Category: Fluids

Marek Misztal, Kenny Erleben, Adam Bargteil, J. Fursund, Brian Bunch Christensen, Andreas Bærentzen, Robert Bridson

In this paper, we present a method for animating multiphase flow of immiscible fluids using unstructured moving meshes. Our underlying discretization is an unstructured tetrahedral mesh, the deformable simplicial complex (DSC), that moves with the flow in a Lagrangian manner. Mesh optimization operations improve element quality and avoid element inversion. In the context of multiphase flow, we guarantee that every element is occupied by a single fluid and, consequently, the interface between fluids is represented by a set of faces in the simplicial complex. This approach ensures that the underlying discretization matches the physics and avoids the additional book-keeping required in grid-based methods where multiple fluids may occupy the same cell. Our Lagrangian approach naturally leads us to adopt a finite element approach to simulation, in contrast to the finite volume approaches adopted by a majority of fluid simulation techniques that use tetrahedral meshes. We characterize fluid simulation as an optimization problem allowing for full coupling of the pressure and velocity fields and the incorporation of a second-order surface energy. We introduce a preconditioner based on the diagonal Schur complement and solve our optimization on the GPU. We provide the results of parameter studies as well as a performance analysis of our method

Multiphase Flow of Immiscible Fluids on Unstructured Moving Meshes

Tyson Brochu, Todd Keeler, Robert Bridson

We present the first quality physics-based smoke animation method which runs in time approximately linear in the size of the rendered two-dimensional visual detail. Our fundamental representation is a closed triangle mesh surface dividing space between clear air and a uniformly smoky region, on which we compute vortex sheet dynamics to accurately solve inviscid buoyant flow. We handle arbitrary moving no-stick solid boundaries and by default handle an infinite domain. The simulation itself runs in time linear to the number of triangles thanks to the use of a well-conditioned integral equation treatment together with a Fast Multipole Method for linear-time summations, providing excellent performance. Basic zero-albedo smoke rendering, with embedded solids, is easy to implement for interactive rates, and the mesh output can also serve as an extremely compact and detailed input to more sophisticated volume rendering.

Linear-Time Smoke Animation with Vortex Sheet Meshes

Morten Bojsen-Hansen, Hao Li, Chris Wojtan

We present a method for recovering a temporally coherent, deforming triangle mesh with arbitrarily changing topology from an incoherent sequence of static closed surfaces. We solve this problem using the surface geometry alone, without any prior information like surface templates or velocity fields. Our system combines a proven strategy for triangle mesh improvement, a robust multi-resolution non-rigid registration routine, and a reliable technique for changing surface mesh topology. We also introduce a novel topological constraint enforcement algorithm to ensure that the output and input always have similar topology. We apply our technique to a series of diverse input data from video reconstructions, physics simulations, and artistic morphs. The structured output of our algorithm allows us to efficiently track information like colors and displacement maps, recover velocity information, and solve PDEs on the mesh as a post process.

Tracking Surfaces with Evolving Topology

Gizem Akinci, Markus Ihmsen, Nadir Akinci, Matthias Teschner

This paper presents a novel method that improves the efficiency of high-quality surface reconstructions for particle-based fluids using Marching Cubes. By constructing the scalar field only in a narrow band around the surface, the computational complexity and the memory consumption scale with the fluid surface instead of the volume. Furthermore, a parallel implementation of the method is proposed. The presented method works with various scalar field construction approaches. Experiments show that our method reconstructs high-quality surface meshes efficiently even on single-core CPUs. It scales nearly linearly on multi-core CPUs and runs up to fifty times faster on GPUs compared to the original scalar field construction approaches.

Parallel Surface Reconstruction for Particle-Based Fluids

Markus Ihmsen, Nadir Akinci, Gizem Akinci, Matthias Teschner

We present a new model for diffuse material, i.e. water–air mixtures, that can be combined with particle-based fluids. Diffuse material is uniformly represented with particles which are classified into spray, foam and air bubbles. Physically motivated rules are employed to generate, advect and dissipate diffuse material. The approach is realized as a post-processing step which enables efficient processing and versatile handling. As interparticle forces and the influence of diffuse material onto the fluid are neglected, large numbers of diffuse particles are efficiently processed to realize highly detailed small-scale effects. The presented results show that our approach can significantly improve the visual realism of large-scale fluid simulations.

Unified Spray, Foam, and Bubbles for Particle-Based Fluids

Tobias Pfaff, Nils Thuerey, Markus Gross

Buoyant turbulent smoke plumes with a sharp smoke-air interface, such as volcanic plumes, are notoriously hard to simulate. The surface clearly shows small-scale turbulent structures which are costly to resolve. In addition, the turbulence onset is directly visible at the interface, and is not captured by commonly used turbulence models. We present a novel approach that employs a triangle mesh as a high-resolution surface representation combined with a coarse Eulerian solver. On the mesh, we solve the interfacial vortex sheet equations, which allows us to accurately simulate buoyancy induced turbulence. For complex boundary conditions we propose an orthogonal turbulence model that handles vortices caused by obstacle interaction. In addition, we demonstrate a re-sampling scheme to remove surfaces that are hidden inside the bulk volume. In this way we are able to achieve highly detailed simulations of turbulent plumes efficiently.

Lagrangian Vortex Sheets for Animating Fluids