Category: Fluids

Jonathan R. Bronson, Joshua A. Levine, Ross T. Whitaker

We introduce a new algorithm for generating tetrahedral meshes that conform to physical boundaries in volumetric domains consisting of multiple materials. The proposed method allows for an arbitrary number of materials, produces high-quality tetrahedral meshes with upper and lower bounds on dihedral angles, and guarantees geometric fidelity. Moreover, the method is combinatoric so its implementation enables rapid mesh construction. These meshes are structured in a way that also allows grading, in order to reduce element counts in regions of homogeneity.

Lattice Cleaving: Conforming Tetrahedral Meshes of Multimaterial Domains with Bounded Quality

Markus Ihmsen, Arthur Wahl, Matthias Teschner

We present an efficient framework for simulating granular material with high visual detail. Our model solves the computationally and numerically critical forces on a coarsely sampled particle simulation. We incorporate a new frictional boundary force into an existing continuum-based method which enables realistic interactions and a more robust simulation. Visual realism is achieved by coupling a set of highly resolved particles with the base simulation at low computational costs. Thereby, visual details can be added which are not resolved by the base simulation.

High-Resolution Simulation of Granular Material with SPH

Gizem Akinci, Nadir Akinci, Markus Ihmsen, Matthias Teschner

In this paper we present an efficient surface reconstruction pipeline for particle-based fluids such as smoothed particle hydrodynamics. After the scalar field computation and the marching cubes based triangulation, we post process the surface mesh by applying surface decimation and subdivision algorithms. In comparison to existing approaches, the decimation step alleviates the particle alignment related bumpiness very efficiently and reduces the number of triangles in flat regions. Later, the subdivision step ensures that the non-smooth regions are smoothed in a performance friendly way which allows our approach to run significantly faster by using lower resolution marching cubes grids. The presented pipeline is applicable to particle position data sets in a frame by frame basis. Throughout the paper, we present both visual and performance comparisons with different parameter settings, and with a state-of-the-art surface reconstruction technique. Our results demonstrate that in comparison to other approaches with comparable surface quality, our pipeline runs 15 to 20 times faster with up to 80% less memory and secondary storage consumption.

An Efficient Surface Reconstruction Pipeline for Particle-Based Fluids

Abhinav Golas, Rahul Narain, Jason Sewall, Pavel Krajcevski, Pradeep Dubey, Ming Lin

Simulating fluids in large-scale scenes with appreciable quality using state-of-the-art methods can lead to high memory and compute requirements. Since memory requirements are proportional to the product of domain dimensions, simulation performance is limited by memory access, as solvers for elliptic problems are not compute-bound on modern systems. This is a significant concern for large-scale scenes. To reduce the memory footprint and memory/compute ratio, vortex singularity bases can be used. Though they form a compact bases for incompressible vector fields, robust and efficient modeling of nonrigid obstacles and free-surfaces can be challenging with these methods.
We propose a hybrid domain decomposition approach that couples Eulerian velocity-based simulations with vortex singularity simulations. Our formulation reduces memory footprint by using smaller Eulerian domains with compact vortex bases, thereby improving the memory/compute ratio, and simulation performance by more than 1000x for single phase flows as well as significant improvements for free-surface scenes. Coupling these two heterogeneous methods also affords flexibility in using the most appropriate method for modeling different scene features, as well as allowing robust interaction of vortex methods with free-surfaces and nonrigid obstacles.

Large-Scale Fluid Simulation using Velocity-Vorticity Domain Decomposition

 

Michael B. Nielsen, Andreas Soderstrom, Robert Bridson

Computer animated ocean waves for feature films are typically carefully choreographed to match the vision of the director and to support the telling of the story. The rough shape of these waves is established in the previsualization (previs) stage, where artists use a variety of modeling tools with fast feedback to obtain the desired look. This poses a challenge to the effects artists who must subsequently match the locked-down look of the previs waves with high-quality simulated or synthesized waves, adding the detail necessary for the final shot. We propose a set of automated techniques for synthesizing Fourier-based ocean waves that match a previs input, allowing
artists to quickly enhance the input wave animation with additional higher frequency detail that moves consistently with the coarse waves, tweak the wave shapes to flatten troughs and sharpen peaks if desired (as is characteristic of deep water waves), and compute a physically reasonable velocity field of the water analytically. These properties are demonstrated with several examples, including a previs scene from a visual effects production environment.

Synthesizing Waves from Animated Height Fields

Karthik Raveendran, Nils Thuerey, Chris Wojtan, Greg Turk

We present an approach for artist-directed animation of liquids using multiple levels of control over the simulation, ranging from the overall tracking of desired shapes to highly detailed secondary effects such as dripping streams, separating sheets of fluid, surface waves and ripples. The first portion of our technique is a volume preserving morph that allows the animator to produce a plausible fluid-like motion from a sparse set of control meshes. By rasterizing the resulting control meshes onto the simulation grid, the mesh velocities act as boundary conditions during the projection step of the fluid simulation. We can then blend this motion together with uncontrolled fluid velocities to achieve a more relaxed control over the fluid that captures natural inertial effects. Our method can produce highly detailed liquid surfaces with control over sub-grid details by using a mesh-based surface tracker on top of a coarse grid-based fluid simulation. We can create ripples and waves on the fluid surface attracting the surface mesh to the control mesh with spring-like forces and also by running a wave simulation over the surface mesh. Our video results demonstrate how our control scheme can be used to create animated characters and shapes that are made of water.

Controlling Liquids Using Meshes

Alfred Barnat, Nancy S. Pollard

Smoke is one of the core phenomena which fluid simulation techniques in computer graphics have attempted to capture. It is both well understood mathematically and important in lending realism to computer generated effects. In an attempt to overcome the diffusion inherent to Eulerian grid-based simulators, a technique has recently been developed which represents velocity using a sparse set of vortex filaments. This has the advantage of providing an easily understandable and controllable model for fluid velocity, but is computationally expensive because each filament affects the fluid velocity over an unbounded region of the simulation space. We present an alternative to existing techniques which merge adjacent filament rings, instead allowing filaments to form arbitrary structures,and we develop a new set of reconnection criteria to take advantage of this filament graph. To complement this technique, we also introduce a method for smoke surface tracking and rendering designed to minimize the number of sample points without introducing excessive diffusion or blurring. Though this representation lends itself to straightforward real-time rendering, we also present a method which renders the thin sheets and curls of smoke as diffuse volumes using any GPU capable of supporting geometry shaders.

Smoke Sheets for Graph-Structured Vortex Filaments

Michael Lentine, Matthew Cong, Saket Patkar, Ron Fedkiw

We provide a novel simulation method for incompressible free surface flows that allows for large time steps on the order of 10-40 times bigger than the typical explicit time step restriction would allow. Although semi-Lagrangian advection allows for this from the standpoint of stability, large time steps typically produce significant visual errors. This was addressed in previous work for smoke simulation using a mass and momentum conserving version of semi-Lagrangian advection, and while its extension to water for momentum conservation for small time steps was addressed, pronounced issues remain when taking large time steps. The main difference between smoke and water is that smoke has a globally defined velocity field whereas water needs to move in a manner uninfluenced by the surrounding air flow, and this poses real issues in determining an appropriate extrapolated velocity field. We alleviate problems with the extrapolated velocity field by not using it when it is incorrect, which we determine via conservative advection of a color function which adds forwardly advected semi-Lagrangian rays to maintain conservation when mass is lost. We note that one might also use a more traditional volume-of-fluid method which is more explicitly focused on the geometry of the interface but can be less visually appealing — it is also unclear how to extend volume-of-fluid methods to have larger time steps. Finally, we prefer the visual smoothness of a particle level set method coupled to a traditional backward tracing semi-Lagrangian advection where possible, only using our forward traced color function solution in areas of the flow where the particle level set method fails due to the extremely large time steps.

Simulating Free Surface Flow with Very Large Timesteps

Nuttapong Chentanez, Matthias Mueller

We present a GPU friendly, Eulerian, free surface fluid simulation method that conserves mass locally and globally without the use of Lagrangian components. Local mass conservation prevents small scale details of the free surface from disappearing, a problem that plagues many previous approaches, while global mass conservation ensures that the total volume of the liquid does not decrease over time. Our method handles moving solid boundaries as well as cells that are partially filled with solids. Due to its stability, it allows the use of large time steps which makes it suitable for both off-line and real-time applications. We achieve this by using density based surface tracking with a novel, unconditionally stable, conservative advection scheme and a novel interface sharpening method. While our approach conserves mass, volume loss is still possible but only temporarily. With constant mass, local volume loss causes a local increase of the density used for surface tracking which we detect and correct over time. We also propose a density post-processing method to reveal sub-grid details of the liquid surface. We show the effectiveness of the proposed method in several practical examples all running either at interactive rates or in real-time.

Mass-Conserving Eulerian Liquid Simulation