Anisotropic screen space rendering for particle-based fluid simulation

This paper proposes a real-time fluid rendering method based on the screen space rendering scheme for particle-based fluid simulation. Our method applies anisotropic transformations to the point sprites to stretch the point sprites along appropriate axes, obtaining smooth fluid surfaces based on the weighted principal components analysis of the particle distribution. Then we combine the processed anisotropic point sprite information with popular screen space filters like curvature flow and narrow-range filters to process the depth information. Experiments show that the proposed method can efficiently resolve the issues of jagged edges and unevenness on the surface that existed in previous methods while preserving sharp high-frequency details

EFFICIENT PARTICLE-BASED VISCOUS FLUID SIMULATION WITH VIDEO-GUIDED REAL-TO-VIRTUAL PARAMETER TRANSFER

Viscous fluids, such as honey and molten chocolate, are common materials frequently seen in our daily life. These viscous fluids exhibit characteristic behaviors. Capturing and understanding such dynamics have been required for various applications. Although recent research made advances in simulating the viscous fluid dynamics, still many challenges are left to be addressed. In this dissertation, I present novel techniques to more efficiently and accurately simulate viscous fluid dynamics and propose a parameter identification framework to facilitate the tedious parameter tuning steps for viscous materials. In fluid simulation, enforcing the incompressibility robustly and efficiently is essential. One known challenge is how to set appropriate boundary conditions for free surfaces and solid boundaries. I propose a new boundary handling approach for an incompressible particle-based solver based on the connectivity analysis for simulation particles. Another challenge is that previously proposed techniques do not scale well. To address this, I propose a new multilevel particle-based solver which constructs the hierarchy of simulation particles. These techniques improve the robustness and efficiency achieving the nearly linear scaling unlike previous approaches. To simulate characteristic behaviors of viscous fluids, such as coiling and buckling phenomena and adhesion to other materials, it is necessary to develop a specialized solver. I propose a stable and efficient particle-based solver for simulating highly viscous fluids by using implicit integration with the full form of viscosity. To simulate more accurate interactions with solid objects, I propose a new two-way fluid-solid coupling method for viscous fluids via the unified minimization. These approaches also improve the robustness and efficiency while generating rotational and sticky behaviors of viscous fluids. One important challenge for the physically-based simulation is that it is not obvious how to choose appropriate material parameters to generate our desirable behaviors of simulated materials. I propose a parameter identification framework that helps to tune material parameters for viscous fluids with example video data captured from real world fluid phenomena. This framework identifies viscosity parameters for the real viscous fluids while estimating the hidden variables for the fluids, and enables the parameter transfer from the real world to virtual environment.Doctor of Philosoph

Computational Fluid Dynamics Modelling of Two-Phase Bubble Columns: A Comprehensive Review

Bubble columns are used in many different industrial applications, and their design and characterisation have always been very complex. In recent years, the use of Computational Fluid Dynamics (CFD) has become very popular in the field of multiphase flows, with the final goal of developing a predictive tool that can track the complex dynamic phenomena occurring in these types of reactors. For this reason, we present a detailed literature review on the numerical simulation of two-phase bubble columns. First, after a brief introduction to bubble column technology and flow regimes, we discuss the state-of-the-art modelling approaches, presenting the models describing the momentum exchange between the phases (i.e., drag, lift, turbulent dispersion, wall lubrication, and virtual mass forces), Bubble-Induced Turbulence (BIT), and bubble coalescence and breakup, along with an overview of the Population Balance Model (PBM). Second, we present different numerical studies from the literature highlighting different model settings, performance levels, and limitations. In addition, we provide the errors between numerical predictions and experimental results concerning global (gas holdup) and local (void fraction and liquid velocity) flow properties. Finally, we outline the major issues to be solved in future studies

Simulating liquids on dynamically warping grids

We introduce dynamically warping grids for adaptive liquid simulation. Our primary contributions are a strategy for dynamically deforming regular grids over the course of a simulation and a method for efficiently utilizing these deforming grids for liquid simulation. Prior work has shown that unstructured grids are very effective for adaptive fluid simulations. However, unstructured grids often lead to complicated implementations and a poor cache hit rate due to inconsistent memory access. Regular grids, on the other hand, provide a fast, fixed memory access pattern and straightforward implementation. Our method combines the advantages of both: we leverage the simplicity of regular grids while still achieving practical and controllable spatial adaptivity. We demonstrate that our method enables adaptive simulations that are fast, flexible, and robust to null-space issues. At the same time, our method is simple to implement and takes advantage of existing highly-tuned algorithms

A stable tensor-based deflection model for controlled fluid simulations

The association between fluids and tensors can be observed in some practical situations, such as diffusion tensor imaging and permeable flow. For simulation purposes, tensors may be used to constrain the fluid flow along specific directions. This work seeks to explore this tensor-fluid relationship and to propose a method to control fluid flow with an orientation tensor field. To achieve our purposes, we expand the mathematical formulation governing fluid dynamics to locally change momentum, deflecting the fluid along intended paths. Building upon classical computer graphics approaches for fluid simulation, the numerical method is altered to accomodate the new formulation. Gaining control over fluid diffusion can also aid on visualization of tensor fields, where the detection and highlighting of paths of interest is often desired. Experiments show that the fluid adequately follows meaningful paths induced by the underlying tensor field, resulting in a method that is numerically stable and suitable for visualization and animation purposes.A associação entre fluidos e tensores pode ser observada em algumas situações práticas, como em ressonância magnética por tensores de difusão ou em escoamento permeável. Para fins de simulação, tensores podem ser usados para restringir o escoamento do fluido ao longo de direções específicas. Este trabalho visa explorar esta relação tensor-fluido e propor um método para controlar o escoamento usando um campo de tensores de orientação. Para atingir nossos objetivos, nós expandimos a formulação matemática que governa a dinâmica de fluidos para alterar localmente o momento, defletindo o fluido para trajetórias desejadas. Tomando como base abordagens clássicas para simulação de fluidos em computação gráfica, o método numérico é alterado para acomodar a nova formulação. Controlar o processo de difusão pode também ajudar na visualização de campos tensoriais, onde frequentemente busca-se detectar e realçar caminhos de interesse. Os experimentos realizados mostram que o fluido, induzido pelo campo tensorial subjacente, percorre trajetórias significativas, resultando em um método que é numericamente estável e adequado para fins de visualização e animação

Book of Abstracts

no abstrac

Droplets, splashes and sprays: highly detailed liquids in visual effects production.

An often misunderstood or under-appreciated feature of the visual effects pipeline is the sheer quantity of components and layers that go into a single shot, or even, single effect. Liquids, often combining waves, splashes, droplets and sprays, are a particular example of this. Whilst there has been a huge amount of research on liquid simulation in the last decade or so, little has been successful in reducing the number of layers or elements required to create a plausible final liquid effect. Furthermore, the finer-scale phenomena of droplets and sprays, often introduced in this layered approach and crucial for plausibility, are some of the least well catered-for in the existing toolkit. In lieu of adequate tooling, creation of these elements relies heavily on non-physical methods, bespoke setups and artistic ingenuity. This project explores physically-based methods for creating these phenomena, demonstrat- ing improved levels of detail and plausibility over existing non-physical approaches. These provide an alternative to existing workflows that are heavily reliant on artistic input, allowing artists to focus efforts on creative direction rather than trying to recreate physical plausibility. We explore various approaches to increasing the level of detail captured in physically-based liquid simulations, developing a collection of tools that improve existing workflows. First, we investigate the potential of a re-simulation approach to increasing artist iteration on fluid simulations using previous simulation data. Following this, a novel droplet interaction model for ballistic particle simulations is developed to introduce higher levels of detail in simulations of liquid droplets and sprays. This allows physically-plausible interactions between droplet particles to be modelled efficiently and helps to create realistic droplet and spray behaviours. Then, to maximise the quality of the results of these and other particle-based simulations, we develop a high quality particle surfacing algorithm to handle the varied nature of inputs common in production. Finally, we discuss the development of an expression language to manipulate point and volume data commonly used in creating these simulations, as well as elsewhere throughout visual effects. This research was driven directly by production requirements in partnership with a world- leading visual effects studio, DNEG. Projects have been developed to immediately integrate into production, using efficient, industry-standard, open technologies such as OpenVDB. It is shown that the toolkit for splashing liquids, even at fine-scales, can be improved by incorporating greater physical motivation further demonstrating the importance of physical simulation in visual effects and suggesting effects similarly reliant on artistic input (e.g. character/skin deformation) may benefit from increased physical correctness