Dissertations / Theses on the topic 'Lagrangian study'

The Southern Ocean is an important region for the sequestration of heat, carbon dioxide and other tracers. The Southern Ocean circulation is typically described in a circumpolarly averaged sense as a Meridional Overturning Circulation (MOC), but the detailed 3-D pathways that make up this circulation remain poorly understood. We use Lagrangian particle trajectories, obtained from eddy permitting numerical models, to map out and quantify different aspects of the 3-D circulation. We first introduce various definitions used to quantify efficient export from the Antarctic Circumpolar Current (ACC) to the subtropical gyres. Using these definitions, we show that the permanent northward export varies by water mass and occurs in localised regions; with 11 key pathways identified. We then examine the dynamics setting the location and efficiency of the identified pathways, which includes the investigation of the role of diapycnal mixing and the impact of short and long time variability in the flow. Although we show that the flow of particles in the 3-D model is predominantly isopycnal, we find that particles that are forced to remain on isopycnals lead to approx. 60% lower export (mainly via three pathways) than identical releases where the diapycnal component of advection is included. Enhanced upward mixing near rough topography, and downward mixing in the southeast Pacific, were shown to be mostly responsible for the export. In addition, we show that most of the export pathways are mainly influenced by timescales from 90 days to 20 years, which suggests that mesoscale eddies are not the leading-order importance in the northward export from the ACC to the subtropical gyres. However, we also find that mesoscale eddies and the mean-ACC flow play a significant role in setting the export from the ACC in some pathways. These results highlight the role of temporal variability and vertical transport in enhancing the northward flow from the ACC by allowing transport across barotropic streamlines and onto more efficiently exporting isopycnals. In addition, the asymmetrical response of the studied quantities emphasises the importance of the three dimensions in understanding the dynamics driving the overturning circulation. We also demonstrated that the annually repeating velocity fields, which are commonly used for trajectory calculations, increase the diapycnal transport of particles and as a consequence, increase the overall 20-year northward export from the ACC by approx. 10%. In the study of the meridional overturning circulation, we diagnose the geographical distribution of the streamwise averaged diffusivity calculated from meridional displacements of the Lagrangian particles. We examine streamwise averaging using both latitude and equivalent latitude and argue that the latter gives a more useful measure. Reconciling tracer and particle horizontal diffusivities, we show that in the ACC, the average diffusivity peaks between 1500m and 2500m with an average value of 1500 m$^{2}$/s and that it is highest near the topographic features. We compare the exact diffusivity and its approximation to show that an assumption of time homogeneity does not hold and therefore that standard expressions for diffusivity that assume time homogeneity are of limited usefulness. Finally, we use the calculated trajectories to provide a streamwise averaged 2-D advection-diffusion model of the Southern Ocean MOC and then examine the extent to which this 2-D model can capture the overall effect of the actual 3-D transport.

In order to understand water quality in the coastal ocean and its effects on human health, the necessity arises to locate the sources of contaminants and track their transport throughout the ocean. Dynamical systems methods are applied to the study of transport of enterococci as an indicator of microbial concentration in the vicinity of Hobie Beach, an urban, subtropical beach in Miami, FL that is used for recreation and bathing on a daily basis. Previous studies on water quality have shown that Hobie Beach has high microbial levels despite having no known point source. To investigate the cause of these high microbial levels, a combination of measured surface drifter trajectories and numerically simulated flows in the vicinity of Hobie Beach is used. The numerically simulated flows are used to identify Lagrangian Coherent Structures (LCSs), which provide a template for transport in the study area. Surface drifter trajectories are shown to be consistent with the simulated flows and the LCS structure. LCSs are then used to explain the persistent water contamination and unusually high concentrations of microbes in the water off of this beach as compared with its neighboring beaches. From the drifter simulations, as well as field experiments, one can see that passive tracers are trapped in the area along the coastline by LCS. The Lagrangian circulation of Hobie Beach, influenced primarily by tide and land geometry causes a high retention rate of water near the shore, and can be used to explain the elevated levels of enterococci in the water.

La perspective lagrangienne, décrivant un écoulement selon les trajectoires de traceurs fluides, est une approche naturelle pour étudier les phénomènes de dispersion dans les écoulements turbulents. En turbulence de paroi, le mouvement des traceurs est influencé par le cisaillement moyen et par une forte inhomogénéité et anisotropie en proche paroi. On étudie les propriétés lagrangiennes d’un écoulement de canal turbulent par simulation numérique directe à un nombre de Reynolds modéré. Les statistiques d’accélération lagrangienne sont comparées aux expériences de suivi de particules réalisées en parallèle à ce travail. Comme en turbulence homogène isotrope (THI), les composantes d’accélération le long des trajectoires lagrangiennes se décorrèlent sur des temps comparables aux plus petites échelles de l’écoulement, tandis que la norme de l’accélération reste corrélée plus longtemps. La persistance d’anisotropie à petite échelle loin de la paroi est constatée par l’existence d’une corrélation croisée non nulle entredeux composantes de l’accélération. On montre que, en conséquence des flux moyens d’énergie cinétique en turbulence de paroi, près des parois les traceurs se déplacent et s’étalent sur des plus grandes distances quand ils sont suivis en arrière dans le temps qu’en avant. La dispersion relative de paires de traceurs est aussi étudiée. Aux temps courts, la séparation des paires est balistique pour toutes les distances à la paroi. Comme en THI, les traceurs se séparent plus rapidement lorsqu’ils sont suivis en arrière dans le temps. Aux temps plus longs, le cisaillement moyen accélère la séparation dans la direction de l’écoulement moyen. Un modèle de cascade balistique initialement proposé pour la THI est adapté aux écoulements inhomogènes
The Lagrangian perspective, describing a flow from the trajectories of fluid tracers, isa natural framework for studying dispersion phenomena in turbulent flows. In wall-boundedturbulence, the motion of fluid tracers is affected by mean shear and by strong inhomogeneityand anisotropy near walls. We investigate the Lagrangian properties of a turbulent channel flowusing direct numerical simulations at a moderate Reynolds number. Lagrangian accelerationstatistics are compared to particle tracking experiments performed in parallel to this work. Asin homogeneous isotropic turbulence (HIT), the acceleration components along Lagrangianpaths decorrelate over time scales representative of the smallest scales of the flow, while theacceleration norm stays correlated for much longer. The persistence of small-scale anisotropy farfrom the wall is demonstrated in the form of a non-zero cross-correlation between accelerationcomponents. As a result of the average fluxes of kinetic energy in wall turbulence, tracers initiallylocated close to the wall travel and spread over longer distances when tracked backwardsin time than forwards. The relative dispersion of tracer pairs is finally investigated. At shorttimes, pair separation is ballistic for all wall distances. As in HIT, relative dispersion is timeasymmetric, with tracers separating faster when tracked backwards in time. At longer times,mean shear dominates leading to rapid separation in the mean flow direction. A ballisticcascade model previously proposed for HIT is adapted to inhomogeneous flows

Conventional morphodynamic models are typically based on a coupled system of hydrodynamic equations, a bed-update equation, and a sediment-transport equation. However, the sediment-transport equation is almost invariably empirical, with numerous options available in the literature. Bed morphological evolution predicted by a conventional model can be very sensitive to the choice of sediment-transport formula. This thesis presents a physics-based model, where the shallow water-sediment-mixture flow is idealised as being divided into two layers of variable (in time and space) densities: the lower layer concerned with bedload transport, and the upper layer representing sediment in suspension. The model is referred to as a Quasi-2-Layer (Q2L) model in order to distinguish it from typical 2-Layer models representing stratified flow by two layers of different but constant and uniform densities. The present model, which does not require the selection of a particular empirical formula for sediment transport rates, is satisfactorily validated against widely used empirical expressions for bedload and total transport rates. Analytical solutions to the model are derived for steady uniform flow over an erodible bed. Case studies show that the Q2L model, in contrast to conventional morphodynamic approaches, yields more realistic results by inherently including the influence of the bed slope on the sediment transport. This conclusion is validated against experimental data from a steep sloping duct. An analytical study using the Q2L model investigates the influence of bed-slope on bedload transport; the resulting expressions are in turn used to modify empirical sediment transport formulae (derived for horizontal beds) in order to render them applicable to arbitrary stream-wise slopes. The Q2L model provides an alternative approach to studying sediment-transport phenomena, whose adequate analysis cannot be undertaken following coniv ventional approaches without further increasing their degree of empiricism. The Q2L model can also lead to the enhancement of conventional morphodynamic models. For coarse sediments and/or relatively low flow velocities, bedload transport is usually responsible for most sediment transport. Bedload transport consists of a combination of particles rolling, sliding and saltating (hopping) along the bed. Hence, saltation models provide considerable insight into near-bed sediment transport. This thesis also presents an analysis of the statistics and mechanics of a saltating particle model. For this purpose, a mathematically simple, computationally efficient, stochastic Lagrangian model has been derived. This model is validated satisfactorily against previously published experimental data on saltation. The model is then employed to derive two criteria aimed at ensuring that statistically convergent results are achieved when similar saltation models are employed. According to the first criterion, 103 hops should be simulated, whilst 104 hops ought to be considered according to the second criterion. This finding is relevant given that previous studies report results after only a few hundred, or less, particle hops have been simulated. The model also investigates sensitivity to the lift force formula, the friction coefficient, and the collision line level. A method is proposed by which to estimate the bedload sediment concentration and transport rate from particle saltation characteristics. This method yields very satisfactory results when compared against widely used empirical expressions for bedload transport, especially when contrasted against previously published saltation-based expressions.

This paper presents a static analysis of functionally graded single and sandwich beams by using an efficient 7DOFs quasi-3D hybrid type theory. The governing equations are derived by employing the principle of virtual works in a weak form and solved by means of the Finite Element Method (FEM). A C1 cubic Hermite interpolation is used for the vertical deflection variables while C0 linear interpolation is employed for the other kinematics variables. Convergence rates are studied in order to validate the finite element technique. Numerical results of the present formulation are compared with analytical and FEM solutions available in the literature.
Revisión por pares

The lack of strong measures to avoid the possible fatal consequences of global warming is pushing researchers to look for other alternatives such as geoengineering.Within geoengineering, this study focuses on the space based solar radiation management methods. More precisely, the project evaluates the feasibilityof implementing a space sun shade near the first Lagrangian point in the Sun-Earth system within a thirty year period time from now. The study isstructured in three main blocks: spacecraft configuration, trajectory definition and launch. An analysis looking at the minimum cost system was carried out,starting with the definition of the mass and size of spacecraft. Furthermore, an optimization of the trajectory was developed in order to minimize the traveltime to the vicinity of the Lagrangian point. The shades will be formed by swarms of 10 000m2 solar sails that will cover an area of 6:3 x 1012 m2 witha total mass of around 5:7 x 1010 kg. The sails will be injected into a LEO and will start a trajectory to the vicinity of the first Lagrangian point that willtake around 2.3 years. The total cost of the project is approximated to be 10 trillion dollars. The mission appears to be feasible from a technological pointof view, with some development needed in the attitude control subsystem. The main challenge will be the launch of all the spacecraft. A space mission of thisdimensions has never been attempted before so it will require a big advance from the launch vehicle industry.
Bristen på åtgärder för att undvika de konsekvenser som den globala uppvärmingen leder till, har drivit forskare att leta efter alternativa lösningar, varav geoengineering är en av dem. Denna studie fokuserar på rymdbaserade strålhanteringsmetoder, mer specifikt på hur huruvida implementationer av solparasoller nära Lagrangepunkten L1 i sol-jord-systemet är möjlig eller ej. Studien är strukturerad i tre huvudsakliga block: rymdskeppskonfiguration, banadefinition och uppskjutning. Med målet att minimera kostnaderna, definierades rymdskeppets utforming, massa och storlek. Vidare så, optimerades vägen till närheten av L1 med avseende på att minimera tiden. Solparasollerna kommer vara placerade i svärmar med en area på 10 000m2 vardera, totalt kommer solparasollerna att täcka en yta av 6:3 x 1012 m2 med en total massa på 5:7 x 1010 kg. Solparasollerna kommer skjutas upp till LEO och därefter starta sin resa till närheten av L1, vilket kommer ta cirka 2.3 år. Totala kostanden för projektet uppskattas till 10 billioner dollar. Efter genomförd studie visades projektet vara genomförbart sett från en teknisk synvinkel, men vidare studier behövs göras för att utveckla och fastställa styrsystemet. Huvudutmaningen kommer att vara uppskjutningen av rymdskeppen, då det kräver stora framsteg och utveckling inom rymdindustrin.

Augmentation of the impact of an explosive is routinely achieved by packing metal particles in the explosive charge. When detonated, the particles in the charge are ejected and dispersed. The ejecta influences the post-detonation combustion processes that bolster the blast wave and determines the total impact of the explosive. Thus, it is vital to understand the dispersal and the combustion of the particles in the post-detonation flow, and numerical simulations have been indispensable in developing important insights. Because of the accuracy of Eulerian-Lagrangian (EL) methods in capturing the particle interaction with the post-detonation mixing zone, EL methods have been preferred over Eulerian-Eulerian (EE) methods. However, in most cases, the number of particles in the flow renders simulations using an EL method unfeasible. To overcome this problem, a combined EE-EL approach is developed by coupling a massively parallel EL approach with an EE approach for granular flows. The overall simulation strategy is employed to simulate the interaction of ambient particle clouds with homogenous explosions and the dispersal of particles after detonation of heterogeneous explosives. Explosives packed with aluminum particles are also considered and the aluminum particle combustion in the post-detonation flow is simulated. The effect of particles, both reactive and inert, on the combustion processes is analyzed. The challenging task of solving for clouds of micron and sub-micron particles in complex post-detonation flows is successfully addressed in this thesis.

The weakness of a conventional Eulerian soot model in capturing primary soot size and its inability to access individual soot information led to the development of a Lagrangian soot tracking (LST) model as reported in this thesis. The LST model aimed to access the history of individual soot particles and capture the soot concentration and primary soot size distribution in high pressure spray flames, under diesel-like conditions. The model was validated in a constant volume spray combustion chamber by comparing the predicted soot volume fraction (SVF), mean primary soot diameter and primary soot size distribution to the experimental data of n-heptane and n-dodecane spray combustion. The inception, surface growth and oxidation models were adopted and modified from the multistep Moss-Brookes (MB) soot model, which was used in this study as the representative of Eulerian soot model. Parametric studies were carried out to investigate the influence of soot surface ageing and oxidation rates on the overall soot formation. Following the parametric study, the developed LST model which incorporated surface ageing effect and higher oxidation rates was implemented to investigate the effect of ambient oxygen and density on soot morphology in n-heptane spray flame. The LST model was shown to have better primary soot size prediction capability while still maintaining comparable performance in predicting SVF with respect to its Eulerian counterpart. The SVF distributions predicted by the LST model qualitatively correspond to the experimental results despite the peak soot location being predicted further downstream by 30 mm. The primary soot size distribution predicted by the LST model had the same order as the measured primary soot size distribution despite predicting larger soot size. The presence of surface ageing factor had a significant effect on the primary soot size distribution whereas only a slight effect on the SVF profile. A maximum soot size reduction of 48% was obtained when incorporating surface ageing effect. The consideration of surface ageing effect led to smaller primary soot size predicted and better agreement with the measured primary soot size distribution. The peak and mean primary soot sizes increased with increasing ambient density, from 14.8 kg/m3 to 30 kg/m3, at the core of spray jet. Meanwhile, the decrease in oxygen level from 21% to 12% at an ambient density of 14.8 kg/m3 caused a non-monotonic effect on the primary soot sizes at the core of spray jet. Trivial differences were predicted when oxygen level decreased from 21% to 15%. However, a significantly smaller primary soot sizes were predicted when oxygen level decreased further to 12%. In addition to net growth rates, soot cloud span and soot age were also found to play an important role in evolution of primary soot size. An increase in ambient oxygen and density resulted in a more upstream first-soot location. The effect of ambient density on soot age was not significant, whereas a lower oxygen level resulted in a longer soot age. A maximum soot age of 0.50 ms was obtained for both 21% and 15% O2 cases at both density levels. As oxygen level decreased to 12%, the maximum soot age increased to 0.58 ms due to lower combustion temperature. Overall, the LST model was shown to perform better in predicting primary soot size and can access information of individual soot particles which are both shortcomings of the Eulerian method. In addition, the LST model was also demonstrated to be able to predict soot age. Apart from playing a role in determining primary soot size, soot age can also serve as a useful parameter to answer various fundamental questions, such as when and where soot particles grow to a certain size, and help in the understanding of fundamental soot processes. Optimisation of the model and extension of its capability to capture soot aggregate structure, size and fractal dimension will be of interest in the future.

The work is focused on experimental study and modelling of spacer grid influence on single- and two-phase flow. In the experimental study a mock-up of a realistic fuel bundle with five spacer grids of thin plate spring construction was investigated. A special pressure measuring technique was used to measure pressure distribution inside the spacer. Five pressure taps were drilled in one of the rods, which could exchange position with other rods, in this way providing a large degree of freedom. Laser Doppler Velocimetry was used to measure mean local axial velocity and its fluctuating component upstream and downstream of the spacer in several subchannels with differing spacer part. The experimental study revealed an interesting behaviour. Subchannels from the interior part of the bundle display a different effect on the flow downstream of the spacer compared to subchannels close to the box wall, even if the spacer part is the same. This behaviour is not reflected in modern correlations. The modelling part, first, consisted in comparing the present experimental data to Computational Fluid Dynamics calculations. It was shown that stand-alone subchannel models could predict the local velocity, but are unreliable in prediction of turbulence enhancement due to spacer. The second part of the modelling consisted in developing a deposition model for increase due to spacer. In this study Lagrangian Particle Tracking (LPT) coupled to Discrete Random Walk (DRW) technique was used to model droplet movements through turbulent flow. The LPT technique has an advantage to model the influence of turbulence structure effect on droplet deposition, in this way presenting a generalized model in view of spacer geometry change. The verification of the applicability of LPT DRW method to model deposition in annular flow at Boiling Water Reactor conditions proved that the method is unreliable in its present state. The model calculations compare reasonably well to air-water deposition data, but display a wrong trend if the fluids have a different density ratio than air-water.

Choosing correct boundary conditions, flow field characteristics and employing right thermal fluid properties can affect the simulation of convection heat transfer using nanofluids. Nanofluids have shown higher heat transfer performance in comparison with conventional heat transfer fluids. The suspension of the nanoparticles in nanofluids creates a larger interaction surface to the volume ratio. Therefore, they can be distributed uniformly to bring about the most effective enhancement of heat transfer without causing a considerable pressure drop. These advantages introduce nanofluids as a desirable heat transfer fluid in the cooling and heating industries. The thermal effects of nanofluids in both forced and free convection flows have interested researchers to a great extent in the last decade. Investigating the interaction mechanisms happening between nanoparticles and base fluid is the main goal of the study. These mechanisms can be explained via different approaches through some theoretical and numerical methods. Two common approaches regarding particle-fluid interactions are Eulerian-Eulerian and Eulerian-Lagrangian. The dominant conceptions in each of them are slip velocity and interaction forces respectively. The mixture multiphase model as part of the Eulerian-Eulerian approach deals with slip mechanisms and somehow mass diffusion from the nanoparticle phase to the fluid phase. The slip velocity can be induced by a pressure gradient, buoyancy, virtual mass, attraction and repulsion between particles. Some of the diffusion processes can be caused by the gradient of temperature and concentration. The discrete phase model (DPM) is a part of the Eulerian-Lagrangian approach. The interactions between solid and liquid phase were presented as forces such as drag, pressure gradient force, virtual mass force, gravity, electrostatic forces, thermophoretic and Brownian forces. The energy transfer from particle to continuous phase can be introduced through both convective and conduction terms on the surface of the particles. A study of both approaches was conducted in the case of laminar and turbulent forced convections as well as cavity flow natural convection. The cases included horizontal and vertical pipes and a rectangular cavity. An experimental study was conducted for cavity flow to be compared with the simulation results. The results of the forced convections were evaluated with data from literature. Alumina and zinc oxide nanoparticles with different sizes were used in cavity experiments and the same for simulations. All the equations, slip mechanisms and forces were implemented in ANSYS-Fluent through some user-defined functions. The comparison showed good agreement between experiments and numerical results. Nusselt number and pressure drops were the heat transfer and flow features of nanofluid and were found in the ranges of the accuracy of experimental measurements. The findings of the two approaches were somehow different, especially regarding the concentration distribution. The mixture model provided more uniform distribution in the domain than the DPM. Due to the Lagrangian frame of the DPM, the simulation time of this model was much longer. The method proposed in this research could also be a useful tool for other areas of particulate systems.
Thesis (PhD)--University of Pretoria, 2016.
Mechanical and Aeronautical Engineering
PhD
Unrestricted

[ES] El principal desafío en los motores turbina de gas empleados en aviación reside en aumentar la eficiencia del ciclo termodinámico manteniendo las emisiones contaminantes por debajo de las rigurosas restricciones. Ésto ha conllevado la necesidad de diseñar nuevas estrategias de inyección/combustión que operan en puntos de operación peligrosos por su cercanía al límite inferior de apagado de llama. En este contexto, el concepto Lean Direct Injection (LDI) ha emergido como una tecnología prometedora a la hora de reducir los óxidos de nitrógeno (NOx) emitidos por las plantas propulsoras de los aviones de nueva generación. En este contexto, la presente tesis tiene como objetivos contribuir al conocimiento de los mecanismos físicos que rigen el comportamiento de un quemador LDI y proporcionar herramientas de análisis para una profunda caracterización de las complejas estructuras de flujo de turbulento generadas en el interior de la cámara de combustión. Para ello, se ha desarrollado una metodología numérica basada en CFD capaz de modelar el flujo bifásico no reactivo en el interior de un quemador LDI académico mediante enfoques de turbulencia U-RANS y LES en un marco Euleriano-Lagrangiano. La resolución numérica de este problema multi-escala se aborda mediante la descripción completa del flujo a lo largo de todos los elementos que constituyen la maqueta experimental, incluyendo su paso por el swirler y entrada a la cámara de combustión. Ésto se lleva a cabo través de dos códigos CFD que involucran dos estrategias de mallado diferentes: una basada en algoritmos de generación y refinamiento automático de la malla (AMR) a través de CONVERGE y otra técnica de mallado estático más tradicional mediante OpenFOAM. Por un lado, se ha definido una metodología para obtener una estrategia de mallado óptima mediante el uso del AMR y se han explotado sus beneficios frente a los enfoques tradicionales de malla estática. De esta forma, se ha demostrado que la aplicabilidad de las herramientas de control de malla disponibles en CONVERGE como el refinamiento fijo (fixed embedding) y el AMR son una opción muy interesante para afrontar este tipo de problemas multi-escala. Los resultados destacan una optimización del uso de los recursos computacionales y una mayor precisión en las simulaciones realizadas con la metodología presentada. Por otro lado, el uso de herramientas CFD se ha combinado con la aplicación de técnicas de descomposición modal avanzadas (Proper Orthogonal Decomposition and Dynamic Mode Decomposition). La identificación numérica de los principales modos acústicos en la cámara de combustión ha demostrado el potencial de estas herramientas al permitir caracterizar las estructuras de flujo coherentes generadas como consecuencia de la rotura de los vórtices (VBB) y de los chorros fuertemente torbellinados presentes en el quemador LDI. Además, la implementación de estos procedimientos matemáticos ha permitido tanto recuperar información sobre las características de la dinámica de flujo como proporcionar un enfoque sistemático para identificar los principales mecanismos que sustentan las inestabilidades en la cámara de combustión. Finalmente, la metodología validada ha sido explotada a través de un Diseño de Experimentos (DoE) para cuantificar la influencia de los factores críticos de diseño en el flujo no reactivo. De esta manera, se ha evaluado la contribución individual de algunos parámetros funcionales (el número de palas del swirler, el ángulo de dichas palas, el ancho de la cámara de combustión y la posición axial del orificio del inyector) en los patrones del campo fluido, la distribución del tamaño de gotas del combustible líquido y la aparición de inestabilidades en la cámara de combustión a través de una matriz ortogonal L9 de Taguchi. Este estudio estadístico supone un punto de partida para posteriores estudios de inyección, atomización y combus
[CA] El principal desafiament als motors turbina de gas utilitzats a la aviació resideix en augmentar l'eficiència del cicle termodinàmic mantenint les emissions contaminants per davall de les rigoroses restriccions. Aquest fet comporta la necessitat de dissenyar noves estratègies d'injecció/combustió que radiquen en punts d'operació perillosos per la seva aproximació al límit inferior d'apagat de flama. En aquest context, el concepte Lean Direct Injection (LDI) sorgeix com a eina innovadora a l'hora de reduir els òxids de nitrogen (NOx) emesos per les plantes propulsores dels avions de nova generació. Sota aquest context, aquesta tesis té com a objectius contribuir al coneixement dels mecanismes físics que regeixen el comportament d'un cremador LDI i proporcionar ferramentes d'anàlisi per a una profunda caracterització de les complexes estructures de flux turbulent generades a l'interior de la càmera de combustió. Per tal de dur-ho a terme s'ha desenvolupat una metodología numèrica basada en CFD capaç de modelar el flux bifàsic no reactiu a l'interior d'un cremador LDI acadèmic mitjançant els enfocaments de turbulència U-RANS i LES en un marc Eulerià-Lagrangià. La resolució numèrica d'aquest problema multiescala s'aborda mitjançant la resolució completa del flux al llarg de tots els elements que constitueixen la maqueta experimental, incloent el seu pas pel swirler i l'entrada a la càmera de combustió. Açò es duu a terme a través de dos codis CFD que involucren estratègies de mallat diferents: una basada en la generación automàtica de la malla i en l'algoritme de refinament adaptatiu (AMR) amb CONVERGE i l'altra que es basa en una tècnica de mallat estàtic més tradicional amb OpenFOAM. D'una banda, s'ha definit una metodologia per tal d'obtindre una estrategia de mallat òptima mitjançant l'ús de l'AMR i s'han explotat els seus beneficis front als enfocaments tradicionals de malla estàtica. D'aquesta forma, s'ha demostrat que l'aplicabilitat de les ferramente de control de malla disponibles en CONVERGE com el refinament fixe (fixed embedding) i l'AMR són una opció molt interessant per tal d'afrontar aquest tipus de problemes multiescala. Els resultats destaquen una optimització de l'ús dels recursos computacionals i una major precisió en les simulacions realitzades amb la metodologia presentada. D'altra banda, l'ús d'eines CFD s'ha combinat amb l'aplicació de tècniques de descomposició modal avançades (Proper Orthogonal Decomposition and Dynamic Mode Decomposition). La identificació numèrica dels principals modes acústics a la càmera de combustió ha demostrat el potencial d'aquestes ferramentes al permetre caracteritzar les estructures de flux coherents generades com a conseqüència del trencament dels vòrtex (VBB) i dels raigs fortament arremolinats presents al cremador LDI. A més, la implantació d'estos procediments matemàtics ha permès recuperar informació sobre les característiques de la dinàmica del flux i proporcionar un enfocament sistemàtic per tal d'identificar els principals mecanismes que sustenten les inestabilitats a la càmera de combustió. Finalment, la metodologia validada ha sigut explotada a traves d'un Diseny d'Experiments (DoE) per tal de quantificar la influència dels factors crítics de disseny en el flux no reactiu. D'aquesta manera, s'ha avaluat la contribución individual d'alguns paràmetres funcionals (el nombre de pales del swirler, l'angle de les pales, l'amplada de la càmera de combustió i la posició axial de l'orifici de l'injector) en els patrons del camp fluid, la distribució de la mida de gotes del combustible líquid i l'aparició d'inestabilitats en la càmera de combustió mitjançant una matriu ortogonal L9 de Taguchi. Aquest estudi estadístic és un bon punt de partida per a futurs estudis de injecció, atomització i combustió en cremadors LDI.
[EN] Aeronautical gas turbine engines present the main challenge of increasing the efficiency of the cycle while keeping the pollutant emissions below stringent restrictions. This has led to the design of new injection-combustion strategies working on more risky and problematic operating points such as those close to the lean extinction limit. In this context, the Lean Direct Injection (LDI) concept has emerged as a promising technology to reduce oxides of nitrogen (NOx) for next-generation aircraft power plants In this context, this thesis aims at contributing to the knowledge of the governing physical mechanisms within an LDI burner and to provide analysis tools for a deep characterisation of such complex flows. In order to do so, a numerical CFD methodology capable of reliably modelling the 2-phase nonreacting flow in an academic LDI burner has been developed in an Eulerian-Lagrangian framework, using the U-RANS and LES turbulence approaches. The LDI combustor taken as a reference to carry out the investigation is the laboratory-scale swirled-stabilised CORIA Spray Burner. The multi-scale problem is addressed by solving the complete inlet flow path through the swirl vanes and the combustor through two different CFD codes involving two different meshing strategies: an automatic mesh generation with adaptive mesh refinement (AMR) algorithm through CONVERGE and a more traditional static meshing technique in OpenFOAM. On the one hand, a methodology to obtain an optimal mesh strategy using AMR has been defined, and its benefits against traditional fixed mesh approaches have been exploited. In this way, the applicability of grid control tools available in CONVERGE such as fixed embedding and AMR has been demonstrated to be an interesting option to face this type of multi-scale problem. The results highlight an optimisation of the use of the computational resources and better accuracy in the simulations carried out with the presented methodology. On the other hand, the use of CFD tools has been combined with the application of systematic advanced modal decomposition techniques (i.e., Proper Orthogonal Decomposition and Dynamic Mode Decomposition). The numerical identification of the main acoustic modes in the chamber have proved their potential when studying the characteristics of the most powerful coherent flow structures of strongly swirled jets in a LDI burner undergoing vortex breakdown (VBB). Besides, the implementation of these mathematical procedures has allowed both retrieving information about the flow dynamics features and providing a systematic approach to identify the main mechanisms that sustain instabilities in the combustor. Last, this analysis has also allowed identifying some key features of swirl spray systems such as the complex pulsating, intermittent and cyclical spatial patterns related to the Precessing Vortex Core (PVC). Finally, the validated methodology is exploited through a Design of Experiments (DoE) to quantify the influence of critical design factors on the non-reacting flow. In this way, the individual contribution of some functional parameters (namely the number of swirler vanes, the swirler vane angle, the combustion chamber width and the axial position of the nozzle tip) into both the flow field pattern, the spray size distribution and the occurrence of instabilities in the combustion chamber are evaluated throughout a Taguchi's orthogonal array L9. Such a statistical study has supposed a good starting point for subsequent studies of injection, atomisation and combustion on LDI burners.
Belmar Gil, M. (2020). Computational study on the non-reacting flow in Lean Direct Injection gas turbine combustors through Eulerian-Lagrangian Large-Eddy Simulations [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/159882
TESIS

Un fluide visqueux newtonien est introduit entre deux ellipses confocables dont les parois interne et externe peuvent être déplacées le long de leur circonférence. Une solution analytique en coordonnées elliptiques a été récemment obtenue à partir des hypothèses du régime de Stokes. Les sections de Poincaré, les points périodiques de l'écoulement et l'évolution d'une tache de traceur, montrent qu'un régime de chaos lagrangien avec de grandes surfaces potentielles de mélange peut se développer dans cette géométrie qui est également un échangeur de chaleur efficace. Le dispositif expérimental a été construit et testé. Les résultats des études théoriques et numériques ont été confrontés avec succès aux résultats expérimentaux par les méthodes suivantes: lignes de courant, déformation de taches de traceur fluorescent, sensibilité aux conditions initiales, test de nombreux profils de vitesses des parois, étude d'écoulements asymptotiques. L’analogie entre les résultats dérives de la géométrie à ellipses concentriques et de la géométrie à cercles excentrés donne des règles générales sur le comportement des mélangeurs bi-dimensionnels annulaires: importance de l'origine relative de l'énergie transférée au fluide, efficacité de l'addition de petits effets inertiels. Des applications directes importantes de cette recherche sont proposées: développement d'une méthode simple d'optimisation de mélangeurs par un choix judicieux de profils de vitesse élémentaires ; développement de nouveaux outils (sections de Poincaré et étude entropique) pour optimiser la conception de nouveaux mélangeurs

In power plants using large utility coal-fired boilers for generation of electricity the coal is pulverised in coal mills and then it has to be pneumatically transported and distributed to a larger number of burners (e.g. 30-40) circumferentially arranged in several rows around the burning chamber of the boiler. Besides the large pipework flow splitting devices are necessary for distribution of an equal amount of pulverised fuel (PF) to each of the burners. So called trifurcators (without inner fittings or guiding vanes) and ''riffle'' type bifurcators are commonly used to split the gas-coal particle flow into two or three pipes/channels with an equal amount of PF mass flow rate in each outflow cross section of the flow splitting device. These PF flow splitting devices are subject of a number of problems. First of all an uneven distribution of PF over the burners of a large utility boiler leads to operational and maintenance problems, increased level of unburned carbon and higher rates of NOX emissions. Maldistribution of fuel between burners caused by non uniform concentration of the PF (particle roping) in pipe and channel bends prior to flow splitting devices leads to uncontrolled differences in the fuel to air ratio between burners. This results in localised regions in the furnace which are fuel rich, where insufficient air causes incomplete combustion of the fuel. Other regions in the furnace become fuel lean, forming high local concentrations of NOX due to the high local concentrations of O2. Otherwise PF maldistribution can impact on power plant maintenance in terms of uneven wear on PF pipework, flow splitters as well as the effects on boiler panels (PF deposition, corrosion, slagging). In order to address these problems in establishing uniform PF distribution over the outlet cross sections of flow splitting devices in the pipework of coal-fired power plants the present paper deals with numerical prediction and analysis of the complex gas and coal particle (PF) flow through trifurcators and ''riffle'' type bifurcators. The numerical investigation is based on a 3-dimensional Eulerian- Lagrangian approach (MISTRAL/PartFlow-3D) developed by Frank et al. The numerical method is capable to predict isothermal, incompressible, steady gas- particle flows in 3-dimensional, geometrically complex flow geometries using boundary fitted, block-structured, numerical grids. Due to the very high numerical effort of the investigated gas-particle flows the numerical approach has been developed with special emphasis on efficient parallel computing on clusters of workstations or other high performance computing architectures. Besides the aerodynamically interaction between the carrier fluid phase and the PF particles the gas-particle flow is mainly influenced by particle-wall interactions with the outer wall boundaries and the inner fittings and guiding vanes of the investigated flow splitting devices. In order to allow accurate quantitative prediction of the motion of the disperse phase the numerical model requires detailed information about the particle-wall collision process. In commonly used physical models of the particle-wall interaction this is the knowledge or experimental prediction of the restitution coefficients (dynamic friction coefficient, coefficient of restitution) for the used combination of particle and wall material, e.g. PF particles on steel. In the present investigation these parameters of the particle-wall interaction model have been obtained from special experiments in two test facilities. Basic experiments to clarify the details of the particle-wall interaction process were made in a test facility with a spherical disk accelerator. This test facility furthermore provides the opportunity to investigate the bouncing process under normal pressure as well as under vacuum conditions, thus excluding aerodynamically influences on the motion of small particles in the near vicinity of solid wall surfaces (especially under small angles of attack). In this experiments spherical glass beads were used as particle material. In a second test facility we have investigated the real impact of non-spherical pulverised fuel particles on a steel/ceramic target. In this experiments PF particles were accelerated by an injector using inert gas like e.g. CO2 or N2 as the carrier phase in order to avoid dust explosion hazards. The obtained data for the particle-wall collision models were compared to those obtained for glass spheres, where bouncing models are proofed to be valid. Furthermore the second test facility was used to obtain particle erosion rates for PF particles on steel targets as a function of impact angles and velocities. The results of experimental investigations has been incorporated into the numerical model. Hereafter the numerical approach MISTRAL/PartFlow-3D has been applied to the PF flow through a ''riffle'' type bifurcator. Using ICEM/CFD-Hexa as grid generator a numerical mesh with approximately 4 million grid cells has been designed for approximation of the complex geometry of the flow splitting device with all its interior fittings and guiding vanes. Based on a predicted gas flow field a large number of PF particles are tracked throughout the flow geometry of the flow-splitter. Besides mean quantities of the particle flow field like e.g. local particle concentrations, mean particle velocities, distribution of mean particle diameter, etc. it is now possible to obtain information about particle erosion on riffle plates and guiding vanes of the flow splitting device. Furthermore the influence of different roping patterns in front of the flow splitter on the uniformness of PF mass flow rate splitting after the bifurcator has been investigated numerically. Results show the efficient operation of the investigated bifurcator in absence of particle roping, this means under conditions of an uniform PF particle concentration distribution in the inflow cross section of the bifurcator. If particle roping occurs and particle concentration differs over the pipe cross section in front of the bifurcator the equal PF particle mass flow rate splitting can be strongly deteriorated in dependence on the location and intensity of the particle rope or particle concentration irregularities. The presented results show the importance of further development of efficient rope splitting devices for applications in coal-fired power plants. Numerical analysis can be used as an efficient tool for their investigation and further optimisation under various operating and flow conditions.

Dans cette thèse, nous implémentons les équations de Saint Venant (SV), ou de Shallow Water, sur des grilles non structurées afin de simuler des écoulements de surface libre sur des bathymétries irrégulières, incluant inondation et d'autres phénomènes complexes qui se produisent généralement dans des applications hydrodynamiques. En particulier, nous voudrions simuler avec précision les tsunamis, la propagation d'onde à grande échelle jusqu'à l'inondation très localisé. À cette fin, nous utilisons deux méthodes qui sont comparées en profondeur le long du manuscrit: la méthode des volumes finis, très populaire dans la communauté hydrodynamique et hydraulique et une technique plus récente appelée Distribution du Résidu appartenant à la classe des schémas upwind multidimensionnels. Pour améliorer la résolution de certaines caractéristiques de l'écoulement telles que le développement du déferlement et les inondations à petite échelle, nous utilisons une adaptation de maillage dynamique basée sur une redistribution des noeuds de maillage, aussi appelé adaptation de type r (r signifiant "relocalisation"). La combinaison appropriée de cette méthode avec le solveur SV est généralement appelée Méthode de Maillage Mobile. Parmi les nombreux algorithmes de maillage mobile disponibles, nous proposons une forme Arbitrary Lagrangian Eulerian (ALE) des équation SV qui permettent de faire évoluer les variables de flux d'une maille à l'autre de manière élégante. Dans ce contexte, nous soulignons les principales contributions de la thèse: Nous montrons l'importance de conserver toutes les propriétés standards d'un solveur Eulérien SWE tel que la préservation du lac au repos et la conservation de la masse également sur des maillages en mouvement. Notre couplage ALE est comparé à l'approche de rezoning, avec une légère augmentation de la performance globale de l'algorithme en termes de précision et de temps CPU. Nous étendons l'approche ALE sur la sphère afin d'inclure l'effet de la courbure terrestre dans la dynamique de propagation des ondes à grande échelle du tsunami. La simulation du tsunami 2011 de Tohoku-Honsu devrait prouver que la méthode de maillage mobile étudiée dans la thèse, bien que simple, pourrait être un bon candidat pour réduire le coût de calcul des simulations de tsunami
In this thesis we implement the Shallow Water equations (SWEs) on unstructured grids in order to simulate free surface flow over irregular bathymetries, wetting/drying and other complex phenomena that typically occurs in hydrodynamic applications. In particular we would to accurately simulate tsunami events, from large scale wave propagation up to localized runup. To this aim we use two methods that are extensively compared along the manuscript: the Finite Volume method, which is very popular in the hydrodynamics and hydraulic community and a more recent technique called Residual Distribution which belongs to the class of multidimensional upwind schemes. To enhance the resolution of important flow feature such as bore development or small scale flooding, we use a dynamic mesh adaptation based on a redistribution of mesh nodes or r-adaptation (r stands for "relocation"). The proper combination of this method with the flow solver is usually referred to as Moving Mesh Method. Among the many different moving mesh algorithms available we propose an Arbitrary Lagrangian Eulerian (ALE) form of the SWEs which elegantly permit to evolve the flow variables from one mesh to the updated one

Ce travail propose une etude numerique et experimentale du comportement d'un ecoulement liquide turbulent charge de particules solides. Le contexte industriel est celui de l'encrassement particulaire des echangeurs de chaleur a plaques. L'observation visuelle ainsi que les mesures de velocimetrie a l'interieur d'une maquette font apparaitre l'existence de structures tourbillonnaires coherentes de grande dimension et confirment le lien etroit entre le depot de particules et la dynamique de l'ecoulement. Les tourbillons contribuent a creer des zones preferentielles de contact entre paroi et particules et faconnent le depot selon un mecanisme precis. Deux processus d'arrachement du depot ont egalement ete mis en evidence. Les phenomenes hydrauliques et les comportements particulaires mis en evidence dans l'etude experimentale sont compares a des cas types dans une etude bibliographique. Une etude numerique basee sur l'utilisation du logiciel trio - developpe au commissariat a l'energie atomique, cea permet de completer l'etude en s'attachant a predire le comportement des particules, couple de facon dynamique a l'ecoulement turbulent. L'approche est basee sur un couplage entre la simulation pseudo-directe de l'ecoulement turbulent et un modele lagrangien pour les trajectoires des particules. Les resultats obtenus ont permis de conclure a un bon accord qualitatif entre les resultats numeriques et les mesures experimentales. La disposition du depot sur la surface du canal est globalement retrouvee par la simulation numerique. L'influence de certains parametres sur le phenomene d'encrassement comme le regime d'ecoulement (re=5000 ou re=10000), la disposition horizontale ou verticale du canal ou le diametre des particules (d#p=100 m ou d#p=25 m) a ete etudiee

La variabilité et la dynamique de l'upwelling côtier du Nord du Pérou au Sud du Chili (50°S) et des courants associés (CPCC Chile Peru coastal Current et PCUC Peru-Chile Undercurrent) ont été étudiées à l'aide de données satellites et de modèles numériques. Les caractéristiques des variations le long de la côte sont séparées par une zone de transition autour de 20°S où les vents, l'upwelling et le PCUC sont particulièrement faibles. Les origines des eaux du PCUC et de l'upwelling ont également été étudiées à l'aide d'outils lagrangiens et les transports ont été quantifiés. Trois sources ont été identifiées : l'EUC (Equatorial Undercurrent), les SSCC (Southern Sub-Surface Counter Current) et une troisième source au Sud du Chili, liée à l'ESPIW (Eastern South Pacific Intermediate Water).

L'adroterapia è il trattamento per la cura dei tumori attraverso l'irraggiamento del volume tumorale con un fascio di protoni o di ioni pesanti. Il vantaggio nell'utilizzare queste particelle consiste nel loro profilo di dose-profondità caratterizzato dal picco di Bragg che permette di massimizzare il danno nella regione tumorale limitando i danni ai tessuti sani limitrofi. Fino ad ora, non è ancora stata fatta una stima completa degli effetti nocivi causati dagli eventi di frammentazione nucleare tra le particelle del fascio e il materiale biologico. L'esperimento FOOT nasce con l'obiettivo di misurare la sezione d'urto differenziale di tutti i prodotti emessi nella frammentazione nucleare tra il fascio e il paziente. E' stato effettuato a tal proposito uno studio sull'identificazione dei frammenti nel quale sono stati stimati il numero atomico z e il numero di massa A. La ricostruzione della carica e della massa è stata effettuata analizzando i frammenti prodotti da un fascio di ossigeno-16 avente energia cinetica 200 MeV/u su un bersaglio di polietilene. I dati sono stati ottenuti attraverso una simulazione effettuata con il codice FLUKA, includendo le risoluzione sperimentali di tutti i rivelatori ottenute da test effettuati su fascio. Sono stati selezionati gli 8 frammenti più comunemente prodotti nell'interazione nucleare tra il fascio e il bersaglio (uno per ogni carica, da z=1 a z=8). La stima di z è stata ottenuta con una risoluzione percentuale inversamente proporzionale al valore della carica stessa, da un 6% per l'idrogeno a un 2% per l'ossigeno. La ricostruzione di A è stata effettuata con metodi diretti e attraverso l'uso di due metodi di fit, minimizzazione del chi quadro e Augmented Lagrangian Method. La migliore risoluzione percentuale di A è risultata del 4% per i frammenti pesanti e del 5% per quelli leggeri. I risultati così ottenuti hanno dimostrato la potenzialità dell'esperimento nell'univoca identificazione dei frammenti considerati.

Animal group formation has often been studied by mathematical biologists through PDE models, producing classical results like traveling and stationary waves. Recently, Eftimie et al. introduced a 1-D PDE model that considers three social interactions between individuals in the relevant neighborhoods, specifically re- pulsion, alignment, and attraction. It takes into account the orientation of the neighbors when consider- ing if they can communicate. This has resulted in exciting new movement behaviors like zig-zag pulses, breathers, and feathers. In this work, we translate the Eftimie model into a Lagrangian implementation. Currently, the results from the Lagrangian formulations show many of the results displayed by Eftimie’s original PDE model, producing patterns like the zig-zag, breather traveling, and stationary pulses. In addi- tion, we model animal movement with an ODE approach to complete the investigation regarding the role of direction-dependent communication mechanism in discrete-space.
Applied Mathematics

碩士
國立成功大學
航空太空工程學系
88
Turbulent dispersion of the dispersed-phase elements in two-phase flows can be performed by probabilistic computation of the particle’s spatial distribution. One way to quantify the turbulent dispersion of particles is to determine how the mean square dispersion of the particles，，changes with time。Nevertheless，very few Lagrangian velocity autocorrelation function information are available in the published literature。 The measured raw data in the well-defined experiment of a planar，two-phase mixing-layer flow which was conducted by Wang and coworkers are used to determine the Lagrangian velocity autocorrelation functions for both the carrier and dispersed phases。The study reveals that the Lagrangian integral time scales along the streamwise (predominant) flow direction，τLi，of the carrier fluid in the two-phase case are largen than those in the single-phase case。Furthermore，the choice of Δτ is important in determination of experimental data of the Lagrangian velocity autocorrelation。The Lagrangian integral time scale for the dispersed phase is different from that for the carrier fluid，and dependent on the particle size。

碩士
國立交通大學
工業工程與管理研究所
86
A Lanrangian relaxation model is studied for interpreting the scheduling behavior on product process structure. Essentially the presentation of the new approach involves two research results. One is the automated generation of process product structre. It means to store the graphic bill of materials (BOM) and routing information in a relational database using linked-lists. Then both are synathesized into a tabular representation of process product structure using union and joint operations. After that, the synthesized table is automatically transformed into a product process structrure in graphic form. The other is the Lagrangian realxation interpretation of the bottlenck schduling techniques. Here the bottleneck on the product process structure is located using capacity analysis. Then feasible schedules of the bottleneck are prpagated along the product process structure. Also the research results has been implemented on a real-time expert system G2 for showing its performance.

碩士
國立臺灣海洋大學
河海工程學系
105
This study develops the total Lagrange formulation of geometrical nonlinear theory for thin-plate structures which is based on total Lagrange virtual work principle. The process of geometrical nonlinear theory includes the stages of correction and prediction. In the stage of correction, first we decide the position-victor and displacement on mid-plane, and then establish strain and curvature of the plane by constitutive law, that we can establish the governing equation and boundary condition in the form which doesn’t have to integrate. In the stage of prediction, we can establish the governing equation and boundary condition in simplified form by using rigid-body motion and ignoring high-order term. Then, we can establish geometrical nonlinear theory of thin-plate structures by using the relation between 1st P.K. stress and 2nd P.K. stress, including the stage of correction and prediction which were established by total Lagrange formulation and update Lagrange formulation. The governing equation which established by static equilibrium is more complicate than by virtual work principle . At last, the moving least square differential quadrature method was used to analyze examples.

碩士
國立臺灣大學
土木工程學研究所
102
In this research, we conduct a method to estimate the moving velocity field using the parameters of the regression function by comparing the radar echo image pairs in adjacent time. The regression function is built from advection equation in polar coordinate, with an assumption that the system velocity of typhoon can be represented by Fourier series. The purpose is to forecast precipitation with the estimated moving velocity fields. The radar echo data in Central Weather Bureau is separated by ten minutes in time and resolution fixed in space. Typhoon behavior is often unable to meet the CFL condition under such data density. This limits the ability of the regression model to estimate the correct velocity field. The estimation method shifts the typhoon features, and is conducted by two parts. The first part is similar to method of Tracking Radar Echo by Correlation (TREC), utilizing Lagrangian scheme, test the displacement by try-and-error method, to track the rough overall shift. The second part is performed after the system is satisfying CFL condition, an Eulerian scheme utilizing advection equation to perform regression, estimate the coefficients of the detailed Fourier velocity function. The system velocity is the summation of the rough overall shifting velocity and the detailed velocity obtained from regression. This method is named Advection Based Lagrangian-Eulerian Regression (ABLER). In this research, the algorithm of our method is verified with observing system experiments (OSE), the data of composed 2-D Gaussian image and simulation results of the WRF model in 2013 Soulik typhoon event are used in the verification.

碩士
國立中興大學
機械工程學系所
101
This study presented numerical study on the motion of drug delivery vehicle (DDV) in blood vessel based the enhanced permeability and retention (EPR) effect due to the tumer-defective blood vessel. Instead of traditional particle tracing, the arbitrary Lagrangian-Eulerian Algorithm (ALE) was employed in this study to account for the force coupling between the moving DDV and blood flow. The ALE and Navier-Stokes modules facilitated by COMSOL software were employed to carry out the numerical simulations. The computational domain was treated as two-dimensional containing three trans-cellular holes caused by the tumor tissue growth. Motions of both spherical and rod-like DDV were considered in this study. The DDV was considered to enter the tumor tissues when it traveled through the trans-cellular holes. The simulated results indicated that the initial locations and size played importation roles on governing the motion of DDV. The DDV had greater chance to travel through the trans-cellular holes when its initial location was close to the vessel wall and its diameter ranged between 20 and 100 nm. The numerical simulations also demonstrated that there was significant difference on the DDV trajectories between ALE and particle tracing computations. The motion characteristics of the rod-like DDV was similar to that of its spherical counterpart except the inclusion of rotational motion. The rod-like DDV had less chance of entering the tumor tissue as compared with the spherical DDV because of its rotational motion. The motion of multiple DDV system in the computation domain was also examined in this study for both types of DDV. By using a pair of DDVs, the numerical results showed that it was more difficult for the DDV to enter the trans-cellular holes due to the mutual interaction between DDVs.

碩士
國立成功大學
交通管理學系碩博士班
100
This thesis investigates the mixed-product assembly line sequencing problem for a door lock manufacturing company in Taiwan. Companies in the door lock industry schedule their production processes to minimize their costs while meeting customer demand. The variance and diversity of lock components complicate the mixed-product assembly line sequencing problem and directly influence the material requirement planning and human resources costs. The assembly line sequencing problem of one of the largest ironware manufacturing companies in Asia, company F, is studied and an integer programming mathematical model with constraints on production lines, labor, warehouse capacity, and order fulfillment rates is formulated to minimize the total cost. A heuristic based on the Lagrangian relaxation principle is devised to solve this mixed-product assembly line sequencing problem. The efficiency of the proposed Lagrangian relaxation heuristic is evaluated by comparing its solutions with those obtained using CPLEX. The results show that the proposed heuristic solves the real-world problem almost 15 times faster than CPLEX in terms of CPU time, with equal solution quality. The salient results and practical issues involved in this unique problem are discussed in detail.

In this work, flow through synthetic arrangements of contacting spheres is studied as a model problem for porous media and packed bed type flows. Direct numerical simulations are performed for moderate pore Reynolds numbers in the range, 10 ≤ Re ≤ 600, where non-linear porescale flow features are known to contribute significantly to macroscale properties of engineering interest. To first choose and validate appropriate computational models for this problem, the relative performance of two numerical approaches involving body conforming and non-conforming grids for simulating porescale flows is examined. In the first approach, an unstructured solver is used with tetrahedral meshes, which conform to the boundaries of the porespace. In the second approach, a fictitious domain formulation (Apte et al., 2009. J Comput. Phys. 228 (8), 2712-2738) is used, which employs non-body conforming Cartesian grids and enforces the no-slip conditions on the pore boundaries implicitly through a rigidity constraint force. Detailed grid convergence studies of both steady and unsteady flow through prototypical arrangements of spheres indicate that for a fixed level of uncertainty, significantly lower grid densities may be used with the fictitious domain approach, which also does not require complex grid generation techniques. Next, flows through both random and structured arrangements of spheres are simulated at pore Reynolds numbers in the steady inertial ( 10 ≲ Re ≲ 200) and unsteady inertial (Re ≈ 600) regimes, and used to analyze the characteristics of porescale vortical structures. Even at similar Reynolds numbers, the vortical structures observed in structured and random packings are remarkably different. The interior of the structured packings are dominated by multi-lobed vortex rings structures that align with the principal axes of the packing, but perpendicular to the mean flow. The random packing is dominated by helical vortices, elongated parallel to the mean flow direction. The unsteady dynamics observed in random and structured arrangements are also distinct, and are linked to the behavior of the porescale vortices. Finally, to investigate the existence and behavior of transport barriers in packed beds, a numerical tool is developed to compute high resolution finite-time Lyapunov exponent (FTLE) fields on-the-fly during DNS of unsteady flows. Ridges in this field are known to correspond to Lagrangian Coherent Structures (LCS), which are invariant barriers to transport and form the skeleton of time dependent Lagrangian fluid motion. The algorithm and its implementation into a parallel DNS solver are described in detail and used to explore several flows, including unsteady inertial flow in a random sphere packing. The resulting FTLE fields unambiguously define the boundaries of dynamically distinct porescale features such as counter rotating helical vortices and jets, and capture time dependent phenomena including vortex shedding at the pore level.
Graduation date: 2013