Molecular simulation is applied to understanding the behaviour of alkane liquids with the eventual goal of being able to predict the viscosity of an arbitrary alkane mixture from first principles. Such prediction would have numerous scientific and industrial applications, as alkanes are the largest component of fuels, lubricants, and waxes; furthermore, they form the backbones of a myriad of organic compounds. This dissertation details the creation of a potential, a model for how the atoms and molecules in the simulation interact, based on a systematic approximation of the quantum mechanical potential energy surface using machine learning. This approximation has the advantage of producing forces and energies of nearly quantum mechanical accuracy at a tiny fraction of the usual cost. It enables accurate simulation of the large systems and long timescales required for accurate prediction of properties such as the density and viscosity. The approach is developed and tested on methane, the simplest alkane, and investigations are made into potentials for longer, more complex alkanes. The results show that the approach is promising and should be pursued further to create an accurate machine learning potential for the alkanes. It could even be extended to more complex molecular liquids in the future.

En simulation de fluides, il est très difficile d'obtenir des simulations contenant un haut niveau de détails efficacement dû à la complexité des phénomènes étudiés. Beaucoup de travaux se sont attaqués à ce problème afin de développer de nouvelles techniques permettant d'augmenter la résolution apparente des fluides à plus faibles coûts de calcul. Les nouvelles méthodes adaptatives ou multi-échelles ont permis de grandement améliorer la qualité visuelle des simulations de fumée et de liquides, mais certains problèmes demeurent toujours ouverts et au centre de nombreuses recherches. L'objectif de ce mémoire est d'élaborer une méthode multi-échelles afin d'augmenter la résolution apparente d'une simulation de liquide basée sur un système de particules déjà existante, un type de simulation très populaire grâce à ses propriétés de conservation d'énergie. Une telle méthode permettrait d'obtenir des simulations de résolution apparente élevée à bien moindres coûts de calcul et permettrait ainsi aux artistes d'obtenir un aperçu de leur simulation plus rapidement, tout en ayant un résultat de haute qualité. Nous présentons une méthode permettant de reconstruire une surface d'une telle simulation qui soit encline à la simulation de dynamique de surface afin d'injecter des détails de hautes fréquences occasionnés par la tension de surface. Notre méthode détecte les endroits sous-résolus de la simulation et y injecte de la turbulence grâce à de multiples oscillateurs à différentes fréquences. Les vagues à hautes fréquences injectées sont alors propagées à l'aide d'une simulation d'onde sur la surface. Notre méthode s'applique totalement en tant que post-traitement et préserve ainsi entièrement le comportement général de la simulation d'entrée tout en augmentant nettement la résolution apparente de la surface de celle-ci. / Accurately simulating the behaviour of fluids remains a difficult problem in computer graphics, and performing these simulations at a high level of detail is particularly challenging due to the complexity of the underlying dynamics of a fluid. A recent and significant body of work targets this problem by trying to augment the apparent resolution of an underlying, lower-resolution simulation, instead of performing a more costly simulation at the full-resolution. Adaptive or multi-scale methods in this area have proven successful for simulations of smoke and liquids, but no comprehensive solution exists. The goal of this thesis is to devise a new multi-scale detail-augmentation technique suitable for application atop existing particle-based fluid simulators. Particle simulations of fluid dynamics are a popular, heavily-used alternative to grid-based simulations due to their ability to better preserve energy, and no detail-augmentation techniques have been devised for this class of simulator. As such, our work would permit digital artists to perform more efficient lower-resolution particle simulations of a liquid, and then layer-on a detailed secondary simulation at a negligible cost. To do so, we present a method for reconstructing the surface of a liquid, during the particle simulation, in a manner that is amenable to high-frequency detail injection due to higher-resolution surface tension effects. Our technique detects potentially under-resolved regions on the initial simulation and synthesizes turbulent dynamics with novel multi-frequency oscillators. These dynamics result in a high-frequency wave simulation that is propagated over the (reconstructed) liquid surface. Our algorithm can be applied as a post-process, completely independent of the underlying simulation code, and so it is trivial to integrate in an existing 3D digital content creation pipeline.

This thesis concentrates on physically based simulation of fluids followed by its photorealistic visualization. It describes one form of Smooth Particle Hydrodynamics methods for viscoelastic fluid simulation and includes its extension for multiple interacting fluids. It also deals with SPH boundary problem and investigates its solution by fixed boundary particles. For visualization of fluids there is a method of Ray Tracing described in detail and it's extended with light absorption in transparent materials. In connection with this method there is also discussed a problem of infinite total reflections and some solution techniques are offered. To extract the surface of the fluid there is used a Marching cubes method and its discussed in terms of Ray Tracing.