered. In the second case, the problem is solved by volume averaging : the effective reactivity is then inferior to the arithmetic average of the reactivities. In the recessive case, the evolution of the surface is computed using an original VOF based code. The effective reactivity is then superior to the non-recessive case and equal to the maximum of the reactivities in a reaction-limited regime. The application to C/C allowed to identify of elementary reactivities by inverse analysis and to compare the ablative behavior of highly non-uniform materials

The main goal of this work is to propose, implement, test, and refine numerical methodologies for computer simulations of two-fluid flows. These methodologies fall into the category of volume tracking methods, with piecewise-linear interface calculation (PLIC). The scope of this work is limited to the laminar, incompressible flow of immiscible, non-reacting Newtonian fluids, without phase change, in planar two-dimensional geometries.
The following new or enhanced procedures are proposed: a parallelogram scheme for multidimensional advection of the volume fraction field, that rigorously conserves mass; a circle fit technique for the orientation of the interface segments and the calculation of curvature; a novel contact angle treatment; and a staggered formulation for volumetric body forces that can accurately balance pressure forces in the vicinity of the interface. In addition, surface-tension-derived and hydrostatic-derived pressure corrections are introduced as a novel means of calculating accurate pressure forces in cells that contain the interface, thereby virtually eliminating parasitic currents, or the non-physical flows that afflict many available volume tracking methods.
A total of six test problems are presented. The first three test problems do not involve surface tension, and are used to demonstrate the ability of the proposed method to accurately simulate two-fluid flows with complex interface deformations. These test problems involve pure advection flows, a collapsing water column, and a Richtmyer-Meshkov instability. The last three test cases are employed to check the effectiveness of the surface tension modelling. Simulations of a static drop indicate that the proposed curvature calculation procedure is of reasonable, but not very high, accuracy, and it is quite successful at maintaining a smooth, high fidelity interface. Next, it is shown that the proposed method can accurately simulate an oscillating bubble. In the final test case, the formation of a meniscus between two parallel plates is simulated. The equilibrium meniscus shape is in good agreement with the analytic meniscus solution. Overall, the proposed method is shown to be capable of accurate and stable simulations of the two-fluid flows considered in this work.

This Thesis describes the numerical simulation of fluid-structure interaction (FSI) problems. A finite-volume based stress analysis code was developed and coupled to an existing in-house CFD code to form a general purpose FSI solver capable of being used with the advanced turbulence and near-wall models developed within the research group. The code has been used to study a number of physiological flows in the present work, although the general nature of the solver allows it to be used for other applications also. By using the same numerical method, implemented in a consistent manner, for both fluid and solid domains, the inefficiencies associated with using separate packages for the fluid and solid were avoided. Separate packages typically store information in different data structures; some form of software interface is required to transfer information between the two packages. This additional software layer, which is called during each FSI iteration, causes a considerable overhead. By using a single numerical mesh across both domains, the inaccuracies associated with boundary interpolation were also avoided. Typically, separate packages use meshes which do not conform at their common boundary. In order to find nodal values of the fluid pressure, say, at the solid nodes, some form of interpolation is necessary. The interpolation leads to the introduction of truncation errors. These improvements allow for more accurate and efficient FSI simulations, particularly transient cases, to be performed. The solid solver was verified against analytical solutions for a number of test cases, including: planar stress distribution in a square plate with a circular hole in the centre; axisymmetric stress in a thick walled cylinder under internal pressure, and unsteady displacement of a cantilevered beam under free vibration. Before coupled analyses were performed, the flow solver was also validated through a number of rigid walled test cases, including steady flow through a stenosed tube and unsteady flow through an aneurysm. Many physiological flows are difficult to capture due to flow separation and early transition to turbulence. The use of a low-Reynolds number k-ε turbulence model was successful at capturing the flow field over a range of physiologically relevant flow rates. Once the solid body and flow solvers had been validated in isolation, they were coupled together and applied to a number of physiological flows, namely: steady flow through an initially straight tube with a compliant wall; steady flow through a compliant stenosis, and unsteady flow through a compliant aneurysm. The results from all three test cases showed good agreement with the available experimental and numerical data in terms of wall deformation. The solid body solver also proved itself to be capable of producing high quality numerical meshes for use in other simulations. The fluid mesh was considered to be a solid body with arbitrary material properties; the required deformation was specified as prescribed displacement boundary conditions. The main benefit of this method, compared to simple elliptical grid generation methods, is that near-wall grid spacing was preserved throughout the coupled simulation.

Classically in an elastohydrodynamic (EHD) problem, the Reynolds equation is the most widely used PDE to describe the behaviour of lubricants in high-pressure non-conforming contacts, and elastic deformation is usually calculated using the Hertzian theory of elastic contacts. This thesis outlines the development of a new method for modelling of fluid-solid interactions in elastohydrodynamic lubrication (EHL) contact based on Finite Volume (FV) techniques. A Computational Fluid Dynamics (CFD) approach to solve the Navier-Stokes equations is implemented to model lubrication in roller bearings using the open-source package OpenFOAM. This has first been applied to simulate full film hydrodynamic lubrication (HL), enabling an accurate description of the flow within the entire domain surrounding the contact region. The rheology is assumed to be non-Newtonian and shear-thinning. The phenomenon of cavitation is modelled by implementing a homogenous equilibrium cavitation model, which maintains specified lubricant saturation pressure in cavitating region. The current fluid solver involves the solution of the full momentum and energy equations, and satisfying continuity. The aim is firstly to demonstrate the range of applicability and the limitations of traditional formulations of the Reynolds equation and secondly to highlight areas where Navier-Stokes based approaches are necessary for accurate solution of lubrication problems. Subsequently, a finite volume solid solver is fully coupled with the fluid solver in a forward iterative manner to take into account elastic deflection effects using Navier-Lamé equation. The advantage of using a single numerical tool enables an internal transfer of information at the fluid-solid interface through one common data structure. The stability of the model, in the presence of high contact pressures, is enhanced by incorporation of multigrid method, implicit coupling and improved mesh adaption and motion techniques. The developed model has been applied to a series of lubricated metal on metal smooth line contact with slide to roll ratios ranging from 0 to 2 and is stable for a wide range of industrial operating conditions (pressures up to 4 GPa). The model is further improved to account for time-dependent transient behaviour of an EHL rough contact. The results for a travelling ridge, dent and sinusoidal wave through EHL conjunction are presented.

The regulation of fluid (water) volume in the body is crucial for tissue homeostasis. The interstitial fluid, which comprises almost 20% of the body fluid, is stored in the loose connective tissue and its volume is actively regulated by components of this tissue. The loose connective tissue provides a path for fluid flow from capillaries to the tissue and lymphatics. This fluid is partially stored in the interstitium and the remainder is directed to the lymphatics. The fibroblasts in the loose connective tissue actively compact the fibrous extracellular matrix (ECM) through mechanotransduction via integrins. This in turn, maintains the interstitial fluid pressure and keeps the ground substance underhydrated. The interstitial fluid pressure is part of the forces that regulate the efflux of fluid from capillaries and keep the ground substance underhydrated. The underhydrated ground substance has a potential to take up fluid 3-fold the plasma volume. Therefore, the active contraction of the ECM via fibroblasts is crucial to prevent the risk of evacuation of fluid from capillaries. During pathologies, such as inflammation and carcinogenesis, the interstitial fluid pressure and hence the interstitial fluid volume is altered. The results presented in this thesis show that the signaling events downstream of αVβ3 integrin, collagen-binding β1 integrins, and platelet-derived growth factor receptor β, that induce cell-mediated matrix contraction, included paired function of PI3K and PLCγ, cofilin activation, actin turnover, and generation of actomyosin forces. Furthermore, the results highlight new potential roles for fibrin and αVβ3 integrins, for instance during clearance of edema. Notably, fibrin extravasation at inflammatory sites induced αVβ3 integrin-dependent matrix contraction, leading to normalization of the altered interstitial fluid volume. It also reprograms the expression of ECM-related genes and hence induces ECM turnover. Taken together, these results provide further insight into the regulatory mechanism through which the loose connective tissue actively regulates the interstitial fluid volume.

A discussion is given of numerical techniques for modelling fluid flows with free surfaces, covering the basic principles behind the schemes and their respective advantages and disadvantages. This leads to the adoption of a two dimensional "Volume of Fluid" (VOF) method for the simulation of wave impacts on vertical walls. The VOF method is described in detail, including enhancements to various sections of the algorithm. The "water piston" theory of air pockets is given, together with a scheme to incorporate the effects into the model. The results of a test problem are given which illustrates the ability of the VOF method to simulate free surface flows, and another test case validates the approach to air pocket modelling. Finally a series of comparisons with experimental results of wave impacts is given; these include time series of surface elevations and wall pressures. The results illustrate the difficulty in making direct comparisons with experiment due to the problem of ensuring identical wave conditions and show that, given this constraint, the VOF model is capable of accurate simulation of impact events.

Carbonated sparkling water has been widely used from ancient age [1]. The original ideacame from natural sparkling water and people believed that taking baths at carbonatedhot springs was good for health and healed their sicknesses. This fact led people to startthinking that sparkling water could have more effective uses. Joseph Priestley success-fully produced artificial carbonated water in 1767 and sparkling water quickly becamewidely spread because it gives people refreshing feeling. The bottled and canned beverageindustry has grown from the 19th century and has become one of the biggest markets inthe world. According to Bloomberg Intelligence and Euromonitor, the global market ofthe carbonated beverages is around 350 billion dollar. One main drawback was that itwas not possible to re-cork the bottle to save the carbonation so that once it was opened,fizz was kept only for a short time. In 1813, the method to dispense a portion of carbon-ated water was invented by Charles Plinth[2]. This was the origin of the Soda Syphon.As the demand of sparkling water increased, the machine with which people could makesparkling water by themselves was introduced. Recently, it has become a very popularhome appliance, especially in Europe and North America. The most common way tocarbonate water is by injecting high-pressure CO2 into a water bottle. However, currentsystems waste a lot of CO2 during this carbonating process. In this thesis, the flow insidethe bottle during the injection of CO2 into water was studied in order to determine the pa-rameters that had most influence on the carbonation process. CFD (Computational FluidDynamics) simulations were performed in STAR-CCM+ of an axisymmetric 2D modeland a 3D model that was a 30 degree wedge of the real bottle shape. The Volume of Fluidmethod was used to solve the multiphase flow of gas and liquid. The RANS approachwas used with k 􀀀ϵ model and implicit time marching. To validate the simulations, axialpropagation of the volume fraction of CO2 was compared with the experimental visual-ization of the CO2 and H2O distribution. At the beginning of the phenomena, the gaspropagation was reasonably predicted and the results capture the features of the bubbleshape. However the results did not perfectly match with the experimental visualization.To seek the reason for the unrealistic results, the grid sensitivity study was performedand to consider the 3D effect the results with the 2D and the 3D model were compared.In addition, the bubble breakup process was deeply investigated.

This study focuses on developing an efficient, fully three dimensional Finite Volume Method for viscoelastic fluid flow problems, and on numerical investigations of some complex rheological phenomena.

This work describes the development of a framework for numerical simulation software, using the finite volume method. A major guideline has been flexibility. The framework is written in C++, making strong use of its object-oriented capabilities. Outlined are the benefits, as well as the pitfalls related to object-oriented programming, if used for numerical simulations. The text explains that flexibility has not only been tried to achieve in terms of software design, but also with respect to the numerical approaches used. The field of application in the scope of this work is computational fluid dynamics. Thus a brief overview over the necessary equations and the employed numerical techniques is given. Furthermore a number of example computations can be found in this text. An important part of this work deals with a novel approach for unstructured mesh generation. The approach is based on multi element type grids and uses level-sets as input to describe the geometry. It is well suited to create anisotropic layers, such as boundary layer grids for fluid dynamics problems. Furthermore it can deal with moving and even topologically changing geometries. In the scope of this work it is limited to two-dimensional problems.

An equal-order co-located Control Volume Finite Element Method (CVFEM) for the prediction of multidimensional, incompressible, viscous fluid flow problems has been formulated. CVFEMs provide the geometric flexibility traditionally associated with finite element methods. In addition, their control volume based formulation facilitates physically meaningful interpretation of the resulting discretization equations.
In the proposed CVFEM, the calculation domain is divided into three-node triangular and four-node tetrahedral finite elements in two- and three-dimensions, respectively. Each element is further subdivided in such a way that upon assembly of all elements, complete control volumes are formed about each node in the calculation domain. Interpolation functions for the dependent variables are prescribed in a manner that is consistent with the physical process they are intended to approximate. In this context, three different interpolation schemes of the convective flux across control volume surfaces are investigated, one of which guarantees positive contributions to the coefficients in the algebraic discretization equations. Appropriate conservation laws are imposed on the control volumes associated with the nodes. The resulting sets of integral conservation equations are then approximated by algebraic discretization equations, using the previously-mentioned interpolation functions. These nonlinear, coupled, algebraic equations are solved by a sequential solution procedure which incorporates Picard iterations.
The proposed method has been implemented into computer programs, and used to solve several test problems. These include convection-diffusion problems, and laminar and turbulent flow problems, in both two- and three-dimensions. The results demonstrate the ability of the proposed CVFEM to accurately solve the mathematical model used in this thesis.
Lastly, the CVFEM was used to predict flows similar to those found in film cooled gas turbine aerofoils. A complementary experimental program was designed and set up to investigate such flows. The numerical predictions were compared to the experimental observations of mean velocities, normal turbulent stresses and one component of the turbulent shear stress. These comparisons indicated that the high Reynolds number $k- epsilon$ turbulence model used in this thesis is unable to capture certain features of the flow. It appears as if a low-Reynolds number turbulence model, with appropriate modifications to account for streamline curvature and non-isotropy of the turbulence in the vicinity of the walls, would be better suited to the prediction of such flows. This new model can be easily incorporated into the CVFEM proposed in this thesis.

La compréhension des écoulements multiphasiques en milieu poreux revêt une importance capitale dans de nombreuses applications industrielles et environnementales, à des échelles spatiales et temporelles variées. Par conséquent, la présente étude propose une modélisation des écoulements multiphasiques en milieu poreux par le biais de la méthode Volume de Fluide, et présente des simulations de digitations de Saffman-Taylor, motivées par l'analyse d'expériences de balayage dans des blocs de grès de Bentheimer quasi bidimensionnels initialement saturés en huile extra-lourde par de l'eau. Le code Gerris, permettant des calculs parallèles efficaces à l'aide d'un maillage de type octree, est utilisé. Des tests de précision et de rapidité de calcul sont réalisés à l'aide de divers niveaux de raffinement, ainsi qu'une comparaison avec des simulations de référence dans la littérature. Des simulations 3D dans des milieux réels numérisés sont réalisés avec des résultats encourageants. Même s'il n'est pas encore possible d'atteindre des nombres capillaires réalistes, des écoulements dans des domaines cubiques de 1 mm de côté sont simulés, avec un temps de calcul raisonnable. Des simulations 2D de digitations visqueuses avec injection centrale ou latérale sont également présentées, basées sur la loi de Darcy. L'aspect fractal des digitations est étudié aussi bien à l'aide de la dimension fractale que de la variation de l'aire des motifs obtenus par rapport à leur périmètre. Enfin, des balayages à l'aide de polymères suivant des balayages à l'eau dans un processus en deux temps sont simulés à partir d'une modélisation darcéenne
Understanding multiphase flow in porous media is of tremendous importance for many industrial and environmental applications at various spatial and temporal scales. The present study consequently focuses on modelling multiphase flows by the Volume-of-Fluid method in porous media and shows simulations of Saffman-Taylor fingering motivated by the analysis of waterflooding experiments of extra-heavy oils in quasi-2D square slab geometries of Bentheimer sandstone. The Gerris code which allows efficient parallel computations with octree mesh refinement is used. It is tested for accuracy and computational speed using several levels of refinement and comparing to reference simulations in the literature. Simulations of real rocks are realised in three dimensions with very promising results. Though it is not yet possible to attain realistic capillary numbers, it is possible to simulate flows in domains of physical size up to 1 mm3 in reasonable CPU time. 2D simulations of viscous fingering with both central and lateral injection are also presented in this study, based on Darcy's law. The fractal aspect of this fingering is studied by considering both its fractal dimension and the variation of the area of the resulting pattern with respect to its arclength. Finally, polymer flooding following waterflooding in a two-step process is simulated with Darcy modelling

This thesis aims to study and explore the drop-weight impact-induced solidification process of an STF, consisting of 58 vol% styrene/acrylate particles in ethylene glycol, in a finite volume. The study begins with the characterisation of the mechanical behaviours (i.e., rheological and confined compression behaviours) of the STF. Low-velocity drop-weight impact experiments are conducted to investigate the effect of the STF’s dimensions in a finite volume on the impact behaviour of the STF. It is found that the impact behaviour is related to both the depth and the diameter of the STF. A new model is therefore proposed that the solidification front of an STF advances linearly to the impact velocity with a constant ratio in both the normal and radial directions, respectively, forming a semi-ellipse-like region which is captured by a direct observation with a high-speed camera. When this front propagates to one boundary, a force transmits back to the impact head. The interaction is detected by the load cell and piezoelectric transducers at the boundaries. Moreover, the coupled Eulerian-Lagrangian model and the volume of fluid model are adopted to simulate the development of the solidification front. In both models, the continuous propagation of the solidification front is depicted by expanding of a high-strain-rate region in all directions. The energy absorption under the drop-weight impact is found to decrease with an increase in the depth or width dimension of the STF before their critical dimension is reached due to the extension of the solidification period. Finally, the displacement-control oscillations are conducted on the STF to further explore its reciprocating performance for characterising the resistant force and energy absorption. It is found that the amplitude of displacement has a clear effect on the resistant force and energy absorption, while the frequency has little influence on them after the activation of the shear thickening.