The knowledge of solid-state second phase heterogeneous nucleation process is limited due to the experimental difficulty, such as tiny length scale, short time period, and high temperature condition. In recent years, some significant breakthroughs in nucleation studies have been achieved by aid of computational techniques. In this study, we apply molecular dynamics (MD) simulations to perform with heterogeneous nucleation occurring at grain boundaries (GB) during the austenite (FCC) phase to ferrite (BCC) phase transformation in a pure Fe polycrystalline system. A neighbor vector analysis (NVA) method has been introduced and it is shown how the NVA can be used to determine the misorientation of grain or interphase boundaries, which allow a further investigation of the boundary structure correlated to interfacial energy and mobility during the nucleation and early grain growth stage. Meanwhile, benefited from the MD technique, the bulk energy, grain boundary energy, and interfacial energy can be individually captured during the simulations, which allow a detail analyze of the shape, critical size and nucleation energy of specific nuclei, through the classical nucleation theory (CNT) and according to a faceted-spherical cap geometric model (FSC). In addition, we also compared the results from the classical approach with a new algorithm that combination of the multi-phase field model (MPFM) and the nudged elastic band (NEB) method to demonstrate the CNT in the solid-state conduction. Finally, we extend our simulation method to a more complex triple GB junction nucleation event, and investigate the non-classical barrier-free nucleation behaviors. The results support the critical informations to clarify the initial state of austenite to ferrite transition, and improve our knowledge of the heterogeneous nucleation process, which help to bridge the gap between the experimental measurements and the theoretical calculations. The simulation method also provided a new approach for studying the complicate heterogeneous nucleation phenomenon in solid-state for a wide variety of polycrystalline material systems. / Thesis / Doctor of Philosophy (PhD)

No description available.

No description available.

The purpose of this bachelor’s thesis is to investigate the uses of spherical robots and on a prototype basis construct and control a dirigible bowling ball. The robot is able to steer left and right after being thrown by the user. This occurs due to a weight displacement inside the ball when a weighted pendulum swings left and right by being radio controlled from afar. The goal of the report is to investigate how well this robot will be able to steer and if it can achievea desired strike. Further, this report will investigate the use of Radio Frequency (RF) signals between a hand controller and the ball and at what distance this method will work. This robot is strictly made for scientific purposes, the authors do not advocate cheating in the sport of bowling in any way. / Syftet med denna kandidatuppsats är att undersöka användningen av sfäriska robotar och på en prototypbasis konstruera och styra ett styrbart bowlingklot. Roboten kommer att kunna styras åt vänster och höger efter att ha kastats avanvändaren. Detta inträffar på grund av en viktförskjutning inuti klotet då en viktad pendel svänger åt vänster och åt höger genom att radiostyras på håll. Målet med rapporten är att undersöka hur bra den här roboten kommer att kunna kontrolleras och om en strike kan uppnås med den nya och samlade kunskapen. Vidare kommer användningen av Radio frekventa (RF)-signaler mellan en handkontroll och klotet att undersökas, hur väl detta kan implementeras och även på hur långt avstånd denna metod kan fungera. Denna robot är endast gjord för vetenskapliga ändamål, författarna förespråkar inte fusk i sporten bowling på något sätt.

<p dir="ltr">Acoustic holography is a powerful tool in the visualization of sound fields and sound sources. It provides engineers and researchers clear insights into sound fields as well as their sound sources. Some widely-used methods include Nearfield Acoustical Holography (NAH), Statistically Optimized Nearfield Acoustic Holography (SONAH) and the Equivalent Source Method (ESM). SONAH and ESM were developed specifically to tackle the intrinsic deficiency of the Fourier-based NAH which requires that the sound field fall to negligible levels at the edges of the measurement aperture, a requirement rarely met in practice. Besides the aforementioned methods, the Inverse Boundary Element Method (IBEM) can be used, given sufficient measurements and computational resources. As useful as they are in visualizing the sound field, none of these methods can provide direct guidance on potential design modifications of the observed structure in order to unequivocally reduce sound power radiation. Acoustic radiation mode analysis has previously been primarily associated with active noise control applications. Since the radiation modes radiate sound power independently, it is only necessary to modify the surface vibration patterns so that they do not couple well with the radiation modes in order to guarantee a reduction of the radiated sound power. Since the radiation modes are orthogonal and complete, they can be used as the basis functions through which the source surface vibration can be described. Therefore, an acoustic holography method based on the acoustic radiation modes will enable the sound power ranking of the modal components of the surface vibration pattern, and in turn, point out the component(s) which should be targeted in order to reduce the overall sound power. However, use of the acoustic radiation modes in the inverse procedure comes with a price: the detailed geometry of the object to be measured must be obtained, thus enabling the calculation of acoustic radiation modes and the modal pressures. But this is not an issue for original equipment manufacturers given that almost all prototypes are now designed with CAD, as is the case with the engine example to be described next.</p><p dir="ltr">In modern engine design, downsizing and reducing weight while still providing an increased amount of power has been a general trend in recent decades. Traditionally, an engine design with superior NVH performance usually comes with a heavier, thus sturdier structure. Therefore, modern engine design requires that NVH be considered in the very early design stage to avoid modifications of engine structures at the last minute, when very few changes can be made. NVH design optimization of engine components has become more practical due to the development of computer software and hardware. However, there is still a need for smarter algorithms to draw a direct relationship between the design and the radiated sound power. At the moment, techniques based on modal acoustic transfer vectors (MATVs) have gained popularity in design optimization for their good performance in sound pressure prediction. Since MATVs are derived based on structural modes, they are not independent with respect to radiated sound power. In contrast, as noted, acoustic radiation modes are an orthogonal set of velocity distributions on the structure’s surface that contribute to the radiated sound power independently. As a result, it is beneficial to describe structural vibration in terms of acoustic radiation modes in order to identify the velocity distributions that contribute the majority of the radiated sound power. Measures can then be taken to modify the identified vibration patterns to reduce their magnitudes, which will in turn result in an unequivocal reduction of the radiated sound power. A workflow of the structural optimization procedure is proposed in this dissertation.</p><p dir="ltr">While acoustic radiation modes have great efficiencies in describing radiated acoustic power, the computation of acoustic radiation modes can be time consuming. In the last chapter of this thesis, a novel way of calculating acoustic radiation modes is proposed, which differs from the traditional singular value decomposition of the power radiation resistance matrix, and which is more efficient than previously proposed procedures. </p><p><br></p>

The present thesis considers the problem of reconstructing a function f that is defined on the d-dimensional unit sphere from its mean values along hyperplane sections. In case of the two-dimensional sphere, these plane sections are circles. In many tomographic applications, however, only limited data is available. Therefore, one is interested in the reconstruction of the function f from its mean values with respect to only some subfamily of all hyperplane sections of the sphere. Compared with the full data case, the limited data problem is more challenging and raises several questions. The first one is the injectivity, i.e., can any function be uniquely reconstructed from the available data? Further issues are the stability of the reconstruction, which is closely connected with a description of the range, as well as the demand for actual inversion methods or algorithms. We provide a detailed coverage and answers of these questions for different families of hyperplane sections of the sphere such as vertical slices, sections with hyperplanes through a common point and also incomplete great circles. Such reconstruction problems arise in various practical applications like Compton camera imaging, magnetic resonance imaging, photoacoustic tomography, Radar imaging or seismic imaging. Furthermore, we apply our findings about spherical means to the cone-beam transform and prove its singular value decomposition. / Die vorliegende Arbeit beschäftigt sich mit dem Problem der Rekonstruktion einer Funktion f, die auf der d-dimensionalen Einheitssphäre definiert ist, anhand ihrer Mittelwerte entlang von Schnitten mit Hyperebenen. Im Fall d=2 sind diese Schnitte genau die Kreise auf der Sphäre. In vielen tomografischen Anwendungen sind aber nur eingeschränkte Daten verfügbar. Deshalb besteht das Interesse an der Rekonstruktion der Funktion f nur anhand der Mittelwerte bestimmter Familien von Hyperebenen-Schnitten der Sphäre. Verglichen mit dem Fall vollständiger Daten birgt dieses Problem mehrere Herausforderungen und Fragen. Die erste ist die Injektivität, also können alle Funktionen anhand der gegebenen Daten eindeutig rekonstruiert werden? Weitere Punkte sind die die Frage nach der Stabilität der Rekonstruktion, welche eng mit einer Beschreibung der Bildmenge verbunden ist, sowie der praktische Bedarf an Rekonstruktionsmethoden und -algorithmen. Diese Arbeit gibt einen detaillierten Überblick und Antworten auf diese Fragen für verschiedene Familien von Hyperebenen-Schnitten, angefangen von vertikalen Schnitten über Schnitte mit Hyperebenen durch einen festen Punkt sowie Kreisbögen. Solche Rekonstruktionsprobleme treten in diversen Anwendungen auf wie der Bildgebung mittels Compton-Kamera, Magnetresonanztomografie, fotoakustischen Tomografie, Radar-Bildgebung sowie der Tomografie seismischer Wellen. Weiterhin nutzen wir unsere Ergebnisse über sphärische Mittelwerte, um eine Singulärwertzerlegung für die Kegelstrahltomografie zu zeigen.

No description available.

The Jupiter Icy Moons Explorer is a planetary exploration mission that aims to study the moons of Jupiter in the planet’s vast magnetosphere. Among the various instruments on board is the Particle Environment Package (PEP), that is led by the Swedish Institute of Space Physics (IRF) in Kiruna. The Jovian plasma Dynamics and Composition analyzer (JDC) is one of six sensors within PEP and focuses on the characterization of positive ions. To be able to measure their three-dimensional distribution and composition, in-situ and in high time resolution, JDC has to cover a large field of view of 2π sr, for the desired energy range, in just a couple of seconds. An electrostatic analyzer within the sensor determines the energy per charge of such particles and a time-of-flight mass spectrometer measures their mass per charge. Constraints on weight and the radiation environment of Jupiter drive the design of the sensor: small and lightweight to allow extra shielding, but still large enough to accomplish measurements in the harsh radiation environment of Jupiter. This work focuses on a new type of compact, electrostatic analyzer using spherical wedges and the start signal generation for the time-of-flight measurement using new venetian blind-type surfaces. Simulations on the electrostatic analyzer showed that the most promising design is a hybrid variant, using an inner shell with spherical wedges and a spheroidal outer shell. A prototype sensor was built and tested with successful results. A reflectron-type time-of-flight cell measures the time it takes for a particle to pass a linear electric field. The time measurement has to be very accurate and requires that all ions enter the reflectron from the same start position. Commonly this is achieved with thin carbon foils of some nanometer thickness to provide a very accurate start position. Upon impact and after leaving a foil, ions generate secondary electrons that act as start signals for the time measurement. Foils require a substantial pre-acceleration of several kilovolts for the ions to penetrate the foil, thus increasing the size and mass of the instrument. When incident ions are reflected at grazing angles from a surface, secondary electrons are released in the same way as with foils. To increase position accuracy during this reflection process, venetian blind-type start surfaces are investigated, where many smaller surfaces replace a large flat surface. The most promising sample was found to be micro pore optics, that were initially designed to focus gamma rays. In several experiments it could be shown that micro pore optics show good reflection properties when used as start surfaces in the time-of-flight measurement. Both improvements allow a more compact and lightweight sensor that can be better shielded against the harsh radiation environment in Jupiter’s system. Jupiter hosts the strongest radiation environment in the solar system, that could kill an unprotected human thousand times over. / JUICE, PEP

Réalisé en cotutelle avec l'Université Bordeaux 1 (France) / En synthèse d'images, le principal calcul à effectuer pour générer une image a été formalisé dans une équation appelée équation du rendu [Kajiya1986]. Cette équation est la intègre la conservation de l'\'energie dans le transport de la lumi\`ere. Elle stipule que l'énergie lumineuse renvoyée, par les objets d'une scène, dans une direction donnée est égale à la somme de l'énergie émise et réfléchie par ceux-ci. De plus, l'énergie réfléchie par un élément de surface est définie comme la convolution de l'éclairement incident avec une fonction de réflectance. Cette dernière modélise le matériau (au sens physique) de l'objet et joue le rôle d'un filtre directionnel et énergétique dans l'équation du rendu, simulant ainsi la manière dont la surface se comporte vis-à-vis d'une réflexion. Dans ce mémoire de thèse, nous introduisons de nouvelles représentations pour la fonction de réflectance ainsi que pour la représentation de l'éclairement incident. Dans la première partie de ce mémoire, nous proposons deux nouveaux modèles pour représenter la fonction de réflectance. Le premier modèle s'inscrit dans une démarche artistique et est destiné à faciliter la création et l'édition des reflets spéculaires. Son principe est de laisser l'utilisateur peindre et esquisser les caractéristiques (forme, couleur, gradient et texture) du reflet spéculaire dans un plan de dessin paramétrisé en fonction de la direction de la réflexion miroir de la lumière. Le but du second modèle est de représenter de manière compacte et efficace les mesures des matériaux isotropes. Pour ce faire, nous introduisons une nouvelle représentation à base de polynômes rationnels. Les coefficients de ces derniers sont obtenus à l'aide d'un processus d'approximation qui garantit une solution optimale au sens de la convergence. Dans la seconde partie de ce mémoire, nous introduisons une nouvelle représentation volumétrique pour l'éclairement indirect représenté directionnellement à l'aide de vecteurs d'irradiance. Nous montrons que notre représentation est compacte et robuste aux variations géométriques et qu'elle peut être utilisée comme système de cache pour du rendu temps réel ou non, ainsi que dans le cadre de la transmission progressive des données (streaming). Enfin, nous proposons deux types de modifications de l'éclairement incident afin de mettre en valeur les détails et les formes d'une surface. Le première modification consiste à perturber les directions de l'éclairement incident tandis que la seconde consiste à en modifier l'intensité. / In image synthesis, the main computation involved to generate an image is characterized by an equation named rendering equation [Kajiya1986]. This equation represents the law of energy conservation. It stipulates that the light emanating from the scene objects is the sum of the emitted energy and the reflected energy. Moreover, the reflected energy at a surface point is defined as the convolution of the incoming lighting with a reflectance function. The reflectance function models the object material and represents, in the rendering equation, a directional and energetic filter that describes the surface behavior regarding the reflection. In this thesis, we introduce new representations for the reflectance function and the incoming lighting. In the first part of this thesis, we propose two new models for the reflectance function. The first model is targeted for artists to help them create and edit highlights. Our main idea is to let the user paint and sketch highlight characteristics (shape, color, gradient and texture) in a plane parametrized by the incident lighting direction. The second model is designed to represent efficiently isotropic material data. To achieve this result, we introduce a new representation of the reflectance function that uses rational polynomials. Their coefficients are computed using a fitting process that guarantees an optimal solution regarding convergence. In the second part of this thesis, we introduce a new volumetric structure for indirect illumination that is directionally represented with irradiance vector. We show that our representation is compact and robust to geometric variations, that it can be used as caching system for interactive and offline rendering and that it can also be transmitted with streaming techniques. Finally, we introduce two modifications of the incoming lighting to improve the shape depiction of a surface. The first modification consists in warping the incoming light directions whereas the second one consists in scaling the intensity of each light source.

Path tracing is a basic, statistically unbiased method for calculating the global illumination in 3D scenes. For practical purposes, the algorithm is too slow, so it is used mainly for theoretical purposes or as a base for more advanced algorithms. This thesis explores the possibility of improving this algorithm by augmenting the sampling part, which computes outgoing directions during ray traversal through the scene. This optimization is accomplished by creating a special data structure in a preprocess step, which describes approximate light distribution in the scene and which then aids the sampling process. The presented algorithm is implemented in the PBRT library.