Results 1  10
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35
WOJTAN C.: Tracking surfaces with evolving topology
 ACM Trans. Graph. (SIGGRAPH
, 2012
"... Figure 1: Our method recovers a sequence of highquality, temporally coherent triangle meshes from any sequence of closed surfaces with arbitrarily changing topology. We reliably extract correspondences from a level set and track textures backwards through a fluid simulation. We present a method for ..."
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Cited by 13 (2 self)
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Figure 1: Our method recovers a sequence of highquality, temporally coherent triangle meshes from any sequence of closed surfaces with arbitrarily changing topology. We reliably extract correspondences from a level set and track textures backwards through a fluid simulation. 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 multiresolution nonrigid 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.
Simulating Liquids and SolidLiquid Interactions with Lagrangian Meshes
"... This paper describes a Lagrangian finite element method that simulates the behavior of liquids and solids in a unified framework. Local mesh improvement operations maintain a highquality tetrahedral discretization even as the mesh is advected by fluid flow. We conserve volume and momentum, locally ..."
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Cited by 13 (1 self)
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This paper describes a Lagrangian finite element method that simulates the behavior of liquids and solids in a unified framework. Local mesh improvement operations maintain a highquality tetrahedral discretization even as the mesh is advected by fluid flow. We conserve volume and momentum, locally and globally, by assigning each element an independent rest volume and adjusting it to correct for deviations during remeshing and collisions. Incompressibility is enforced with pernode pressure values, and extra degrees of freedom are selectively inserted to prevent pressure locking. Topological changes in the domain are explicitly treated with local mesh splitting and merging. Our method models surface tension with an implicit formulation based on surface energies computed on the boundary of the volume mesh. With this method we can model elastic, plastic, and liquid materials in a single mesh, with no need for explicit coupling. We also model heat diffusion and thermoelastic effects, which allow us to simulate phase changes.
C.: Explicit mesh surfaces for particle based fluids
 Computer Graphics Forum (Proc. Eurographics
, 2012
"... Figure 1: A drop falling into a shallow pool creates a water crown. We introduce the idea of using an explicit triangle mesh to track the air/fluid interface in a smoothed particle hydrodynamics (SPH) simulator. Once an initial surface mesh is created, this mesh is carried forward in time using near ..."
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Cited by 12 (2 self)
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Figure 1: A drop falling into a shallow pool creates a water crown. We introduce the idea of using an explicit triangle mesh to track the air/fluid interface in a smoothed particle hydrodynamics (SPH) simulator. Once an initial surface mesh is created, this mesh is carried forward in time using nearby particle velocities to advect the mesh vertices. The mesh connectivity remains mostly unchanged across timesteps; it is only modified locally for topology change events or for the improvement of triangle quality. In order to ensure that the surface mesh does not diverge from the underlying particle simulation, we periodically project the mesh surface onto an implicit surface defined by the physics simulation. The mesh surface gives us several advantages over previous SPH surface tracking techniques. We demonstrate a new method for surface tension calculations that clearly outperforms the state of the art in SPH surface tension for computer graphics. We also demonstrate a method for tracking detailed surface information (like colors) that is less susceptible to numerical diffusion than competing techniques. Finally, our temporallycoherent surface mesh allows us to simulate highresolution surface wave dynamics without being limited by the particle resolution of the SPH simulation.
Articulated Swimming Creatures
"... We present a general approach to creating realistic swimming behavior for a given articulated creature body. The two main components of our method are creature/fluid simulation and the optimization of the creature motion parameters. We simulate twoway coupling between the fluid and the articulated ..."
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Cited by 12 (3 self)
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We present a general approach to creating realistic swimming behavior for a given articulated creature body. The two main components of our method are creature/fluid simulation and the optimization of the creature motion parameters. We simulate twoway coupling between the fluid and the articulated body by solving a linear system that matches acceleration at fluid/solid boundaries and that also enforces fluid incompressibility. The swimming motion of a given creature is described as a set of periodic functions, one for each joint degree of freedom. We optimize over the space of these functions in order to find a motion that causes the creature to swim straight and stay within a given energy budget. Our creatures can perform path following by first training appropriate turning maneuvers through offline optimization and then selecting between these motions to track the given path. We present results for a clownfish, an eel, a sea turtle, a manta ray and a frog, and in each case the resulting motion is a good match to the realworld animals. We also demonstrate a plausible swimming gait for a fictional creature that has no realworld counterpart.
Multiphase Flow of Immiscible Fluids on Unstructured Moving Meshes
"... Figure 1: Multiple fluids with different viscosity coefficients and surface tension densities splashing on the bottom of a cylindrical container. Observe that the simulation has no problem dealing with thin sheets. In this paper, we present a method for animating multiphase flow of immiscible fluids ..."
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Cited by 10 (3 self)
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Figure 1: Multiple fluids with different viscosity coefficients and surface tension densities splashing on the bottom of a cylindrical container. Observe that the simulation has no problem dealing with thin sheets. 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 bookkeeping required in gridbased 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 secondorder 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
Animating Bubble Interactions in a Liquid Foam
"... Figure 1: Coke foam. By representing foam geometry using a weighted Voronoi diagram, our particlebased algorithm can efficiently provide bubble features in existing liquid animation. This example contains up to 100K bubbles and each frame takes less than 20 seconds to simulate. Links: DL PDF Web Vi ..."
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Cited by 9 (1 self)
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Figure 1: Coke foam. By representing foam geometry using a weighted Voronoi diagram, our particlebased algorithm can efficiently provide bubble features in existing liquid animation. This example contains up to 100K bubbles and each frame takes less than 20 seconds to simulate. Links: DL PDF Web Video 1
Mass and Momentum Conservation for Fluid Simulation
, 2011
"... Momentum conservation has long been used as a design principle for solid simulation (e.g. collisions between rigid bodies, massspring elastic and damping forces, etc.), yet it has not been widely used for fluid simulation. In fact, semiLagrangian advection does not conserve momentum, but is still ..."
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Cited by 9 (2 self)
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Momentum conservation has long been used as a design principle for solid simulation (e.g. collisions between rigid bodies, massspring elastic and damping forces, etc.), yet it has not been widely used for fluid simulation. In fact, semiLagrangian advection does not conserve momentum, but is still regularly used as a bread and butter method for fluid simulation. In this paper, we propose a modification to the semiLagrangian method in order to make it fully conserve momentum. While methods of this type have been proposed earlier in the computational physics literature, they are not necessarily appropriate for coarse grids, large time steps or inviscid flows, all of which are common in graphics applications. In addition, we show that the commonly used vorticity confinement turbulence model can be modified to exactly conserve momentum as well. We provide a number of examples that illustrate the benefits of this new approach, both in conserving fluid momentum and passively advected scalars such as smoke density. In particular, we show that our new method is amenable to efficient smoke simulation with one time step per frame, whereas the traditional nonconservative semiLagrangian method experiences serious artifacts when run with these large time steps, especially when object interaction is considered.
Discrete Viscous Sheets
"... Figure 1: A thin sheet of molten chocolate enrobes a spherical truffle. As viscous sheets deform they exhibit behaviors that combine both the fluidity of liquids, and the buckling and wrinkling instabilities of thin materials, as evidenced here in the beautiful spindly legs. We present the first red ..."
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Cited by 8 (1 self)
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Figure 1: A thin sheet of molten chocolate enrobes a spherical truffle. As viscous sheets deform they exhibit behaviors that combine both the fluidity of liquids, and the buckling and wrinkling instabilities of thin materials, as evidenced here in the beautiful spindly legs. We present the first reduceddimensional technique to simulate the dynamics of thin sheets of viscous incompressible liquid in three dimensions. Beginning from a discrete Lagrangian model for elastic thin shells, we apply the StokesRayleigh analogy to derive a simple yet consistent model for viscous forces. We incorporate nonlinear surface tension forces with a formulation based on minimizing discrete surface area, and preserve the quality of triangular mesh elements through local remeshing operations. Simultaneously, we track and evolve the thickness of each triangle to exactly conserve liquid volume. This approach enables the simulation of extremely thin sheets of viscous liquids, which are difficult to animate with existing volumetric approaches. We demonstrate our method with examples of several characteristic viscous sheet behaviors, including stretching, buckling, sagging, and wrinkling.
WOJTAN C.: Liquid surface tracking with error compensation
 ACM Trans. Graph
, 2013
"... Figure 1: Our method permits highresolution tracking of a lowresolution fluid simulation, without any visual or topological artifacts. The original simulation (a) exhibits sharp details and lowresolution banding artifacts. Smoothing the surface tracker (b) hides the artifacts but corrodes importa ..."
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Cited by 5 (1 self)
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Figure 1: Our method permits highresolution tracking of a lowresolution fluid simulation, without any visual or topological artifacts. The original simulation (a) exhibits sharp details and lowresolution banding artifacts. Smoothing the surface tracker (b) hides the artifacts but corrodes important surface features. We propose a smoothing technique (c) that preserves sharp details while selectively removing surface tracking artifacts, and a force generation method (d) that removes visual artifacts with strategically placed surface waves. Our algorithms are general and apply to both level sets as well as meshbased surface tracking techniques. Our work concerns the combination of an Eulerian liquid simulation with a highresolution surface tracker (e.g. the level set method or a Lagrangian triangle mesh). The naive application of a highresolution surface tracker to a lowresolution velocity field can produce many visually disturbing physical and topological artifacts that limit their use in practice. We address these problems by defining an error function which compares the current state of the surface tracker to the set of physically valid surface states. By reducing this error with a gradient descent technique, we introduce a novel physicsbased surface fairing method. Similarly, by treating this error function as a potential energy, we derive a new surface correction force that mimics the vortex sheet equations. We demonstrate our results with both level set and meshbased surface trackers.
An adaptive discretization of incompressible flow using a multitude of moving Cartesian grids
"... We present a novel method for discretizing a multitude of moving and overlapping Cartesian grids each with an independently chosen cell size to address adaptivity. Advection is handled with first and second order accurate semiLagrangian schemes in order to alleviate any time step restriction associ ..."
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Cited by 4 (3 self)
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We present a novel method for discretizing a multitude of moving and overlapping Cartesian grids each with an independently chosen cell size to address adaptivity. Advection is handled with first and second order accurate semiLagrangian schemes in order to alleviate any time step restriction associated with small grid cell sizes. Likewise, an implicit temporal discretization is used for the parabolic terms, such as the heat equation and NavierStokes viscosity. The most intricate aspect of any such discretization is the method used in order to solve the elliptic equation for the NavierStokes pressure or that resulting from the temporal discretization of parabolic terms. We address this by first removing any degrees of freedom which duplicately cover spatial regions due to overlapping grids, and then providing a discretization for the remaining degrees of freedom adjacent to these regions. We observe that a robust second order accurate symmetric positive definite readily preconditioned discretization can be obtained by constructing a local Voronoi region on the fly for each degree of freedom in question in order to obtain both its stencil (logically connected neighbors) and stencil weights. We independently demonstrate each aspect of our approach on test problems in order to show efficacy and convergence before finally addressing a number of common test cases for incompressible flow with potentially moving solid bodies. 1.