Pore-scale modeling of two-phase flow instabilities in porous media

Les problèmes d'écoulements multiphasiques en média poreux sont d'un grand intérêt pour de nombreuses applications scientifiques et techniques ; comme la séquestration de C02, l'extraction de pétrole et la dépollution des aquifères. La complexité intrinsèque des systèmes multiphasiques et l'hétérogénéité des formations géologiques sur des échelles multiples représentent un challenge majeur pour comprendre et modéliser les déplacements immiscibles dans les milieux poreux. Les descriptions à l'échelle supérieure basées sur la généralisation de l'équation de Darcy sont largement utilisées, mais ces méthodes sont sujettes à limitations pour les écoulements présentant de l'hystérèse. Les avancées récentes en terme de performances computationnelles et le développement de méthodes précises pour caractériser l'espace interstitiel ainsi que la distribution des phases ont favorisé l'utilisation de modèles qui permettent une résolution fine à l'échelle du pore. Ces modèles offrent un aperçu des caractéristiques de l'écoulement qui ne peuvent pas être facilement observées en laboratoire et peuvent être utilisé pour expliquer la différence entre les processus physiques et les modèles à l'échelle macroscopique existants. L'objet premier de la thèse se porte sur la simulation numérique directe : les équations de Navier-Stokes sont résolues dans l'espace interstitiel et la méthode du volume de fluide (VOF) est employée pour suivre l'évolution de l'interface. Dans VOF, la distribution des phases est décrite par une fonction fluide pour l'ensemble du domaine et des conditions aux bords particulières permettent la prise en compte des propriétés de mouillage du milieu poreux. Dans la première partie de la thèse, nous simulons le drainage dans une cellule Hele-Shaw 2D avec des obstacles cylindriques. Nous montrons que l'approche proposée est applicable même pour des ratios de densité et de viscosité très importants et permet de modéliser la transition entre déplacement stable et digitation visqueuse. Nous intéressons ensuite à l'interprétation de la pression capillaire à l'échelle macroscopique. Nous montrons que les techniques basées sur la moyenne spatiale de la pression présentent plusieurs limitations et sont imprécises en présence d'effets visqueux et de piégeage. Au contraire, une définition basée sur l'énergie permet de séparer les contributions capillaires des effets visqueux. La seconde partie de la thèse est consacrée à l'investigation des effets d'inertie associés aux reconfigurations irréversibles du ménisque causé par l'interface des instabilités. Comme prototype pour ces phénomènes, nous étudions d'abord la dynamique d'un ménisque dans un pore angulaire. Nous montrons que, dans un réseau de pores cubiques, les sauts et reconfigurations sont si fréquents que les effets d'inertie mènent à différentes configurations des fluides. A cause de la non-linéarité du problème, la distribution des fluides influence le travail des forces de pression, qui, à son tour, provoque une chute de pression dans la loi de Darcy. Cela suggère que ces phénomènes devraient être pris en compte lorsque que l'on décrit l'écoulement multiphasique en média poreux à l'échelle macroscopique. La dernière partie de la thèse s'attache à démontrer la validité de notre approche par une comparaison avec des expériences en laboratoire : un drainage instable dans un milieu poreux quasi 2D (une cellule Hele-Shaw avec des obstacles cylindriques). Plusieurs simulations sont tournées sous différentes conditions aux bords et en utilisant différents modèles (modèle intégré 2D et modèle 3D) afin de comparer certaines quantités macroscopiques avec les observations au laboratoire correspondantes. Malgré le challenge de modéliser des déplacements instables, où, par définition, de petites perturbations peuvent grandir sans fin, notre approche numérique apporte de résultats satisfaisants pour tous les cas étudiés. - Problems involving multiphase flow in porous media are of great interest in many scientific and engineering applications including Carbon Capture and Storage, oil recovery and groundwater remediation. The intrinsic complexity of multiphase systems and the multi scale heterogeneity of geological formations represent the major challenges to understand and model immiscible displacement in porous media. Upscaled descriptions based on generalization of Darcy's law are widely used, but they are subject to several limitations for flow that exhibit hysteric and history- dependent behaviors. Recent advances in high performance computing and the development of accurate methods to characterize pore space and phase distribution have fostered the use of models that allow sub-pore resolution. These models provide an insight on flow characteristics that cannot be easily achieved by laboratory experiments and can be used to explain the gap between physical processes and existing macro-scale models. We focus on direct numerical simulations: we solve the Navier-Stokes equations for mass and momentum conservation in the pore space and employ the Volume Of Fluid (VOF) method to track the evolution of the interface. In the VOF the distribution of the phases is described by a fluid function (whole-domain formulation) and special boundary conditions account for the wetting properties of the porous medium. In the first part of this thesis we simulate drainage in a 2-D Hele-Shaw cell filled with cylindrical obstacles. We show that the proposed approach can handle very large density and viscosity ratios and it is able to model the transition from stable displacement to viscous fingering. We then focus on the interpretation of the macroscopic capillary pressure showing that pressure average techniques are subject to several limitations and they are not accurate in presence of viscous effects and trapping. On the contrary an energy-based definition allows separating viscous and capillary contributions. In the second part of the thesis we investigate inertia effects associated with abrupt and irreversible reconfigurations of the menisci caused by interface instabilities. As a prototype of these phenomena we first consider the dynamics of a meniscus in an angular pore. We show that in a network of cubic pores, jumps and reconfigurations are so frequent that inertia effects lead to different fluid configurations. Due to the non-linearity of the problem, the distribution of the fluids influences the work done by pressure forces, which is in turn related to the pressure drop in Darcy's law. This suggests that these phenomena should be taken into account when upscaling multiphase flow in porous media. The last part of the thesis is devoted to proving the accuracy of the numerical approach by validation with experiments of unstable primary drainage in a quasi-2D porous medium (i.e., Hele-Shaw cell filled with cylindrical obstacles). We perform simulations under different boundary conditions and using different models (2-D integrated and full 3-D) and we compare several macroscopic quantities with the corresponding experiment. Despite the intrinsic challenges of modeling unstable displacement, where by definition small perturbations can grow without bounds, the numerical method gives satisfactory results for all the cases studied.

Predicting spinor condensate dynamics from simple principles

We study the spin dynamics of quasi-one-dimensional F=1 condensates both at zero and finite temperatures for arbitrary initial spin configurations. The rich dynamical evolution exhibited by these nonlinear systems is explained by surprisingly simple principles: minimization of energy at zero temperature and maximization of entropy at high temperature. Our analytical results for the homogeneous case are corroborated by numerical simulations for confined condensates in a wide variety of initial conditions. These predictions compare qualitatively well with recent experimental observations and can, therefore, serve as a guidance for ongoing experiments.

Mesoscale numerical analysis of the historical November 1982 heavy precipitation event over Andorra (Eastern Pyrenees)

From 6 to 8 November 1982 one of the most catastrophic flash-flood events was recorded in the Eastern Pyrenees affecting Andorra and also France and Spain with rainfall accumulations exceeding 400 mm in 24 h, 44 fatalities and widespread damage. This paper aims to exhaustively document this heavy precipitation event and examines mesoscale simulations performed by the French Meso-NH non-hydrostatic atmospheric model. Large-scale simulations show the slow-evolving synoptic environment favourable for the development of a deep Atlantic cyclone which induced a strong southerly flow over the Eastern Pyrenees. From the evolution of the synoptic pattern four distinct phases have been identified during the event. The mesoscale analysis presents the second and the third phase as the most intense in terms of rainfall accumulations and highlights the interaction of the moist and conditionally unstable flows with the mountains. The presence of a SW low level jet (30 m s-1) around 1500 m also had a crucial role on focusing the precipitation over the exposed south slopes of the Eastern Pyrenees. Backward trajectories based on Eulerian on-line passive tracers indicate that the orographic uplift was the main forcing mechanism which triggered and maintained the precipitating systems more than 30 h over the Pyrenees. The moisture of the feeding flow mainly came from the Atlantic Ocean (7-9 g kg-1) and the role of the Mediterranean as a local moisture source was very limited (2-3 g kg-1) due to the high initial water vapour content of the parcels and the rapid passage over the basin along the Spanish Mediterranean coast (less than 12 h).

Sensitivity of single-voxel 1H-MRS in investigating the metabolism of the activated human visual cortex at 7 T.

Proton magnetic resonance spectroscopy (1H-MRS) has been used in a number of studies to noninvasively assess the temporal changes of lactate in the activated human brain. However, the results have not been consistent. The aim of the present study was to test the sensitivity of 1H-MRS during functional experiments at the highest magnetic field currently available for human studies (7 T). Stability and reproducibility of the measurements were evaluated from LCModel analysis of time series of spectra measured during a visual stimulation paradigm and by examination of the difference between spectra obtained at rest and during activation. The sensitivity threshold to detect concentration changes was 0.2 micromol/g for most of the quantified metabolites. The possible variations of metabolite concentrations during visual stimulation were within the same range (+/-0.2 micromol/g). In addition, the influence of a small line-narrowing effect due to the blood oxygenation level-dependent (BOLD) T2* changes on the estimated concentrations was simulated. Quantification of metabolites was, in general, not affected beyond 1% by line-width changes within 0.5 Hz.

On the convergence of EM-like algorithms for image segmentation using Markov random fields.

Inference of Markov random field images segmentation models is usually performed using iterative methods which adapt the well-known expectation-maximization (EM) algorithm for independent mixture models. However, some of these adaptations are ad hoc and may turn out numerically unstable. In this paper, we review three EM-like variants for Markov random field segmentation and compare their convergence properties both at the theoretical and practical levels. We specifically advocate a numerical scheme involving asynchronous voxel updating, for which general convergence results can be established. Our experiments on brain tissue classification in magnetic resonance images provide evidence that this algorithm may achieve significantly faster convergence than its competitors while yielding at least as good segmentation results.

Coordination on Networks: Does Topology Matter?

Effective coordination is key to many situations that affect the well-being of two or more humans. Social coordination can be studied in coordination games between individuals located on networks of contacts. We study the behavior of humans in the laboratory when they play the Stag Hunt game - a game that has a risky but socially efficient equilibrium and an inefficient but safe equilibrium. We contrast behavior on a cliquish network to behavior on a random network. The cliquish network is highly clustered and resembles more closely to actual social networks than the random network. In contrast to simulations, we find that human players dynamics do not converge to the efficient outcome more often in the cliquish network than in the random network. Subjects do not use pure myopic best-reply as an individual update rule. Numerical simulations agree with laboratory results once we implement the actual individual updating rule that human subjects use in our laboratory experiments.

Impingement jet cooling of end windings in a high-speed electric machine

The rotational speed of high-speed electric machines is over 15 000 rpm. These machines are compact in size when compared to the power rate. As a consequence, the heat fluxes are at a high level and the adequacy of cooling becomes an important design criterion. In the high-speed machines, the air gap between the stator and rotor is a narrow flow channel. The cooling air is produced with a fan and the flow is then directed to the air gap. The flow in the gap does not provide sufficient cooling for the stator end windings, and therefore additional cooling is required. This study investigates the heat transfer and flow fields around the coil end windings when cooling jets are used. As a result, an innovative and new assembly is introduced for the cooling jets, with the benefits of a reduced amount of hot spots, a lower pressure drop, and hence a lower power need for the cooling fan. The gained information can also be applied to improve the cooling of electric machines through geometry modifications. The objective of the research is to determine the locations of the hot spots and to find out induced pressure losses with different jet alternatives. Several possibilities to arrange the extra cooling are considered. In the suggested approach cooling is provided by using a row of air jets. The air jets have three main tasks: to cool the coils effectively by direct impingement jets, to increase and cool down the flow that enters the coil end space through the air gap, and to ensure the correct distribution of the flow by forming an air curtain with additional jets. One important aim of this study is the arrangement of cooling jets in such manner that hot spots can be avoided to wide extent. This enables higher power density in high-speed motors. This cooling system can also be applied to the ordinary electric machines when efficient cooling is needed. The numerical calculations have been performed using a commercial Computational Fluid Dynamics software. Two geometries have been generated: cylindrical for the studied machine and Cartesian for the experimental model. The main parameters include the positions, arrangements and number of jets, the jet diameters, and the jet velocities. The investigated cases have been tested with two widely used turbulence models and using a computational grid of over 500 000 cells. The experimental tests have been made by using a simplified model for the end winding space with cooling jets. In the experiments, an emphasis has been given to flow visualisation. The computational analysis shows good agreement with the experimental results. Modelling of the cooling jet arrangement enables also a better understanding of the complex system of heat transfer at end winding space.

Effect of neighborhood and plot size on experiments with multiple-harvest oleraceous crops

The objective of this work was to determine the efficiency of the Papadakis method on the quality evaluation of experiments with multiple-harvest oleraceous crops, and on the estimate of the covariate and the ideal plot size. Data from nine uniformity trials (five with bean pod, two with zucchini, and two with sweet pepper) and from one experiment with treatments (with sweet pepper) were used. Through the uniformity trials, the best way to calculate the covariate was defined and the optimal plot size was calculated. In the experiment with treatments, analyses of variance and covariance were performed, in which the covariate was calculated by the Papadakis method, and experimental precision was evaluated based on four statistics. The use of analysis of covariance with the covariate obtained by the Papadakis method increases the quality of experiments with multiple-harvest oleraceous crops and allows the use of smaller plot sizes. The best covariate is the one that considers a neighboring plot of each side of the reference plot.

Analysis of Industrial Granular Flow Applications by Using Advanced Collision Models

Granular flow phenomena are frequently encountered in the design of process and industrial plants in the traditional fields of the chemical, nuclear and oil industries as well as in other activities such as food and materials handling. Multi-phase flow is one important branch of the granular flow. Granular materials have unusual kinds of behavior compared to normal materials, either solids or fluids. Although some of the characteristics are still not well-known yet, one thing is confirmed: the particle-particle interaction plays a key role in the dynamics of granular materials, especially for dense granular materials. At the beginning of this thesis, detailed illustration of developing two models for describing the interaction based on the results of finite-element simulation, dimension analysis and numerical simulation is presented. The first model is used to describing the normal collision of viscoelastic particles. Based on some existent models, more parameters are added to this model, which make the model predict the experimental results more accurately. The second model is used for oblique collision, which include the effects from tangential velocity, angular velocity and surface friction based on Coulomb's law. The theoretical predictions of this model are in agreement with those by finite-element simulation. I n the latter chapters of this thesis, the models are used to predict industrial granular flow and the agreement between the simulations and experiments also shows the validation of the new model. The first case presents the simulation of granular flow passing over a circular obstacle. The simulations successfully predict the existence of a parabolic steady layer and show how the characteristics of the particles, such as coefficients of restitution and surface friction affect the separation results. The second case is a spinning container filled with granular material. Employing the previous models, the simulation could also reproduce experimentally observed phenomena, such as a depression in the center of a high frequency rotation. The third application is about gas-solid mixed flow in a vertically vibrated device. Gas phase motion is added to coherence with the particle motion. The governing equations of the gas phase are solved by using the Large eddy simulation (LES) and particle motion is predicted by using the Lagrangian method. The simulation predicted some pattern formation reported by experiment.

Fluid flow and dispersion predictions in porous media by utilizing advanced numerical methods

Diplomityön tavoitteena oli tarkastella numeerisen virtauslaskennan avulla virtaukseen liittyviä ilmiöitä ja kaasun dispersiota. Diplomityön sisältö on jaettu viiteen osaan; johdantoon, teoriaan, katsaukseen virtauksen mallinnukseen huokoisessa materiaalissa liittyviin tutkimusselvityksiin, numeeriseen mallinnukseen sekä tulosten esittämiseen ja johtopäätöksiin. Diplomityön alussa kiinnitettiin huomiota erilaisiin kokeellisiin, numeerisiin ja teoreettisiin mallinnusmenetelmiin, joilla voidaan mallintaa virtausta huokoisessa materiaalissa. Kirjallisuusosassa tehtiin katsaus aikaisemmin julkaistuihin puoliempiirisiin ja empiirisiin tutkimusselvityksiin, jotka liittyvät huokoisen materiaalin aiheuttamaan painehäviöön. Numeerisessa virtauslaskenta osassa rakennettiin ja esitettiin huokoista materiaalia kuvaavat numeeriset mallit käyttäen kaupallista FLUENT -ohjelmistoa. Työn lopussa arvioitiin teorian, numeerisen virtauslaskennan ja kokeellisten tutkimusselvitysten tuloksia. Kolmiulotteisen huokoisen materiaalinnumeerisessa mallinnuksesta saadut tulokset vaikuttivat lupaavilta. Näiden tulosten perusteella tehtiin suosituksia ajatellen tulevaa virtauksen mallinnusta huokoisessa materiaalissa. Osa tässä diplomityössä esitetyistä tuloksista tullaan esittämään 55. Kanadan Kemiantekniikan konferenssissa Torontossa 1619 Lokakuussa 2005. ASME :n kansainvälisessä tekniikan alan julkaisussa. Työ on hyväksytty esitettäväksi esitettäväksi laskennallisen virtausmekaniikan (CFD) aihealueessa 'Peruskäsitteet'. Lisäksi työn yksityiskohtaiset tulokset tullaan lähettämään myös CES:n julkaisuun.

Étude des mécanismes de transfert des fibres en sciences forensiques

RESUME Les fibres textiles sont des produits de masse utilisés dans la fabrication de nombreux objets de notre quotidien. Le transfert de fibres lors d'une action délictueuse est dès lors extrêmement courant. Du fait de leur omniprésence dans notre environnement, il est capital que l'expert forensique évalue la valeur de l'indice fibres. L'interprétation de l'indice fibres passe par la connaissance d'un certain nombre de paramètres, comme la rareté des fibres, la probabilité de leur présence par hasard sur un certain support, ainsi que les mécanismes de transfert et de persistance des fibres. Les lacunes les plus importantes concernent les mécanismes de transfert des fibres. A ce jour, les nombreux auteurs qui se sont penchés sur le transfert de fibres ne sont pas parvenus à créer un modèle permettant de prédire le nombre de fibres que l'on s'attend à retrouver dans des circonstances de contact données, en fonction des différents paramètres caractérisant ce contact et les textiles mis en jeu. Le but principal de cette recherche est de démontrer que la création d'un modèle prédictif du nombre de fibres transférées lors d'un contact donné est possible. Dans le cadre de ce travail, le cas particulier du transfert de fibres d'un tricot en laine ou en acrylique d'un conducteur vers le dossier du siège de son véhicule a été étudié. Plusieurs caractéristiques des textiles mis en jeu lors de ces expériences ont été mesurées. Des outils statistiques (régression linéaire multiple) ont ensuite été utilisés sur ces données afin d'évaluer l'influence des caractéristiques des textiles donneurs sur le nombre de fibres transférées et d'élaborer un modèle permettant de prédire le nombre de fibres qui vont être transférées à l'aide des caractéristiques influençant significativement le transfert. Afin de faciliter la recherche et le comptage des fibres transférées lors des expériences de transfert, un appareil de recherche automatique des fibres (liber finder) a été utilisé dans le cadre de cette recherche. Les tests d'évaluation de l'efficacité de cet appareil pour la recherche de fibres montrent que la recherche automatique est globalement aussi efficace qu'une recherche visuelle pour les fibres fortement colorées. Par contre la recherche automatique perd de son efficacité pour les fibres très pâles ou très foncées. Une des caractéristiques des textiles donneurs à étudier est la longueur des fibres. Afin de pouvoir évaluer ce paramètre, une séquence d'algorithmes de traitement d'image a été implémentée. Cet outil permet la mesure de la longueur d'une fibre à partir de son image numérique à haute résolution (2'540 dpi). Les tests effectués montrent que les mesures ainsi obtenues présentent une erreur de l'ordre du dixième de millimètre, ce qui est largement suffisant pour son utilisation dans le cadre de cette recherche. Les résultats obtenus suite au traitement statistique des résultats des expériences de transfert ont permis d'aboutir à une modélisation du phénomène du transfert. Deux paramètres sont retenus dans le modèle: l'état de la surface du tissu donneur et la longueur des fibres composant le tissu donneur. L'état de la surface du tissu est un paramètre tenant compte de la quantité de fibres qui se sont détachées de la structure du tissu ou qui sont encore faiblement rattachées à celle-ci. En effet, ces fibres sont les premières à se transférer lors d'un contact, et plus la quantité de ces fibres par unité de surface est importante, plus le nombre de fibres transférées sera élevé. La longueur des fibres du tissu donneur est également un paramètre important : plus les fibres sont longues, mieux elles sont retenues dans la structure du tissu et moins elles se transféreront. SUMMARY Fibres are mass products used to produce numerous objects encountered everyday. The transfer of fibres during a criminal action is then very common. Because fibres are omnipresent in our environment, the forensic expert has to evaluate the value of the fibre evidence. To interpret fibre evidence, the expert has to know some parameters as frequency of fibres,' probability of finding extraneous fibres by chance on a given support, and transfer and persistence mechanisms. Fibre transfer is one of the most complex parameter. Many authors studied fibre transfer mechanisms but no model has been created to predict the number of fibres transferred expected in a given type of contact according to parameters as characteristics of the contact and characteristics of textiles. The main purpose of this research is to demonstrate that it is possible to create a model to predict the number of fibres transferred during a contact. In this work, the particular case of the transfer of fibres from a knitted textile in wool or in acrylic of a driver to the back of a carseat has been studied. Several characteristics of the textiles used for the experiments were measured. The data obtained were then treated with statistical tools (multiple linear regression) to evaluate the influence of the donor textile characteristics on the number of úbers transferred, and to create a model to predict this number of fibres transferred by an equation containing the characteristics having a significant influence on the transfer. To make easier the searching and the counting of fibres, an apparatus of automatic search. of fibers (fiber finder) was used. The tests realised to evaluate the efficiency of the fiber finder shows that the results obtained are generally as efficient as for visual search for well-coloured fibres. However, the efficiency of automatic search decreases for pales and dark fibres. One characteristic of the donor textile studied was the length of the fibres. To measure this parameter, a sequence of image processing algorithms was implemented. This tool allows to measure the length of a fibre from it high-resolution (2'540 dpi) numerical image. The tests done shows that the error of the measures obtained are about some tenths of millimetres. This precision is sufficient for this research. The statistical methods applied on the transfer experiment data allow to create a model of the transfer phenomenon. Two parameters are included in the model: the shedding capacity of the donor textile surface and the length of donor textile fibres. The shedding capacity of the donor textile surface is a parameter estimating the quantity of fibres that are not or slightly attached to the structure of the textile. These fibres are easily transferred during a contact, and the more this quantity of fibres is high, the more the number of fibres transferred during the contact is important. The length of fibres is also an important parameter: the more the fibres are long, the more they are attached in the structure of the textile and the less they are transferred during the contact.

Virtauslaskennankäyttö keskipakopumpun eroosiokulumisen mallintamisessa

Diplomityössä kehitetään malli tiheän kaksifaasivirtauksen aiheuttaman eroosiokulumi-sen mallintamiseksi, ratkaistaan virtauskenttä kahdessa erilaisessa keskipakopumpussa, sovelletaan kehitettyä mallia, sekävertaillaan mallin antamia tuloksia käytännön kokeissa saavutettuihin tuloksiin. Työssä on erityisenä mielenkiinnon kohteena savukaasupesurin pumppu. Työn alkuosa sisältää tarkemman kuvauksen savukaasupesurin toi-minnasta. Numeerinen ratkaisu ja laskentahilan generointi suoritetaan ANSYS CFX- ja Turbo-Grid-ohjelmistoilla. Laskennassa virtauksen Navier-Stokesin yhtälöt on aikakeskiarvo-tettu ja ratkaistu käyttäen kontrollitilavuusmenetelmää. Tiheiden kaksifaasivirtausten eroosiokulumista on mallinnettu tekijän kehittämällä mallilla, jonka käytännön toteutus-ta ei kuitenkaan saateta julkiseksi, koodin kehittämisessä käytetty teoria on kuitenkin esitetty työssä. Työn piirissä tehtiin myös kulumiskokeita Sulzer Pumps Finlandin Karhulan tehtailla, sekä vertailtiin simuloinnin tuloksia aikaisemmissa kokeissa saavutettuihin. Koejärjestelyt kuvataan työssä.

Numerical Modelling of Combustion and Emission Formation in a Diesel Engine with Different Fuel Injection Characteristics

Päästöjen vähentäminen on ollut viime vuosina tärkeässä osassa polttomoottoreita kehitettäessä.Monet viralliset tahot asettavat uusia tiukempia päästörajoituksia. Päästörajatovat tyypillisesti olleet tiukimmat autoteollisuuden valmistamille pienille nopeakäyntisille diesel-moottoreille, mutta viime aikoina paineita on kohdistunut myös suurempiin keskinopeisiin ja hidaskäyntisiin diesel-moottoreihin. Päästörajat ovat erilaisia riippuen moottorin tyypistä, käytetystä polttoaineesta ja paikasta missä moottoria käytetään johtuen erilaisista paikallisista laeista ja asetuksista. Eniten huomiota diesel-moottorin päästöissä täytyy kohdistaa typen oksideihin, savun muodostukseen sekä partikkeleihin. Laskennallisen virtausmekaniikan (CFD) avulla on hyvät mahdollisuudet tutkia diesel-moottorin sylinterissä tapahtuvia ilmiöitä palamisen aikana. CFD on hyödyllinen työkalu arvioitaessa moottorin suorituskykyä ja päästöjen muodostumista. CFD:llä on mahdollista testata erilaisten parametrien ja geometrioiden vaikutusta ilman kalliita moottorinkoeajoja. CFD:tä voidaan käyttää myös opetustarkoituksessa lisäämään paloprosessin tuntemusta. Tulevaisuudessa palamissimuloinnit CFD:llä tulevat epäilemättä olemaan tärkeä osa moottorin kehityksessä. Tässä diplomityössä on tehty palamissimuloinnit kahteen erilaisilla poittoaineenruiskutuslaitteistoilla varustettuun Wärtsilän keskinopeaan diesel-moottoriin. W46 moottorin ruiskutuslaitteisto on perinteinen mekaanisesti ohjattu pumppusuutin ja W46-CR moottorissa on elektronisesti ohjattu 'common rail' ruiskutuslaitteisto. Näiden moottorien ja käytössä olevien ruiskutusprofiilien lisäksi on simuloinneilla testattu erilaisia uusia ruiskutusprofiileja, jotta erityyppisten profiilien hyvät ja huonot ominaisuudet tulisivat selville. Matalalla kuormalla kiinnostuksen kohteena on nokipäästöjen muodostus ja täydellä kuormalla NOx-päästöjen muodostus ja polttoaineen kulutus. Simulointien tulokset osoittivat, että noen muodostusta matalalla kuormalla voidaan selvästi vähentää monivaiheisella ruiskutuksella, jossa yksi ruiskutusjakso jaetaan kahteen tai useampaan jaksoon. Erityisen tehokas noen vähentämisessä vaikuttaa olevan ns. jälkiruiskutus (post injection). Matalat NOx-päästöt ja hyvä polttoaineen kulutus täydellä kuormalla on mahdollista saavuttaaasteittain nostettavalla ruiskutusnopeudella.

Motion artifact reduction and vessel enhancement for free-breathing navigator-gated coronary MRA using 3D k-space reordering.

Breathing-induced bulk motion of the myocardium during data acquisition may cause severe image artifacts in coronary magnetic resonance angiography (MRA). Current motion compensation strategies include breath-holding or free-breathing MR navigator gating and tracking techniques. Navigator-based techniques have been further refined by the applications of sophisticated 2D k-space reordering techniques. A further improvement in image quality and a reduction of relative scanning duration may be expected from a 3D k-space reordering scheme. Therefore, a 3D k-space reordered acquisition scheme including a 3D navigator gated and corrected segmented k-space gradient echo imaging sequence for coronary MRA was implemented. This new zonal motion-adapted acquisition and reordering technique (ZMART) was developed on the basis of a numerical simulation of the Bloch equations. The technique was implemented on a commercial 1.5T MR system, and first phantom and in vivo experiments were performed. Consistent with the results of the theoretical findings, the results obtained in the phantom studies demonstrate a significant reduction of motion artifacts when compared to conventional (non-k-space reordered) gating techniques. Preliminary in vivo findings also compare favorably with the phantom experiments and theoretical considerations. Magn Reson Med 45:645-652, 2001.