Verification and Validation of Numerical Modelling Approaches Pertinent to Stomach Modelling

The digestive system is vital to the human body. Over many decades, scientists have been investigating the food breakdown mechanisms inside the stomach through in vivo human and animal studies and in vitro experiments. Due to recent improvements in computing speed and algorithm development, computational modelling has become a viable option to investigate in-body processes. Such in silico models are more easily controlled to investigate individual variables, do not require invasive physical experiments, and can provide valuable insights into the local physics of gastric flow. There is a huge potential for numerical approaches in stomach modelling as they can provide a comprehensive understanding of the complex flow and chemistry in the stomach. However, to make sure the numerical methods are accurate and reliable, rigorous verification and validation are essential as part of model development. A significant focus of this thesis was on verifying and validating the numerical modelling approaches pertinent to stomach modellin

Developments of numerical methods for linear and nonlinear fluid-solid interaction dynamics with applications

This paper presents a review on some developments of numerical methods for linear and nonlinear fluid-solid interaction (FSI) problems with their applications in engineering. The discussion covers the four types of numerical methods: 1) mixed finite element (FE)-substructure-subdomain model to deal with linear FSI in a finite domain, such as sloshing, acoustic-structure interactions, pressure waves in fluids, earthquake responses of chemical vessels, dam-water couplings, etc.; 2) mixed FE-boundary element (BE) model to solve linear FSI with infinite domains, for example, VLFS subject to airplane landing impacts, ship dynamic response caused by cannon / missile fire impacts, etc.; 3) mixed FE-finite difference (FD) / volume (FV) model for nonlinear FSI problems with no separations between fluids and solids and breaking waves; 4) mixed FE-smooth particle (SP) method to simulate nonlinear FSI problems with f-s separations as well as breaking waves. The partitioned iteration approach is suggested in base of available fluid and solid codes to separately solve their governing equations in a time step, and then through reaching its convergence in coupling iteration to forward until the problem solved. The selected application examples include air-liquid-shell three phases interactions, LNG ship-water sloshing; acoustic analysis of air-building interaction system excited by human foot impacts; transient dynamic response of an airplane-VLFS-water interaction system excited by airplane landing impacts; turbulence flow-body interactions; structure dropping down on the water surface with breaking waves, etc. The numerical results are compared with the available experiment or numerical data to demonstrate the accuracy of the discussed approaches and their values for engineering applications. Based on FSI analysis, linear and nonlinear wave energy harvesting devices are listed to use the resonance in a linear system and the periodical solution in a nonlinear system, such as flutter, to effectively harvest wave energy. There are 231 references are given in the paper, which provides very useful resources for readers to further investigate their interesting approaches

Fluid-structure interaction models on the hydroelastic analysis of containerships in waves

Commercial vessels have recently been increasing in size to meet the fast-growing demand for transportation and operations. However, this trend may result in more flexible or "softer" hulls. The flexible hull structure and high operational speed requirements bring the ship's natural frequency closer to the wave encounter frequency, increasing the probability of resonance or high-frequency vibrations. Therefore, hydroelastic effects and relevant loads should be considered when designing wave loads and evaluating the strength of large ships. A robust numerical model is in search of ship designers and regulators, intended to predict the impact of hydroelasticity in the initial stages of design as per the design regulations, where there exists a greater opportunity to make modifications and utilise high-fidelity tools to verify the performance of advanced designs. This study aims to fill this gap by performing robust numerical investigations based on open-source software on the seakeeping and hydroelastic analysis of a monohull under wave excitations. Firstly, a detailed literature review is presented to overview the previous theoretical and numerical methods for ship hydroelasticity. This review also includes a general comparison between these hydroelastic techniques and discusses the differences. Following this, two fully coupled CFD-based unsteady FSI numerical frameworks are established: coupled CFD-FEA and CFD-DMB methods, respectively. The physical principle of these FSI models is to treat a ship’s surface hull as an elastic body and interact with its surrounding flow field to form a fully coupled system. Taking advantage of the present numerical models, the hydroelastic behaviours of a containership, such as its vertical bending displacement and corresponding bending moment, can be quantified, and the “springing” and “whipping” behaviour can be measured. It is believed that the present FSI model will exhibit more advantages over the traditional rigid-body method in the ship seakeeping field. Later, the presented CFD-DMB model is further extended for its application to irregular extreme waves and damaged ship conditions. The results achieved from these studies could also help to assess the structural integrity and longitudinal strength of a ship (intact or damaged), which serves as an improved technique for regulations to evaluate conventional ship designs. Finally, the results drawn from each chapter of this thesis are summarised and discussed, and recommendations are made for future research.Commercial vessels have recently been increasing in size to meet the fast-growing demand for transportation and operations. However, this trend may result in more flexible or "softer" hulls. The flexible hull structure and high operational speed requirements bring the ship's natural frequency closer to the wave encounter frequency, increasing the probability of resonance or high-frequency vibrations. Therefore, hydroelastic effects and relevant loads should be considered when designing wave loads and evaluating the strength of large ships. A robust numerical model is in search of ship designers and regulators, intended to predict the impact of hydroelasticity in the initial stages of design as per the design regulations, where there exists a greater opportunity to make modifications and utilise high-fidelity tools to verify the performance of advanced designs. This study aims to fill this gap by performing robust numerical investigations based on open-source software on the seakeeping and hydroelastic analysis of a monohull under wave excitations. Firstly, a detailed literature review is presented to overview the previous theoretical and numerical methods for ship hydroelasticity. This review also includes a general comparison between these hydroelastic techniques and discusses the differences. Following this, two fully coupled CFD-based unsteady FSI numerical frameworks are established: coupled CFD-FEA and CFD-DMB methods, respectively. The physical principle of these FSI models is to treat a ship’s surface hull as an elastic body and interact with its surrounding flow field to form a fully coupled system. Taking advantage of the present numerical models, the hydroelastic behaviours of a containership, such as its vertical bending displacement and corresponding bending moment, can be quantified, and the “springing” and “whipping” behaviour can be measured. It is believed that the present FSI model will exhibit more advantages over the traditional rigid-body method in the ship seakeeping field. Later, the presented CFD-DMB model is further extended for its application to irregular extreme waves and damaged ship conditions. The results achieved from these studies could also help to assess the structural integrity and longitudinal strength of a ship (intact or damaged), which serves as an improved technique for regulations to evaluate conventional ship designs. Finally, the results drawn from each chapter of this thesis are summarised and discussed, and recommendations are made for future research

Numerical simulation of fast transient phenomena in fluid-structure systems

In this master thesis the Fast Transient Fluid-Structure interaction is studied. The goal is to understand the numerical methods behind the models that simulate this phenomena, in order to be able to reproduce them in two examples

Unified Lagrangian formulation for fluid and solid mechanics, fluid-structure interaction and coupled thermal problems using the PFEM

The objective of this thesis is the derivation and implementation of a unified Finite Element formulation for the solution of uid and solid mechanics, Fluid-Structure Interaction (FSI) and coupled thermal problems. The unified procedure is based on a stabilized velocity-pressure Lagrangian formulation. Each time step increment is solved using a two-step Gauss-Seidel scheme: first the linear momentum equations are solved for the velocity increments, next the continuity equation is solved for the pressure in the updated configuration. The Particle Finite Element Method (PFEM) is used for the fluid domains, while the Finite Element Method (FEM) is employed for the solid ones. As a consequence, the domain is remeshed only in the parts occupied by the fluid. Linear shape functions are used for both the velocity and the pressure fields. In order to deal with the incompressibility of the materials, the formulation has been stabilized using an updated version of the Finite Calculus (FIC) method. The procedure has been derived for quasi-incompressible Newtonian fluids. In this work, the FIC stabilization procedure has been extended also to the analysis of quasi-incompressible hypoelastic solids. Specific attention has been given to the study of free surface flow problems. In particular, the mass preservation feature of the PFEM-FIC stabilized procedure has been deeply studied with the help of several numerical examples. Furthermore, the conditioning of the problem has been analyzed in detail describing the effect of the bulk modulus on the numerical scheme. A strategy based on the use of a pseudo bulk modulus for improving the conditioning of the linear system is also presented. The unified formulation has been validated by comparing its numerical results to experimental tests and other numerical solutions for fluid and solid mechanics, and FSI problems. The convergence of the scheme has been also analyzed for most of the problems presented. The unified formulation has been coupled with the heat tranfer problem using a staggered scheme. A simple algorithm for simulating phase change problems is also described. The numerical solution of several FSI problems involving the temperature is given. The thermal coupled scheme has been used successfully for the solution of an industrial problem. The objective of study was to analyze the damage of a nuclear power plant pressure vessel induced by a high viscous fluid at high temperature, the corium. The numerical study of this industrial problem has been included in the thesis.El objectivo de la presente tesis es la derivación e implementación de una formulación unificada con elementos finitos para la solución de problemas de mecánica de fluidos y de sólidos, interacción fluido-estructura (Fluid-Structure Interaction (FSI)) y con acoplamiento térmico. El método unificado està basado en una formulación Lagrangiana estabilizada y las variables incognitas son las velocidades y la presión. Cada paso de tiempo se soluciona a través de un esquema de dos pasos de tipo Gauss-Seidel. Primero se resuelven las ecuaciones de momento lineal por los incrementos de velocidad, luego se calculan las presiones en la configuración actualizada usando la ecuación de continuidad. Para los dominios fluidos se utiliza el método de elementos finitos de partículas (Particle Finite Element Method (PFEM)) mientras que los sólidos se solucionan con el método de elementos finitos (Finite Element Method (FEM)). Por lo tanto, se ramalla sólo las partes del dominio ocupadas por el fluido. Los campos de velocidad y presión se interpolan con funciones de forma lineales. Para poder analizar materiales incompresibles, la formulación ha sido estabilizada con una nueva versión del método Finite Calculus (FIC). La técnica de estabilización ha sido derivada para fluidos Newtonianos casi-incompresibles. En este trabajo, la estabilización con FIC se usa también para el análisis de sólidos hipoelásticos casi-incompresibles. En la tesis se dedica particular atención al estudio de flujo con superficie libre. En particular, se analiza en profundidad el tema de las pérdidas de masa y se muestra con varios ejemplos numéricos la capacidad del método de garantizar la conservación de masa en problemas de flujos en supeficie libre. Además se estudia con detalle el condicionamiento del esquema numérico analizando particularmente el efecto del módulo de compresibilidad. Se presenta también una estrategia basada en el uso de un pseudo módulo de compresibilidad para mejorar el condicionamiento del problema. La formulación unificada ha sido validada comparando sus resultados numéricos con pruebas de laboratorio y resultados numéricos de otras formulaciones. En la mayoría de los ejemplos también se ha estudiado la convergencia del método. En la tesis también se describe una estrategia segregada para el acoplamiento de la formulación unificada con el problema de transmisión de calor. Además se presenta una simple estrategia para simular el cambio de fase. El esquema acoplado ha sido utilizado para resolver varios problemas de FSI donde se incluye la temperatura y su efecto. El esquema acoplado con el problema térmico ha sido utilizado con éxito para resolver un problema industrial. El objetivo del estudio era la simulación del daño y la fusión de la vasija de un reactor nuclear provocados por el contacto con un fluido altamente viscoso y a gran temperatura. En la tesis se describe con detalle el estudio numérico realizado para esta aplicación industrialPostprint (published version

Study on wave-induced kinematic responses and flexures of ice floe by Smoothed Particle Hydrodynamics

In this paper, a Smoothed Particle Hydrodynamics (SPH) method is extended to simulate the wave–ice interactions. The main contribution of this work is that an improved fluid-ice interface treatment scheme is included into SPH code to deal with the contact between the fluid and ice particles. The proposed SPH approach is applied to model two typical patterns of the wave–ice interaction process: the kinematic response of a small ice floe and the flexural motion of a sea ice floe, induced by water waves. To verify the accuracy of SPH method for the wave–ice interactions, the numerical results of SPH are compared with corresponding experimental data. The good agreement between the two demonstrates that the SPH method can be a useful numerical tool for simulating the wave-induced motion and bending deformation of an ice floe

Two case studies of SPH modelling in biological system: large intestine and deep vein valves

Computational Fluid Dynamics (CFD) techniques has proven its adaptiveness and mutuality in various application for diversified fields. At the moment, the investigation of physical mechanics developed by CFD in physiological transportation has arisen great attention. This thesis aims at deepening the insight into the interactive mechanism between the periodic deformable human conduits and the conveyed fluid content. To start with a coupled method consists of Smoothed Particle Hydrodynamics (SPH) method and Coarse-Grained Molecular Dynamics (CGMD) I Mass-spring modelling (MSM) is adopted as it has proven its potentials in dealing with the transportation inside the deformable solid boundaries. The fluid part is simply simulated by the SPH method while the soft solid boundaries are mimicked by another one to yield the optimal utility of the coupled method. Our models developed for Gastric Intestine (GI) system are first validated by comparisons with PET experimental outcomes and are applied to the practical usage. Quantitative analysis is made among simulations to illustrate the mixing and transportation of food propelled by the muscle peristalsis. Then the method is further extended into the study of the venous system and the results indicate a good interpretation of the pathological study of the valve dysfunction and the venous development

An immersed computational framework for multiphase fluid-structure interaction.

"The objective of this thesis is to further extend the application range of immersed computational approaches in the context of hydrodynamics and present a novel general framework for the simulation of fluid-structure interaction problems involving rigid bodies, flexible solids and multiphase flows. 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. The new framework will be capable of coping with large density ratios, multiphase flows and will be focussed on hydrodynamic problems. The two main challenges to be addressed are: - the representation, evolution and compatibility of the multiple fluid-solid interface - the proposition of unified framework containing multiphase flows, flexible structures and rigid bodies with possibly large density ratios First, a new variation of the original IBM is presented by rearranging the governing equations which define the behaviour of the multiple physics involved. The formulation is compatibile with the "one-fluid" equation for two phase flows and can deal with large density ratios with the help of an anisotropic Poisson solver. Second, deformable structures and fluid are modelled in a identical manner except for the deviatoric part of the Cauchy stress tensor. The challenging part is the calculation of the deviatoric part the Cauchy stress in the structure, which is expressed as a function of the deformation gradient tensor. The technique followed In this thesis is that original ISP, but re-expressed in terms of the Cauchy stress tensor. Any immersed rigid body is considered as an incompressible non-viscous continuum body with an equivalent internal force field which constrains the velocity field to satisfy the rigid body motion condition. The "rigid body" spatial velocity is evaluated by means of a linear least squares projection of the background fluid velocity, whilst the immersed force field emerges as a result of the linear momentum conversation equation. This formulation is convenient for arbitrary rigid shapes around a fixed point and the most general translation- rotation. A characteristic or indicator function, defined for each interacting continuum phase, evolves passively with the velocity field. Generally, there are two families of algorithms for the description of the interfaces, namely, Eulerian grid based methods (interface tracking). In this thesis, the interface capturing Level Set method is used to capture the fluid-fluid interface, due to its advantages to deal with possible topological changes. In addiction, an interface tracking Lagrangian based meshless technique is used for the fluid-structure interface due to its benefits at the ensuring mass preservation. From the fluid discretisation point of view, the discretisation is based on the standard Marker-and-Cell method in conjunction with a fractional step approach for the pressure/velocity decoupling. The thesis presents a wide range of applications for multiphase flows interacting with a variety of structures (i.e. rigid and deformable) Several numerical examples are presented in order to demonstrate the robustness and applicability of the new methodology. (Abstract shortened by ProQuest.).

Numerical modelling of the Oldroyd-B fluid

The purpose of this thesis is to develop a 3D finite element model of the Oldroyd-B fluid for use in a complex geometry. The model is developed in deal.ii, which is a C++ finite element library. In addition to the standard finite element approach for the momentum equation, the discontinuous Galerkin method is used for the constitutive relation of the fluid model, with the extra stress as the unknown variable. The model developed is verified by using the symmetric “flow over a cylinder” benchmark problem. The effect of using piecewise-constant discontinuous and bilinear discontinuous elements for the extra stress field is investigated. The the results of the scheme are compared to those found in literature. The model is implemented in the solution of a complex problem of blood flow in an arteriovenous fistula, using geometry acquired from MRI data. A resistance boundary condition is used for the outlets. The flow profiles obtained from using both the Newtonian and Oldroyd-B fluids are validated against velocity encoded MRI and also compared to Fluid-Structure Interaction results for Newtonian fluids, from the literature. The effect of using a viscoelastic fluid on the flow profile and wall shear stresses are investigated. The results from this work show that using a viscoelastic fluid, rather than a Newtonian fluid, provides additional details regarding the wall shear stress in the arteriovenous fistula