Fluid Simulation by the Smoothed Particle Hydrodynamics Method: A Survey.

This paper presents a survey of Smoothed Particle Hydrodynamics (SPH) and its use in computational fluid dynamics. As a truly mesh-free particle method based upon the Lagrangian formulation, SPH has been applied to a variety of different areas in science, computer graphics and engineering. It has been established as a popular technique for fluid based simulations, and has been extended to successfully simulate various phenomena such as multi-phase flows, rigid and elastic solids, and fluid features such as air bubbles and foam. Various aspects of the method will be discussed: Similarities, advantages and disadvantages in comparison to Eulerian methods; Fundamentals of the SPH method; The use of SPH in fluid simulation; The current trends in SPH. The paper ends with some concluding remarks about the use of SPH in fluid simulations, including some of the more apparent problems, and a discussion on prospects for future work

Regional Time Stepping for SPH

International audienceThis paper presents novel and efficient strategies to spatially adapt the amount of computational effort applied based on the local dynamics of a free surface flow, for classic weakly compressible SPH (WCSPH). Using a convenient and readily parallelizable block-based approach, different regions of the fluid are assigned differing time steps and solved at different rates to minimize computational cost. We demonstrate that our approach can achieve about two times speed-up over the standard method even in highly dynamic scenes

An Efficient Sleepy Algorithm for Particle-Based Fluids

We present a novel Smoothed Particle Hydrodynamics (SPH) based algorithm for efficiently simulating compressible and weakly compressible particle fluids. Prior particle-based methods simulate all fluid particles; however, in many cases some particles appearing to be at rest can be safely ignored without notably affecting the fluid flow behavior. To identify these particles, a novel sleepy strategy is introduced. By utilizing this strategy, only a portion of the fluid particles requires computational resources; thus an obvious performance gain can be achieved. In addition, in order to resolve unphysical clumping issue due to tensile instability in SPH based methods, a new artificial repulsive force is provided. We demonstrate that our approach can be easily integrated with existing SPH based methods to improve the efficiency without sacrificing visual quality

A Unified Particle System Framework for Multi-Phase, Multi-Material Visual Simulations

We introduce a unified particle framework which integrates the phase-field method with multi-material simulation to allow modeling of both liquids and solids, as well as phase transitions between them. A simple elasto-plastic model is used to capture the behavior of various kinds of solids, including deformable bodies, granular materials, and cohesive soils. States of matter or phases, particularly liquids and solids, are modeled using the non-conservative Allen-Cahn equation. In contrast, materials---made of different substances---are advected by the conservative Cahn-Hilliard equation. The distributions of phases and materials are represented by a phase variable and a concentration variable, respectively, allowing us to represent commonly observed fluid-solid interactions. Our multi-phase, multi-material system is governed by a unified Helmholtz free energy density. This framework provides the first method in computer graphics capable of modeling a continuous interface between phases. It is versatile and can be readily used in many scenarios that are challenging to simulate. Examples are provided to demonstrate the capabilities and effectiveness of this approach

Pairwise Force SPH Model for Real-Time Multi-Interaction Applications

In this paper, we present a novel pairwise-force smoothed particle hydrodynamics (PF-SPH) model to allow modeling of various interactions at interfaces in real time. Realistic capture of interactions at interfaces is a challenging problem for SPH-based simulations, especially for scenarios involving multiple interactions at different interfaces. Our PF-SPH model can readily handle multiple kinds of interactions simultaneously in a single simulation; its basis is to use a larger support radius than that used in standard SPH. We adopt a novel anisotropic filtering term to further improve the performance of interaction forces. The proposed model is stable; furthermore, it avoids the particle clustering problem which commonly occurs at the free surface. We show how our model can be used to capture various interactions. We also consider the close connection between droplets and bubbles, and show how to animate bubbles rising in liquid as well as bubbles in air. Our method is versatile, physically plausible and easy-to-implement. Examples are provided to demonstrate the capabilities and effectiveness of our approach

Interactive simulation and rendering of fluids on graphics hardware

Computational uid dynamics can be used to reproduce the complex motion of fluids for use in computer graphics, but the simulation and rendering are both highly computationally intensive. In the past performing these tasks on the CPU could take many minutes per frame, especially for large scale scenes at high levels of detail, which limited their usage to offline applications such as in film and media. However, using the massive parallelism of GPUs, it is nowadays possible to produce uid visual effects in real time for interactive applications such as games. We present such an interactive simulation using the CUDA GPU computing environment and OpenGL graphics API. Smoothed Particle Hydrodynamics (SPH) is a popular particle-based fluid simulation technique that has been shown to be well suited to acceleration on the GPU. Our work extends an existing GPU-based SPH implementation by incorporating rigid body interaction and rendering. Solid objects are represented using particles to accumulate hydrodynamic forces from surrounding fluid, while motion and collision handling are handled by the Bullet Physics library on the CPU. Our system demonstrates two-way coupling with multiple objects floating, displacing fluid and colliding with each other. For rendering we compare the performance and memory consumption of two approaches, splatting and raycasting, we also describe the visual characteristics of each. In our evaluation we consider a target of between 24 and 30 fps to be sufficient for smooth interaction and aim to determine the performance impact of our new features. We begin by establishing a performance baseline and find that the original system runs smoothly up to 216,000 fluid particles but after introducing rendering this drops to 27,000 particles with the rendering taking up the majority of the frame time in both techniques. We find that the most significant limiting factor to splatting performance to be the onscreen area occupied by fluid while the raycasting performance is primarily determined by the resolution of the 3D texture used for sampling. Finally we find that performing solid interaction on the CPU is a viable approach that does not introduce significant overhead unless solid particles vastly outnumber fluid ones

Animated surfaces in physically-based simulation

Physics-based animation has become a ubiquitous element in all application areas of computer animation, especially in the entertainment sector. Animation and feature films, video games, and advertisement contain visual effects using physically-based simulation that blend in seamlessly with animated or live-action productions. When simulating deformable materials and fluids, especially liquids, objects are usually represented by animated surfaces. The visual quality of these surfaces not only depends on the actual properties of the surface itself but also on its generation and relation to the underlying simulation. This thesis focuses on surfaces of cloth simulations and fluid simulations based on Smoothed Particle Hydrodynamics (SPH), and contributes to improving the creation of animations by specifying surface shapes, modeling contact of surfaces, and evaluating surface effects of fluids. In many applications, there is a reference given for a surface animation in terms of its shape. Matching a given reference with a simulation is a challenging task and similarity is often determined by visual inspection. The first part of this thesis presents a signature for cloth animations that captures characteristic shapes and their temporal evolution. It combines geometric features with physical properties to represent accurately the typical deformation behavior. The signature enables calculating similarities between animations and is applied to retrieve cloth animations from collections by example. Interactions between particle-based fluids and deformable objects are usually modeled by sampling the deformable objects with particles. When interacting with cloth, however, this would require resampling the surface at large planar deformations and the thickness of cloth would be bound to the particle size. This problem is addressed in this thesis by presenting a two-way coupling technique for cloth and fluids based on the simulation mesh of the textile. It allows robust contact handling and intuitive control of boundary conditions. Further, a solution for intersection-free fluid surface reconstruction at contact with thin flexible objects is presented. The visual quality of particle-based fluid animation highly depends on the properties of the reconstructed surface. An important aspect of the reconstruction method is that it accurately represents the underlying simulation. This thesis presents an evaluation of surfaces at interfaces of SPH simulations incorporating the connection to the simulation model. A typical approach in computer graphics is compared to surface reconstruction used in material sciences. The behavior of free surfaces in fluid animations is highly influenced by surface tension. This thesis presents an evaluation of three types of surface tension models in combination with different pressure force models for SPH to identify the individual characteristics of these models. Systematic tests using a set of benchmark scenes are performed to reveal strengths and weaknesses, and possible areas of applications.Physikalisch basierte Animationen sind ein allgegenwärtiger Teil in jeglichen Anwendungsbereichen der Computeranimation, insbesondere dem Unterhaltungssektor. Animations- und Spielfilme, Videospiele und Werbung enthalten visuelle Effekte unter Verwendung von physikalisch basierter Simulation, die sich nahtlos in Animations- oder Realfilme einfügen. Bei der Simulation von deformierbaren Materialien und Fluiden, insbesondere Flüssigkeiten, werden die Objekte gewöhnlich durch animierte Oberflächen dargestellt. Die visuelle Qualität dieser Oberflächen hängt nicht nur von den Eigenschaften der Fläche selbst ab, sondern auch von deren Erstellung und der Verbindung zu der zugrundeliegenden Simulation. Diese Dissertation widmet sich Oberflächen von Textil- und Fluidsimulationen mit der Methode der Smoothed Particle Hydrodynamics (SPH) und leistet einen Beitrag zur Verbesserung der Erstellung von Animationen durch die Beschreibung von Oberflächenformen, der Modellierung von Kontakt von Oberflächen und der Evaluierung von Oberflächeneffekten von Fluiden. In vielen Anwendungen gibt es eine Referenz für eine Oberflächenanimation, die ihre Form beschreibt. Das Abgleichen einer Referenz mit einer Simulation ist eine große Herausforderung und die Ähnlichkeit wird häufig visuell überprüft. Im ersten Teil der Dissertation wird eine Signatur für Textilanimationen vorgestellt, die charakteristische Formen und ihre zeitliche Veränderung erfasst. Sie ist eine Kombination aus geometrischen Merkmalen und physikalischen Eigenschaften, um das typische Deformationsverhalten genau zu repräsentieren. Die Signatur erlaubt es, Ähnlichkeiten zwischen Animationen zu berechnen, und wird angewendet, um Textilanimationen aus Kollektionen anhand eines Beispiels aufzufinden. Interaktionen zwischen partikelbasierten Fluiden und deformierbaren Objekten werden gewöhnlich durch das Abtasten des deformierbaren Objekts mit Partikeln modelliert. Bei der Interaktion mit Textilien würde dies jedoch ein neues Abtasten bei großen planaren Deformation erfordern und die Stärke des Textils wäre an die Partikelgröße gebunden. Mit diesem Problem befasst sich diese Dissertation und stellt eine Technik für die wechselseitige Kopplung zwischen Textilien und Fluiden vor, die auf dem Simulationsnetz des Textils beruht. Diese erlaubt eine robuste Kontaktbehandlung und intuitive Kontrolle von Randbedingungen. Des Weiteren wird ein Lösungsansatz für eine durchdringungsfreie Oberflächenrekonstruktion beim Kontakt mit dünnen flexiblen Objekten präsentiert. Die visuelle Qualität von partikelbasierten Fluidanimationen hängt stark von den Eigenschaften der rekonstruierten Oberfläche ab. Wichtig bei Rekonstruktionsmethoden ist, dass sie die zugrundeliegende Simulation genau repräsentieren. Die Dissertation präsentiert eine Evaluierung von Oberflächen an Grenzflächen, die den Zusammenhang zum Simulationsmodell miteinbezieht. Ein typischer Ansatz aus der Computergrafik wird mit der Oberflächenrekonstruktion in der Werkstoffkunde verglichen. Das Verhalten von freien Oberflächen in Fluidanimationen wird stark von der Oberflächenspannung beeinflusst. In dieser Dissertation wird eine Evaluierung von drei Oberflächenspannungsmodellen in Kombination mit verschiedenen Druckmodellen für SPH präsentiert, um die Charakteristika der jeweiligen Modelle zu identifizieren. Es werden systematische Tests mit Hilfe von Benchmark-Tests durchgeführt, um Stärken, Schwächen und mögliche Anwendungsbereiche deutlich zu machen