DNS of multiphase flows: study of atomization and free-surface phenomena

The present thesis focuses on the numerical analysis of some diverse physical set-up that involve the interaction of two -or three immiscible and incompressible phases. The simulations are carried out by means of finite-volume algorithms developed on the in-house Computational Fluid Dynamics platform TermoFluids, developed by the Heat and Mass Transfer Technological Center (CTTC). They are intended to give detailed insights on the physics of the analyzed phenomena by carrying out Direct Numerical Simulations (DNS). In the context of multiphase flows, namely, Computational Multi-Fluid Dynamics (CMFD) field, DNS means that all the interfacial and turbulent scales of the phenomenon must be fully resolved. In the Introduction, a general overview of the engineering applications and the computational methods related to multiphase flows is proposed. The various types of physics analyzed in this work and the numerical approaches applied here to carry out efficient simulations are introduced. In Chapter 2, a low-dissipation convection scheme for the stable discretization of multiphase flow by means of interface-capturing schemes is analyzed. The hybrid form of the convective operator proposed incorporates localized low-dispersion characteristics to limit the growth of spurious flow solutions. Moreover, in comparison to pure-dissipative schemes, the discretization aims at minimizing the differences in kinetic energy preservation with respect to the continuous governing equations. This property plays a fundamental role in the case of flows presenting significant levels of turbulence. The simulation of a turbulent 2D coaxial jet with the low-dissipation convection scheme demonstrates its capability of solving correctly the two-phase turbulent problems. In Chapter 3, all the work carried out on the simulation of two-phase flow with the aid of Adaptive Mesh Refinement (AMR) strategies is described. The model is globally addressed at improving the representation of interfacial and turbulent scales in general multiphase flows. It is first applied to the simulation of simple multiphase phenomena, as 2D and 3D rising bubbles, to demonstrate the convergence of the method and the important computational savings in comparison to static mesh computations. However, its adoption becames essential in the simulation of instability and break-up phenomena, where the necessity of representing accurately the complex structures that appear at the interface, as ligaments and droplets, make the simulation particularly expensive in terms of computational cost. In Chapter 4, we analyze in detail the simulations of 3-D atomizing phenomena, including the coaxial jet case, characterized by the parallel injection of high speed liquid and gas fluxes, and the liquid spray case, characterized by the injection of a high speed liquid inside a still air chamber. In Chapter 5, an original single-phase scheme for the DNS of free-surface problems on 3-D unstructured meshes is presented. The scheme is based on a novel treatment of the interface for the deactivation of the light-phase, allowing an optimization of the classic two-phase model for the cases in which the influence of the lighter phase is negligible. Consequently, the model is particularly addressed at analyzing problems involving the movement of free-surfaces, as the evolution of waves on the sea, and their interaction with fixed and moving obstacles. Some practical cases of application are proposed, as the evaluation of stresses on an object due to the action of a dam-break event, and the interaction of linear waves with an oscillating water column device. In the same Chapter we describe the procedure to couple the single-phase model to the Immersed Boundary Method. The method is aimed at representing the interaction of a solid moving with prescribed velocity and the two-phase flow. The most significant example consists in the simulation of a sliding wedge into a liquid basin.Esta tesis se focaliza en la simulación numérica de algunos set-up físicos que involucran la interacción entre dos o tres fluidos incompresibles y immiscibles. Las simulaciones se realizan por medio de algoritmos de volúmenes-finitos desarrollados en la plataforma propia de Fluido-Dinámica Computacional (CFD) denominada TermoFluids, desarrollada en el Centro Tecnologico de Trasferencia de Calor (CTTC). Las simulaciones quieren estudiar en detalle la física de los fenómenos analizados, realizando su Simulación Numérica Directa (DNS). En el contexto de los flujos multifase, DNS significa que todas las escalas interfaciales y turbulentas del fenómeno han de ser totalmente resueltas. En la Introducción, se propone una panorámica general de las aplicaciones de ingeniería y de los métodos computacionales relacionados con flujos multifases. Se introducen los varios tipos de física analizados en este trabajo y las estrategias numéricas aplicadas aquí para efectuar su simulación de manera eficiente. En el Capitulo 2 se analiza un esquema convectivo de baja-disipación para la discretization de flujo multifase por medio de métodos de interface-capturing. La forma híbrida del operador convectivo propuesto incorpora la característica de una baja dispersión localizada, focalizada en limitar el crecimiento de soluciones numéricas espurias. Además, en comparación con métodos disipativos puros, la discretización apunta a minimizar las diferencias en la conservación de energía cinética en respeto a las ecuaciones continuas que gobiernan el flujo. Esta propiedad juega un papel fundamental en el caso de flujo caracterizado por un alto nivel de turbulencia. La simulación de un jet 2D coaxial turbulento con el método convectivo de baja disipación demuestra su capacidad de resolver correctamente un flujo de dos fases turbulentos. En el Capitulo 3 se reporta todo el trabajo realizado sobre la simulación de flujo multifase con el auxilio de técnicas de refinamiento adaptativo de malla (AMR). El modelo es globalmente dirigido a la mejora de la representación de las escalas turbulentas y interfaciales en flujos multifases en general. Se aplica inicialmente a la simulación de flujos sencillos, como unos casos de burbujas flotantes 2D y 3D, demostrando la convergencia del método y los importantes ahorros computacional en comparación con los cálculos de mallas estáticas. La adopción de la técnica se hace esencial en la simulación de fenómenos de inestabilidad y de ruptura, donde la necesidad de representar sacramentalmente las estructuras complejas que aparecen en la interfaz, como ligamentos o pequeñas gotas, hacen que la simulación sea particularmente pesada en términos de coste computacional. En el Capitulo 4 se reportan en detalle las simulaciones de fenómenos de atomización 3D. Esas incluyen el caso del jet coaxial, caracterizado por la inyección paralela de flujos de aire y liquido de altas velocidades, y el caso del spray liquido, que consiste en la inyección de un liquido dentro de una cámara de aire. En el Capitulo 5 se presenta un esquema de single-phase original, para el DNS de problemas de superficie libre en mallas 3D no-estructuradas. El esquema se basa en un nuevo tratamiento de la interfase para la desactivación de la fase ligera, permitiendo la optimización del solver clásico de dos fases para los casos en que la influencia de la fase mas ligera sea despreciable. En consecuencia, el modelo es particularmente indicado para la análisis de problemas que involucran el movimiento de superficies libres, como la evolución de olas en la superficie marina y su interacción con obstáculos fijos o muebles. Se proponen algunos casos prácticos de aplicación, como la evaluación de las fuerzas sobre un objeto debidos a un episodio de dam-break, o el estudio de las olas generadas por el impacto de un solido deslizante (representado integrando la tecnica de Immersed Boundary con el presente metodo de single-phase) con un embalse de agua.Postprint (published version

DNS of multiphase flows: study of atomization and free-surface phenomena

The present thesis focuses on the numerical analysis of some diverse physical set-up that involve the interaction of two -or three immiscible and incompressible phases. The simulations are carried out by means of finite-volume algorithms developed on the in-house Computational Fluid Dynamics platform TermoFluids, developed by the Heat and Mass Transfer Technological Center (CTTC). They are intended to give detailed insights on the physics of the analyzed phenomena by carrying out Direct Numerical Simulations (DNS). In the context of multiphase flows, namely, Computational Multi-Fluid Dynamics (CMFD) field, DNS means that all the interfacial and turbulent scales of the phenomenon must be fully resolved. In the Introduction, a general overview of the engineering applications and the computational methods related to multiphase flows is proposed. The various types of physics analyzed in this work and the numerical approaches applied here to carry out efficient simulations are introduced. In Chapter 2, a low-dissipation convection scheme for the stable discretization of multiphase flow by means of interface-capturing schemes is analyzed. The hybrid form of the convective operator proposed incorporates localized low-dispersion characteristics to limit the growth of spurious flow solutions. Moreover, in comparison to pure-dissipative schemes, the discretization aims at minimizing the differences in kinetic energy preservation with respect to the continuous governing equations. This property plays a fundamental role in the case of flows presenting significant levels of turbulence. The simulation of a turbulent 2D coaxial jet with the low-dissipation convection scheme demonstrates its capability of solving correctly the two-phase turbulent problems. In Chapter 3, all the work carried out on the simulation of two-phase flow with the aid of Adaptive Mesh Refinement (AMR) strategies is described. The model is globally addressed at improving the representation of interfacial and turbulent scales in general multiphase flows. It is first applied to the simulation of simple multiphase phenomena, as 2D and 3D rising bubbles, to demonstrate the convergence of the method and the important computational savings in comparison to static mesh computations. However, its adoption becames essential in the simulation of instability and break-up phenomena, where the necessity of representing accurately the complex structures that appear at the interface, as ligaments and droplets, make the simulation particularly expensive in terms of computational cost. In Chapter 4, we analyze in detail the simulations of 3-D atomizing phenomena, including the coaxial jet case, characterized by the parallel injection of high speed liquid and gas fluxes, and the liquid spray case, characterized by the injection of a high speed liquid inside a still air chamber. In Chapter 5, an original single-phase scheme for the DNS of free-surface problems on 3-D unstructured meshes is presented. The scheme is based on a novel treatment of the interface for the deactivation of the light-phase, allowing an optimization of the classic two-phase model for the cases in which the influence of the lighter phase is negligible. Consequently, the model is particularly addressed at analyzing problems involving the movement of free-surfaces, as the evolution of waves on the sea, and their interaction with fixed and moving obstacles. Some practical cases of application are proposed, as the evaluation of stresses on an object due to the action of a dam-break event, and the interaction of linear waves with an oscillating water column device. In the same Chapter we describe the procedure to couple the single-phase model to the Immersed Boundary Method. The method is aimed at representing the interaction of a solid moving with prescribed velocity and the two-phase flow. The most significant example consists in the simulation of a sliding wedge into a liquid basin.Esta tesis se focaliza en la simulación numérica de algunos set-up físicos que involucran la interacción entre dos o tres fluidos incompresibles y immiscibles. Las simulaciones se realizan por medio de algoritmos de volúmenes-finitos desarrollados en la plataforma propia de Fluido-Dinámica Computacional (CFD) denominada TermoFluids, desarrollada en el Centro Tecnologico de Trasferencia de Calor (CTTC). Las simulaciones quieren estudiar en detalle la física de los fenómenos analizados, realizando su Simulación Numérica Directa (DNS). En el contexto de los flujos multifase, DNS significa que todas las escalas interfaciales y turbulentas del fenómeno han de ser totalmente resueltas. En la Introducción, se propone una panorámica general de las aplicaciones de ingeniería y de los métodos computacionales relacionados con flujos multifases. Se introducen los varios tipos de física analizados en este trabajo y las estrategias numéricas aplicadas aquí para efectuar su simulación de manera eficiente. En el Capitulo 2 se analiza un esquema convectivo de baja-disipación para la discretization de flujo multifase por medio de métodos de interface-capturing. La forma híbrida del operador convectivo propuesto incorpora la característica de una baja dispersión localizada, focalizada en limitar el crecimiento de soluciones numéricas espurias. Además, en comparación con métodos disipativos puros, la discretización apunta a minimizar las diferencias en la conservación de energía cinética en respeto a las ecuaciones continuas que gobiernan el flujo. Esta propiedad juega un papel fundamental en el caso de flujo caracterizado por un alto nivel de turbulencia. La simulación de un jet 2D coaxial turbulento con el método convectivo de baja disipación demuestra su capacidad de resolver correctamente un flujo de dos fases turbulentos. En el Capitulo 3 se reporta todo el trabajo realizado sobre la simulación de flujo multifase con el auxilio de técnicas de refinamiento adaptativo de malla (AMR). El modelo es globalmente dirigido a la mejora de la representación de las escalas turbulentas y interfaciales en flujos multifases en general. Se aplica inicialmente a la simulación de flujos sencillos, como unos casos de burbujas flotantes 2D y 3D, demostrando la convergencia del método y los importantes ahorros computacional en comparación con los cálculos de mallas estáticas. La adopción de la técnica se hace esencial en la simulación de fenómenos de inestabilidad y de ruptura, donde la necesidad de representar sacramentalmente las estructuras complejas que aparecen en la interfaz, como ligamentos o pequeñas gotas, hacen que la simulación sea particularmente pesada en términos de coste computacional. En el Capitulo 4 se reportan en detalle las simulaciones de fenómenos de atomización 3D. Esas incluyen el caso del jet coaxial, caracterizado por la inyección paralela de flujos de aire y liquido de altas velocidades, y el caso del spray liquido, que consiste en la inyección de un liquido dentro de una cámara de aire. En el Capitulo 5 se presenta un esquema de single-phase original, para el DNS de problemas de superficie libre en mallas 3D no-estructuradas. El esquema se basa en un nuevo tratamiento de la interfase para la desactivación de la fase ligera, permitiendo la optimización del solver clásico de dos fases para los casos en que la influencia de la fase mas ligera sea despreciable. En consecuencia, el modelo es particularmente indicado para la análisis de problemas que involucran el movimiento de superficies libres, como la evolución de olas en la superficie marina y su interacción con obstáculos fijos o muebles. Se proponen algunos casos prácticos de aplicación, como la evaluación de las fuerzas sobre un objeto debidos a un episodio de dam-break, o el estudio de las olas generadas por el impacto de un solido deslizante (representado integrando la tecnica de Immersed Boundary con el presente metodo de single-phase) con un embalse de agua

Unified one-fluid formulation for incompressible flexible solids and multiphase flows: Application to hydrodynamics using the immersed structural potential method (ISPM)

In this paper, we present a two-dimensional computational framework for the simulation of fluid-structure interaction problems involving incompressible flexible solids and multiphase flows, further extending the application range of classical immersed computational approaches to the context of hydrodynamics. The proposed method aims to overcome shortcomings such as the restriction of having to deal with similar density ratios among different phases or the restriction to solve single-phase flows. First, a variation of classical immersed techniques, pioneered with the immersed boundary method (IBM), is presented by rearranging the governing equations, which define the behaviour of the multiple physics involved. The formulation is compatible with the “one-fluid” formulation for two-phase flows and can deal with large density ratios with the help of an anisotropic Poisson solver. Second, immersed deformable structures and fluid phases are modelled in an identical manner except for the computation of the deviatoric stresses. The numerical technique followed in this paper builds upon the immersed structural potential method developed by the authors, by adding a level set–based method for the capturing of the fluid-fluid interfaces and an interface Lagrangian-based meshless technique for the tracking of the fluid-structure interface. The spatial discretisation is based on the standard marker-and-cell method used in conjunction with a fractional step approach for the pressure/velocity decoupling, a second-order time integrator, and a fixed-point iterative scheme. The paper presents a wide d range of two-dimensional applications involving multiphase flows interacting with immersed deformable solids, including benchmarking against both experimental and alternative numerical schemes

Development of a Particle Interaction Kernel Function in MPS Method for Simulating Incompressible Free Surface Flow

We aimed to derive a kernel function that accounts for the interaction among moving particles within the framework of particle method. To predict a computationally more accurate moving particle solution for the Navier-Stokes equations, kernel function is a key to success in the development of interaction model. Since the smoothed quantity of a scalar or a vector at a spatial location is mathematically identical to its collocated value provided that the kernel function is chosen to be the Dirac delta function, our guideline is to derive the kernel function that is closer to the delta function as much as possible. The proposed particle interaction model using the newly developed kernel function will be validated through the two investigated Navier-Stokes problems which have either the semianalytical or the benchmark solutions

Numerical simulation of multiphase flows : level-set techniques

This thesis aims at developing numerical methods based on level-set techniques suitable for the direct numerical simulation (DNS) of free surface and interfacial flows, in order to be used on basic research and industrial applications. First, the conservative level-set method for capturing the interface between two fluids is combined with a variable density projection scheme in order to simulate incompressible two-phase flows on unstructured meshes. All equations are discretized by using a finite-volume approximation on a collocated grid arrangement. A high order scheme based on a flux limiter formulation, is adopted for approximating the convective terms, while the diffusive fluxes are centrally differenced. Gradients are computed by the least-squares approach, whereas physical properties are assumed to vary smoothly in a narrow band around the interface to avoid numerical instabilities. Surface tension force is calculated according to the continuous surface force approach. The numerical method is validated against experimental and numerical data reported in the scientific literature. Second, the conservative level-set method is applied to study the gravity-driven bubbly flow. Unlike the cases presented in the first part, a periodic boundary condition is applied in the vertical direction, in order to mimic a channel of infinite length. The shape and terminal velocity of a single bubble which rises in a quiescent liquid are calculated and validated against experimental results reported in the literature. In addition, different initial arrangements of bubble pairs were considered to study its hydrodynamic interaction, and, finally the interaction of multiple bubbles is explored in a periodic vertical duct, allowing their coalescence. In the third part of this thesis, a new methodology is presented for simulation of surface-tension-driven interfacial flows by combining volume-of-fluid with level-set methods. The main idea is to benefit from the advantage of each strategy, which is to minimize mass loss through the volume-of-fluid method, and to keep a fine description of the interface curvature using a level-set function. With the information of the interface given by the volume-of-fluid method, a signed distance function is reconstructed following an iterative geometric algorithm, which is used to compute surface tension force. This numerical method is validated on 2D and 3D test cases well known in the scientific literature. The simulations reveal that numerical schemes afford qualitatively similar results to those obtained by the conservative level-set method. Mass conservation is shown to be excellent, while geometrical accuracy remains satisfactory even for the most complex cases involving topology changes. In the fourth part of the thesis a novel multiple marker level-set method is presented. This method is deployed to perform numerical simulation of deformable fluid particles without numerical coalescence of their interfaces, which is a problem inherent to standard interface tracking methodologies (e.g. level-set and volume of fluid). Each fluid particle is described by a separate level-set function, thus, different interfaces can be solved in the same control volume, avoiding artificial and potentially unphysical coalescence of fluid particles. Therefore, bubbles or droplets are able to approach each other closely, within the size of one grid cell, and can even collide. The proposed algorithm is developed in the context of the conservative levelset method, whereas, surface tension is modeled by the continuous surface force approach. The pressure-velocity coupling is solved by the fractional-step projection method. For validation of the proposed numerical method, the gravity-driven impact of a droplet on a liquid-liquid interface is studied; then, the binary droplet collision with bouncing outcome is examined, and finally, it is applied on simulation of gravity-driven bubbly flow in a vertical column. The study of these cases contributed to shed some light into physics present in bubble and droplet flows.Ésta tesis se enfoca en el desarrollo de métodos numéricos basados en la aplicación de técnicas level-set para la Simulación Numérica Directa (DNS) de flujos interfaciales y flujos de superficie libre, con el objetivo de ser usados tanto en investigación básica como en aplicaciones industriales. Primero, el método level-set conservativo desarrollado para la captura de interfaces entre dos fluidos, es combinado con un esquema de proyección adaptado para un fluido de densidad variable, con el objetivo de simular flujos de dos fases en mallas no estructuradas. Todas las ecuaciones son discretizadas mediante una aproximación de volúmenes finitos sobre un arreglo de malla colocada. Un esquema de alto orden cuya formulación se basa en el uso de limitadores de flujo, es usado para la discretización de los términos convectivos, mientras que los flujos difusivos son calculados mediante diferencias centradas. Los gradientes son calculados mediante el método de los mínimos cuadrados, en tanto que se asume que las propiedades físicas varían suavemente en una zona estrecha alrededor de la interface con el objetivo de evitar inestabilidades numéricas. La tensión superficial es incorporada mediante el enfoque de la fuerza superficial continua. El método numérico es validado con respecto a los datos experimentales y numéricos reportados en la literatura científica. Segundo, el método level-set conservativo es aplicado en el estudio del flujo de burbujas conducidas por la gravedad. A diferencia de los casos precedentes, se aplica una condición de frontera periódica en la dirección vertical, con el objetivo de simular un canal de longitud infinita. La forma y velocidad terminal de una burbuja ascenciendo en un líquido inicialmente en reposo son calculadas y contrastadas con los resultados reportados en la literatura. Adicionalmente se estudia la interacción hidrodinámica de un par de burbujas para diferentes configuraciones, y finalmente se explora la interacción de un emjambre de burbujas ascendiendo en un canal vertical. En la tercera parte de ésta tesis, se presenta una nueva metodología para la simulación de flujos interfaciales conducidos por la tensión superficial, mediante la combinación de los métodos volume-of-fluid y level-set. La idea principal se basa en usar el método volume-of-fluid para advectar la interface, minimizando las pérdidas de masa, mientras que las propiedades geométricas de la interface se calculan a partir de una función level-set obtenida mediante un algoritmo geométrico iterativo. La propiedades geométricas así calculadas son usadas para el cómputo de la tensión superficial. El método numérico es validado mediante casos bi y tri-dimensionales bien conocidos en la literatura científica. La conservación de la masa es excelente en tanto que la precisión del método es altamente satisfactoria incluso en los casos más complejos. En la cuarta parte de ésta tesis se presenta un nuevo método level-set de múltiples marcadores. Éste método es diseñado para llevar a cabo simulaciones numéricas de partículas de fluido deformables, evitando la coalescencia numérica de las interfaces. Cada partícula de fluido es capturada por una función level-set distinta, así, diferentes interfaces pueden ser resueltas en el mismo volumen de control, evitando la coalescencia artificial y potencialmente no-física de las partículas fluidas. Por lo tanto, las burbujas (o gotas) pueden acercarce y colisionar. El algoritmo es propuesto en el contexto del método level-set conservativo, mientras que la tensión superficial se resuelve mediante una adaptación del enfoque de la fuerza superficial continua. Para su validación, se estudia el impacto conducido por la gravedad de una gota sobre una interface líquido-líquido; luego, se estudia la collisión de dos gotas con salida rebotante, y finalmente el método numérico es aplicado para la simulación de un enjambre de burbujas sin coalescencia numérica

Survivability of Wave Energy Converter and Mooring Coupled System using CFD

Full version unavailable due to 3rd party copyright restrictions.This thesis discusses the development of a Numerical Wave Tank (NWT) capable of describing the coupled behaviour of Wave Energy Converters (WECs) and their moorings under extreme wave loading. The NWT utilises the open-source Computational Fluid Dynamics (CFD) software OpenFOAM(R) to solve the fully nonlinear, incompressible, Reynolds-Averaged Navier-Stokes (RANS) equations for air and water using the Finite Volume Method (FVM) and a Volume of Fluid (VOF) treatment of the interface. A method for numerically generating extreme waves is devised, based on the dispersively-focused NewWave theory and using the additional toolbox waves2Foam. A parametric study of the required mesh resolution shows that steeper waves require finer grids for mesh independence. Surface elevation results for wave-only cases closely match those from experiments, although an improved definition of the flow properties is required to generate very steep focused waves. Predictions of extreme wave run-up and pressure on the front of a fixed truncated cylinder compare well with physical measurements; the numerical solution successfully predicts the secondary loading cycle associated with the nonlinear ringing effect and shows a nonlinear relationship between incident crest height and horizontal load. With near perfect agreement during an extreme wave event, the reproduction of the six degree of freedom (6DOF) motion and load in the linearly-elastic mooring of a hemispherical-bottomed buoy significantly improves on similar studies from the literature. Uniquely, this study compares simulations of two existing WEC designs with scale-model tank tests. For the Wavestar machine, a point-absorber constrained to pitch motion only, results show good agreement with physical measurements of pressure, force and float motion in regular waves, although the solution in the wake region requires improvement. Adding bespoke functionality, a point-absorber designed by Seabased AB, consisting of a moored float and Power Take-Off (PTO) with limited stroke length, translator and endstop, is modelled in large regular waves. This represents a level of complexity not previously attempted in CFD and the 6DOF float motion and load in the mooring compare well with experiments. In conclusion, the computational tool developed here is capable of reliably predicting the behaviour of WEC systems during extreme wave events and, with some additional parameterisation, could be used to assess the survivability of WEC systems at full-scale before going to the expense of deployment at sea.Engineering and Physical Sciences Research Council (EPSRC) via SuperGen UK Centre for Marine Energy Research (UKCMER

A multimaterial Eulerian approach for fluid-solid interaction

This thesis is devoted to understanding and modeling multimaterial interactions, and to develop accordingly a robust scheme taking into account the largest variety of those, with a particular interest in resolving solid/fluid configurations. This very general frame of studies can be tackled with numerous different approaches as several issues arise and need to be addressed before attempting any modelisation of these problems. A first questioning should be the frame of reference to be used for the materials considered. Eulerian shock-capturing schemes have advantages for modeling problems involving complex non-linear wave structures and large deformations. If originally reserved mostly to fluids components, recent work has focused on extending Eulerian schemes to other media such as solid dynamics, as long as the set of equations employed is written under a hyperbolic system of conservation laws. Another matter of interest when dealing with multiple immiscible materials it the necessity to include some means of tracking material boundaries within a numerical scheme. Interface tracking methods based on the use of level set functions are an attractive alternative for problems with sliding interfaces since it allows discontinuous velocity profiles at the material boundaries whilst employing fixed grids. However, its intrinsic lack of variables conservation needs to be circumvented by applying an appropriate fix near the interface, where cells might comprise multiple components. Another requirement is the ability to correctly predict the physical interaction at the interface between the materials. For that purpose, the Riemann problem corresponding to the interfacial conditions needs to be formulated and solved. This implies in turn the need of appropriate Riemann solvers; if they are largely available when the materials are identical (i.e. governed by the same set of equations), a specific Riemann solver will be developed to account for fluid/solid interaction. Eventually, these newly developed methods will be tested on a wide range of different multimaterial problems, involving several materials undergoing large deformations. The materials used, whether modelling fluid/fluid or solid/fluid interactions, will be tested using various initial conditions from both sides of the interface, to demonstrate the robustness of the solver and its flexibility. These testcases will be carried out in 1D, 2D and 3D frames, and compared to exact solutions or other numerical experiments conducted in previous studies.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Navier-Stokes simulations of steep breaking water waves with a coupled air-water interface

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 2005.Includes bibliographical references (p. 367-378).Wave breaking on the ocean surface significantly facilitates the transfer of mass, momentum, heat and energy across the air-sea interface. In the context of the near field flow about a surface ship, the breaking bow wave is a key element to the bubbly signature and an appreciable portion of the wave drag of the ship. Yet, despite its direct effect on many aspects of ocean engineering, this phenomenon is not well understood even at a basic level. Most of the knowledge has been contributed by experiments in the laboratory and the field although results are often limited due to the difficulty in taking measurements of local quantities during the breaking event. Numerical solution of the breaking wave problem has generally been limited to the pre-breaking phase as it avoids complex mechanisms such as surface re-entry, spray formation, air entrainment and strong turbulence. Additionally, relatively few experimental or numerical studies exist which dynamically couple the air-water interface. The objective of this thesis is to contribute to the knowledge of steep breaking waves in the context of the coupled air-water interface. Of central importance are basic kinematics and dynamics, the rate of energy dissipation and energy flux at the interface during the breaking event.(cont.) To this end, a systematic study of a range of breaking waves is performed by direct numerical simulation (DNS) of the Navier- Stokes equations using an Eulerian interface capturing method. The advantage of the DNS approach is that all physical scales are resolved and no turbulence closure models are necessary. However, because of this, DNS is limited to the study to moderate Reynolds numbers with a relatively high computational cost for each simulation. For this reason, this study is limited to two-dimensional flows at Reynolds number 0(10³). The interface capturing method used is a modified form of the level set method which is better suited for simulating coupled air-water flows. The level set method provides a natural numerical treatment of the coupled air-water interface through complex surface topology changes. Thus, no ad-hoc treatment of the air-water interface during the breaking event is necessary. The key findings of this thesis represent new contributions to the study of breaking waves in three distinct areas. The first is the kinematics and dynamics of deep water breaking waves for both spilling and plunging types. For the waves in this study, there was no indication of flow reversal or separation in the water while the air flow showed separation on the front face of the wave and over the crest.(cont. ) Localized shear regions are found in spilling breaking waves and curvature effects are identified as the dominant mechanism of vorticity generation in both types of breaking waves. The second area is the energy dissipated by breaking waves. The volumetric dissipation rates as well as its spatial variation for both air and water are presented for the range of waves in this study. While the water volume experienced an increase in dissipation rate during the breaking event, the increase is more pronounced in the air volume to the point that it becomes the same order of magnitude as that in the water for some waves. The amount of energy in the wave lost due to breaking is quantified as a function of the energy in the wave prior to breaking. A threshold below which waves do not break is identified and qualitative comparisons to experiment are made when applicable. The third area is the transfer of energy at the air-water interface during breaking which is an aspect of the breaking process that has not received much attention in the literature. In this thesis, the formulation of a term in the energy equation which accounts for the energy flux rate at the air-water interface is presented. The waves in this numerical study give evidence that this quantity is appreciable.(cont.) Although the calculation of this term is sensitive to errors associated with the conservation of energy, values as high as 25% of the energy lost to breaking are found. At the Reynolds numbers in this study, the dominant mechanism for each type of wave is identified as inviscid for spilling breaking waves and viscous for plunging breaking waves. This numerical effort has contributed to the basic knowledge of wave breaking at moderate Reynolds numbers. Through the inclusion of the coupled air-water interface, unique insight to the kinematics, dynamics, dissipation and energy fluxes of breaking waves was obtained. The information gained in this study provides an initial step towards physics-based turbulence models for the study of wave breaking at larger scales.by Kelli L. Hendrickson.Sc.D

Fracture-Based Fabrication of a Size-Controllable Micro/Nanofluidic Platform for Mapping of DNA/Chromatin.

In the years since the launch of The Human Genome Project (HGP), which significantly increased our understanding of biological inheritance by revealing the structure and function of genetic material, tangential research efforts have revealed mechanisms of inheritance that extend beyond the sequence of nucleic acids within an individual’s genome. The study of these mechanisms, referred to as epigenetics, now lies at the frontier of biomedical research. While much is known regarding genetic inheritance, the complexity of chromosome structure and lack of appropriate methodologies have long hindered mechanistic dissection of epigenetic inheritance. The work in this dissertation seeks three fundamental objectives: (1) the development of appropriate tools for chromatin mapping, (2) the identification of a well-defined model system, and (3) the use of ‘super-resolution imaging’. First, a unique micro/nanofluidics platform was developed utilizing fracture-based fabrication techniques. The use of such techniques, combined with the careful selection of appropriate materials, enabled the formation of channels with dimensions that could be modified by simply modifying the magnitude of the uniaxial strain applied. By integrating stress focusing notch micro-features into the soft elastomer, polydimethylsiloxane (PDMS), nano-scale fractures were generated at desired positions, producing an array of nano-channels. These adjustable channels were then utilized to achieve the efficient pre-concentration, capturing, and linearization of DNA and chromatin via nano-confinement and a squeezing flow. In the tuneable channel device, DNA molecules were pre-concentrated up to 10,000 fold at the defined position using electrophoresis, and were successfully trapped and linearized up to its contour length for epigenetic marker profiling. Finally, Tetrahymena was selected as an optimal biological system, and was used to elucidate the spatial distribution of histones along replicated DNA, as well as to characterize specific histone-DNA interactions occurring during replication by the super-resolution microscopy. This multi-disciplinary dissertation project provides insight into both the unknown epigenetic changes occurring during DNA replication, and the biological machinery underlying fundamental DNA-histone interactions. The application of this adjustable fluidics platform to other biological model species may provide a means to establish other epigenetic marker maps including patterns of post-translational modifications of histone and DNA methylation to study yet unknown epigenetic mechanisms.PHDBiomedical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/110347/1/introbc_1.pd