<p>One of the most conspicuous problems in space physics for the last decades has been to theoretically describe how the large parallel electric fields on auroral field lines can be generated. There is strong observational evidence of such electric fields, and stationary theory supports the need for electric fields accelerating electrons to the ionosphere where they generate auroras. However, dynamic models have not been able to reproduce these electric fields. This thesis sheds some light on this incompatibility and shows that the missing ingredient in previous dynamic models is a correct description of the electron temperature. As the electrons accelerate towards the ionosphere, their velocity along the magnetic field line will increase. In the converging magnetic field lines, the mirror force will convert much of the parallel velocity into perpendicular velocity. The result of the acceleration and mirroring will be a velocity distribution with a significantly higher temperature in the auroral acceleration region than above. The enhanced temperature corresponds to strong electron pressure gradients that balance the parallel electric fields. Thus, in regions with electron acceleration along converging magnetic field lines, the electron temperature increase is a fundamental process and must be included in any model that aims to describe the build up of parallel electric fields. The development of such a model has been hampered by the difficulty to describe the temperature variation. This thesis shows that a local equation of state cannot be used, but the electron temperature variations must be descibed as a nonlocal response to the state of the auroral flux tube. The nonlocal response can be accomplished by the particle-fluid model presented in this thesis. This new dynamic model is a combination of a fluid model and a Particle-In-Cell (PIC) model and results in large parallel electric fields consistent with in-situ observations.</p>

Generating natural phenomena in a virtual world has a number of practical applications. Thanks to the rich and complicated details in the real world, the goal of realistically and efficiently reproducing natural phenomena is well known as an open problem for graphics researchers. In this dissertation, three different issues in modeling liquid animations have been addressed. First, a virtual surface method is proposed to account for surface tension effects and their interactions with solid surfaces in physically based fluid simulation. This allows us to generate various surface tension behaviors in small scale liquid. The second issue that is addressed is how to make small scale fluid simulation more efficient. The proposed solution is a general shallow wave equation model, extended from the original shallow wave equations. By simplifying 3D incompressible fluid dynamics into 2D, small scale liquid can be stably and efficiently simulated over arbitrarily curved surfaces using implicit numerical schemes. The third contribution is a novel hybrid framework that combines image based reconstruction techniques with physically based fluid simulation. While image based methods cannot correctly generate fluid animations alone frame by frame, physics is used as a refinement tool to enforce physical soundness by propagating shape information back and forth in space and time. In this way, water animations can be realistically and faithfully generated from images without error accumulation or stability issues.

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BARBOSA, Charles Welton Ferreira. Uma análise comparativa de métodos aproximativos baseados em smoothed particle hydrodynamics para animação de fluidos. 2013. 59 f. : Dissertação (mestrado) - Universidade Federal do Ceará, Centro de Ciências, Departamento de Computação, Fortaleza-CE, 2013. / Submitted by guaracy araujo (guaraa3355@gmail.com) on 2016-06-14T18:05:54Z No. of bitstreams: 1 2013_dis_cwfbarbosa.pdf: 4026755 bytes, checksum: 3808122d6233e5855d6891854317c373 (MD5) / Approved for entry into archive by guaracy araujo (guaraa3355@gmail.com) on 2016-06-14T18:07:17Z (GMT) No. of bitstreams: 1 2013_dis_cwfbarbosa.pdf: 4026755 bytes, checksum: 3808122d6233e5855d6891854317c373 (MD5) / Made available in DSpace on 2016-06-14T18:07:17Z (GMT). No. of bitstreams: 1 2013_dis_cwfbarbosa.pdf: 4026755 bytes, checksum: 3808122d6233e5855d6891854317c373 (MD5) Previous issue date: 2013 / Animations of fluids, such as water or smoke, are used to add details to virtual games or films. Various methods exist for simulating fluids using particle systems, including the Smoothed Particle Hydrodynamics (SPH) method. The SPH method is a good choice for simulating fluids for its easy description and implementation. This work describes some methods based on SPH for general fluid simulation, and for the interaction between fluids and solids. These methods are analysed and compared on their realism, and a few weak points of each method are presented. This analysis may be used for the creation of better methods based on SPH, and for the realistic animation of fluids / Animações de fluidos, como a água ou fumaça, são utilizadas para introduzir detalhes em jogos virtuais ou filmes. Vários métodos existem para a simulação de fluidos utilizando sistemas de partículas e, em especial, utilizando o método Smoothed Particle Hydrodynamics, ou SPH. O método SPH é uma boa escolha para a simulação de fluidos por sua fácil descrição e implementação. Este trabalho apresenta alguns métodos baseados no SPH para a simulação de fluidos gerais, e para a interação entre fluidos e sólidos. Esses métodos são analisados e comparados relativos ao seu realismo, apresentando alguns pontos fracos de cada método. Essa análise pode ser utilizada para guiar a implementação de melhores métodos baseados no SPH, e para a animação realista de fluidos.

Este trabalho aborda a utilização do método de Lattice Boltzmann como ferramenta de simulação para a área de dinâmica de fluidos. Além disso, apresenta o programa LBSim desenvolvido durante o período de pesquisa, construído para ser flexível e extensível (através do emprego de técnicas de orientação a objetos) e com uma interface gráfica mais amigável do que outros projetos semelhantes. Como resultado deste trabalho, o software LBSim implementa uma série de módulos diferentes que utilizam o método de Lattice Boltzmann como base, permitindo a simulação de casos monofásico, multifásico, multicomponente, com suporte à gravidade, meios porosos, difusão, transferência de calor e paredes deslizantes. / This work approaches the use of the Lattice Boltzmann method as a simulation tool for the fluid dynamics area. Beyond that, it presents the LBSim software, developed during the period of the research and built to be flexible and extensible (through the application of techniques of the object oriented paradigm) and with a graphical interface more friendly than other similar projects. As a result of this work, the LBSim software implements a series of different modules that uses the Lattice Boltzmann method as a base, allowing the simulations of the following cases: monophase, multiphase, multicomponent, with support of gravity, porous media, diffusion, heat transfer and moving walls.

The primary concern of this work is real-time fluid simulation on modern programmable graphics hardware. It starts by introducing fundamental fluid simulation principles with focus on Smoothed particle hydrodynamics technique. The following discussion then provides a brief introduction to OpenCL as well as contemporary GPU hardware and outlines their programming specifics in comparison with CPUs. Finally, the last two chapters of this work, detail the problem analysis and its implementation.

L’animation basée sur la physique peut générer des systèmes aux comportements complexes et réalistes. Malheureusement, contrôler de tels systèmes est une tâche ardue. Dans le cas de la simulation de fluide, le processus de contrôle est particulièrement complexe. Bien que de nombreuses méthodes et outils ont été mis au point pour simuler et faire le rendu de fluides, trop peu de méthodes offrent un contrôle efficace et intuitif sur une simulation de fluide. Étant donné que le coût associé au contrôle vient souvent s’additionner au coût de la simulation, appliquer un contrôle sur une simulation à plus haute résolution rallonge chaque itération du processus de création. Afin d’accélérer ce processus, l’édition peut se faire sur une simulation basse résolution moins coûteuse. Nous pouvons donc considérer que la création d’un fluide contrôlé peut se diviser en deux phases: une phase de contrôle durant laquelle un artiste modifie le comportement d’une simulation basse résolution, et une phase d’augmentation de détail durant laquelle une version haute résolution de cette simulation est générée. Cette thèse présente deux projets, chacun contribuant à l’état de l’art relié à chacune de ces deux phases. Dans un premier temps, on introduit un nouveau système de contrôle de liquide représenté par un modèle particulaire. À l’aide de ce système, un artiste peut sélectionner dans une base de données une parcelle de liquide animé précalculée. Cette parcelle peut ensuite être placée dans une simulation afin d’en modifier son comportement. À chaque pas de simulation, notre système utilise la liste de parcelles actives afin de reproduire localement la vision de l’artiste. Une interface graphique intuitive a été développée, inspirée par les logiciels de montage vidéo, et permettant à un utilisateur non expert de simplement éditer une simulation de liquide. Dans un second temps, une méthode d’augmentation de détail est décrite. Nous proposons d’ajouter une étape supplémentaire de suivi après l’étape de projection du champ de vitesse d’une simulation de fumée eulérienne classique. Durant cette étape, un champ de perturbations de vitesse non-divergent est calculé, résultant en une meilleure correspondance des densités à haute et à basse résolution. L’animation de fumée résultante reproduit fidèlement l’aspect grossier de la simulation d’entrée, tout en étant augmentée à l’aide de détails simulés. / Physics-based animation can generate dynamic systems of very complex and realistic behaviors. Unfortunately, controlling them is a daunting task. In particular, fluid simulation brings up particularly difficult problems to the control process. Although many methods and tools have been developed to convincingly simulate and render fluids, too few methods provide efficient and intuitive control over a simulation. Since control often comes with extra computations on top of the simulation cost, art-directing a high-resolution simulation leads to long iterations of the creative process. In order to shorten this process, editing could be performed on a faster, low-resolution model. Therefore, we can consider that the process of generating an art-directed fluid could be split into two stages: a control stage during which an artist modifies the behavior of a low-resolution simulation, and an upresolution stage during which a final high-resolution version of this simulation is driven. This thesis presents two projects, each one improving on the state of the art related to each of these two stages. First, we introduce a new particle-based liquid control system. Using this system, an artist selects patches of precomputed liquid animations from a database, and places them in a simulation to modify its behavior. At each simulation time step, our system uses these entities to control the simulation in order to reproduce the artist’s vision. An intuitive graphical user interface inspired by video editing tools has been developed, allowing a nontechnical user to simply edit a liquid animation. Second, a tracking solution for smoke upresolution is described. We propose to add an extra tracking step after the projection of a classical Eulerian smoke simulation. During this step, we solve for a divergence-free velocity perturbation field resulting in a better matching of the low-frequency density distribution between the low-resolution guide and the high-resolution simulation. The resulting smoke animation faithfully reproduces the coarse aspect of the low-resolution input, while being enhanced with simulated small-scale details.

Two of the methods common in video game fluid simulation are SmoothedParticle Hydrodynamics (SPH), and Position Based Dynamics (PBD). They are both Lagrangian methods of fluid simulation. SPH has been used for many years in offline simulations and has truthful visuals, but is not as stable as the newer method PBD when using larger timesteps. SPH also tends to become unstable during compression. In this report both methods have been tested on different scenarios as the methods’ performance and visual depend on the scenario used. Additionally, the size of the particle radius was varied when comparing Compressible SPH (CSPH), Weak Compressible SPH (WCSPH), and PBD. From these tests, the conclusion could be drawn that CSPH performed slightly better than PBD regarding frames per second (FPS) in all cases except one. However, WCSPH and sometimes CSPH had stability issues. The stability of PBD and its possibility for larger timesteps with only minor FPS difference lead to the conclusion that PBD is overall the more suitable method for fluid simulation in video games. / Två av metoderna som är vanliga vid vätskesimulering i videospel är SmoothedParticle Hydrodynamics (SPH) och Position Based Dynamics (PBD). De är båda Lagrangiska metoder för vätskesimulering. SPH har använts i många år i offline-simuleringar och har realistiskt utseende, men är inte lika stabil som den nyare metoden PBD vid användning av större tidssteg. SPH tenderar också att bli instabil under kompression. Båda metoderna blev testade i olika scenarion eftersom deras prestanda och utseende beror på det använda scenariot. Storleken av partikelradien har också varierat när Compressible SPH (CSPH), Weak Compressible SPH (WCSPH) och PBD jämfördes. Från dessa tester kunde man se att CSPH presterade lite bättre än PBD gällande bilder per sekund (FPS) i alla fall utom ett. Däremot hade WCSPH och ibland CSPH stabilitetsproblem. Stabiliteten av PBD och dess möjlighet att ta större tidssteg med endast minimala FPS skillnader ledde till slutsatsen att PBD är överlag den mer lämpliga metoden för vätskesimulering i videospel.

Le phénomène connu sous l'appellation "méandrement" (ou meandering) désigne les variations aléatoires de la trajectoire du sillage aérodynamique d'une éolienne. Ce phénomène est responsable de contraintes mécaniques particulières sur les éoliennes positionnées dans le sillage d'autres éoliennes et joue donc rôle dans la conception et dans la prévision de production des parcs éoliens.Ce travail propose d'étudier le méandrement par des moyens expérimentaux et numériques. La problématique est traitée en deux parties, la première portant sur la modélisation de l'écoulement de couche limite atmosphérique, avec une attention particulière portée à la modélisation des grandes échelles de la turbulence atmosphérique. La seconde partie porte sur l'étude du sillage d'un disque actuateur soumis à un écoulement atmosphérique. Chacune de ces parties comporte un volet expérimental et un volet numérique. La modélisation numérique instationnaire de l'écoulement atmosphérique fait intervenir une technique de génération stochastique de champs de vitesse turbulente avec évolution temporelle, spécialement développée au cours de la présente thèse et à laquelle un chapitre spécifique est dédié.L'un des principaux résultats est que le méandrement du sillage est fortement corrélé avec les grandes échelles de la turbulence atmosphérique. / The phenomenon known as meandering describes the unsteady trajectory variations of the wake of a wind turbine. This phenomenon is responsible for specific mechanical stresses on turbines positioned in the wake of other turbines. As such, this phenomenon must be accounted for in the design and operation of wind turbine plants.This work uses numerical fluid simulation and wind tunnel testing in order to study the meandering of the wake of a wind turbine. The subject is discussed in two parts. The first part discusses the modeling of the atmospheric boundary layer, with a focus on the large scales of the atmospheric turbulence. The second part is a study of the behavior of the wake of an actuator disc model in atmospheric wind conditions.Both parts include experimental and numerical work. The numerical simulation of the atmospheric boundary layer involves the generation of synthetic turbulent velocity time series by mean of a stochastic technique developed during this thesis, to which a chapter is dedicated.One of the main results of this work is that the meandering is highly correlated with the large scales of the atmospheric turbulence.

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