An Extended Partitioned Method for Conservative Solid-Fluid Coupling

Muzaffer Akbay, Nicholas Nobles, Victor Zoran, Tamar Shinar

We present a novel extended partitioned method for two-way solid-fluid coupling, where the fluid and solid solvers are treated as black boxes with limited exposed interfaces, facilitating modularity and code reusability. Our method achieves improved stability and extended range of applicability over standard partitioned approaches through three techniques. First, we couple the black-box solvers through a small, reduced-order monolithic system, which is constructed on the fly from input/output pairs generated by the solid and fluid solvers. Second, we use a conservative, impulse-based interaction term to couple the solid and fluid rather than typical pressure-based forces. We show that both of these techniques significantly improve stability and reduce the number of iterations needed for convergence. Finally, we propose a novel boundary pressure projection method that allows for the partitioned simulation of a fully enclosed fluid coupled to a dynamic solid, a scenario that has been problematic for partitioned methods. We demonstrate the benefits of our extended partitioned method by coupling Eulerian fluid solvers for smoke and water to Lagrangian solid solvers for volumetric and thin deformable and rigid objects in a variety of challenging scenarios. We further demonstrate our method by coupling a Lagrangian SPH fluid solver to a rigid body solver

An Extended Partitioned Method for Conservative Solid-Fluid Coupling

Posted by christopherbatty on July 15th, 2018 | Comments Off on An Extended Partitioned Method for Conservative Solid-Fluid Coupling Posted in Deformables, Fluids, Rigid bodies

HairControl: A Tracking Solution for Directable Hair Simulation

Antoine Milliez, Bob Sumner, Markus Gross, Bernhard Thomaszewski

We present a method for adding artistic control to physics-based hair simulation. Taking as input an animation of a coarse set of guide hairs, we constrain a subsequent higher-resolution simulation of detail hairs to follow the input motion in a spatially-averaged sense. The resulting high-resolution motion adheres to the artistic intent but is enhanced with detailed deformations and dynamics generated by physics-based simulation. The technical core of our approach is formed by a set of tracking constraints, requiring the center of mass of a given subset of detail hair to maintain its position relative to a reference point on the corresponding guide hair. As a crucial element of our formulation, we introduce the concept of dynamically changing constraint targets that allow reference points to slide along the guide hairs to provide sufficient flexibility for natural deformations. We furthermore propose to regularize the null space of the tracking constraints based on variance minimization, effectively controlling the amount of spread in the hair. We demonstrate the ability of our tracking solver to generate directable yet natural hair motion on a set of targeted experiments and show its application to production-level animations.

HairControl: A Tracking Solution for Directable Hair Simulation

Posted by christopherbatty on July 14th, 2018 | Comments Off on HairControl: A Tracking Solution for Directable Hair Simulation Posted in Hair/Rods/Rope

Projective peridynamics for modeling versatile elastoplastic materials

Xiaowei He, Huamin Wang, Enhua Wu

Unified simulation of versatile elastoplastic materials and different dimensions offers many advantages in animation production, contact handling, and hardware acceleration. The unstructured particle representation is particularly suitable for this task, thanks to its simplicity. However, previous meshless techniques either need too much computational cost for addressing stability issues, or lack physical meanings and fail to generate interesting deformation behaviors, such as the Poisson effect. In this paper, we study the development of an elastoplastic model under the state-based peridynamics framework, which uses integrals rather than partial derivatives in its formulation. To model elasticity, we propose a unique constitutive model and an efficient iterative simulator solved in a projective dynamics way. To handle plastic behaviors, we incorporate our simulator with the Drucker-Prager yield criterion and a reference position update scheme, both of which are implemented under peridynamics. Finally, we show how to strengthen the simulator by position-based constraints and spatially varying stiffness models, to achieve incompressibility, particle redistribution, cohesion, and friction effects in viscoelastic and granular flows. Our experiments demonstrate that our unified, meshless simulator is flexible, efficient, robust, and friendly with parallel computing.

Projective peridynamics for modeling versatile elastoplastic materials

Posted by christopherbatty on July 14th, 2018 | Comments Off on Projective peridynamics for modeling versatile elastoplastic materials Posted in Deformables

Cosserat Rods with Projective Dynamics

Carlota Soler, Tobias Martin, Olga Sorkine-Hornung

We present a novel method to simulate Cosserat rods with Projective Dynamics (PD). The proposed method is both numerically robust and accurate with respect to the underlying physics, making it suitable for a variety of applications in computer graphics and related disciplines. Cosserat theory assigns an orientation frame to each point and is thus able to realistically simulate stretching and shearing effects, in addition to bending and twisting. Within the PD framework, it is possible to obtain accurate simulations given the implicit integration over time and its decoupling of the local-global solve. In the proposed method, we start from the continuous formulation of the Cosserat theory and derive the constraints for the PD solver. We extend the standard definition of PD and add body orientations as system variables. Thus, we include the preservation of angular momentum, so tha twisting and bending can be accurately simulated. Our formulation allows the simulation of different bending behaviors with respect to a user-defined Young’s modulus, the radius of the rod’s cross-section, and material density. We show how different material specifications in our simulations converge within a few iterations to a reference solution, generated with a high-precision finite element method. Furthermore, we demonstrate mesh independence of our formulation: Refining the simulation mesh still results in the same characteristic motion, which is in contrast to previous position based method.

Posted by christopherbatty on July 2nd, 2018 | Comments Off on Cosserat Rods with Projective Dynamics Posted in Hair/Rods/Rope, Rods

A Temporally Adaptive Material Point Method with Regional Time Stepping

Yu Fang, Yuanming Hu, Shi-Min Hu, Chenfanfu Jiang

Spatially and temporally adaptive algorithms can substantially improve the computational efficiency of many numerical schemes in computational mechanics and physics-based animation. Recently, a crucial need for temporal adaptivity in the Material Point Method (MPM) is emerging due to the potentially substantial variation of material stiffness and velocities in multi-material scenes. In this work, we propose a novel temporally adaptive symplectic Euler scheme for MPM with regional time stepping (RTS), where different time steps are used in different regions. We design a time stepping scheduler operating at the granularity of small blocks to maintain a natural consistency with the hybrid particle/grid nature of MPM. Our method utilizes the Sparse Paged Grid (SPGrid) data structure and simultaneously offers high efficiency and notable ease of implementation with a practical multi-threaded particle-grid transfer strategy. We demonstrate the efficacy of our asynchronous MPM method on various examples including elastic objects, granular media, and fluid.

A Temporally Adaptive Material Point Method with Regional Time Stepping

Posted by christopherbatty on July 2nd, 2018 | Comments Off on A Temporally Adaptive Material Point Method with Regional Time Stepping Posted in Deformables, Fluids

Time-Domain Parallelization for Accelerating Cloth Simulation

Junbang Liang, Ming C. Lin

Cloth simulations, widely used in computer animation and apparel design, can be computationally expensive for real-time applications. Some parallelization techniques have been proposed for visual simulation of cloth using CPU or GPU clusters and often rely on parallelization using spatial domain decomposition techniques that have a large communication overhead. In this paper, we propose a novel time-domain parallelization technique that makes use of the two-level mesh representation to resolve the time-dependency issue and develop a practical algorithm to smooth the state transition from the corresponding coarse to fine meshes. A load estimation and a load balancing technique used in online partitioning are also proposed to maximize the performance acceleration. Our method achieves a nearly linear performance scaling on manycore clusters and outperforms spatial-domain parallelization on a diverse set of benchmarks.

Time-Domain Parallelization for Accelerating Cloth Simulation

Posted by christopherbatty on July 2nd, 2018 | Comments Off on Time-Domain Parallelization for Accelerating Cloth Simulation Posted in Cloth, Deformables

Coupled Fluid Density and Motion from Single Views

Marie-Lena Eckert, Wolfgang Heidrich, Nils Thuerey

We present a novel method to reconstruct a fluid’s 3D density and motion based on just a single sequence of images. This is rendered possible by using powerful physical priors for this strongly under-determined problem. More specifically, we propose a novel strategy to infer density updates strongly coupled to previous and current estimates of the flow motion. Additionally, we employ an accurate discretization and depth-based regularizers to compute stable solutions. Using only one view for the reconstruction reduces the complexity of the capturing setup drastically and could even allow for online video databases or smart-phone videos as inputs. The reconstructed 3D velocity can then be flexibly utilized, e.g., for re-simulation, domain modification or guiding purposes. We will demonstrate the capacity of our method with a series of synthetic test cases and the reconstruction of real smoke plumes captured with a Raspberry Pi camera.

Coupled Fluid Density and Motion from Single Views

Posted by christopherbatty on June 19th, 2018 | Comments Off on Coupled Fluid Density and Motion from Single Views Posted in Fluids

An Efficient Solver for Two-way Coupling Rigid Bodies with Incompressible Flow

Mridul Aanjaneya

We present an efficient solver for monolithic two-way coupled simulation of rigid bodies with incompressible fluids that is robust to poor conditioning of the coupled system in the presence of large density ratios between the solid and the fluid. Our method leverages ideas from the theory of Domain Decomposition, and uses a hybrid combination of direct and iterative solvers that exploits the low-dimensional nature of the solid equations. We observe that a single Multigrid V-cycle for the fluid equations serves as a very effective preconditioner for solving the Schur-complement system using Conjugate Gradients, which is the main computational bottleneck in our pipeline. We use spectral analysis to give some theoretical insights behind this observation. Our method is simple to implement, is entirely assembly-free besides the solid equations, allows for the use of large time steps because of the monolithic formulation, and remains stable even when the iterative solver is terminated early. We demonstrate the efficacy of our method on several challenging examples of two-way coupled simulation of smoke and water with rigid bodies. To illustrate that our method is applicable to other problems, we also show an example of underwater bubble simulation.

An Efficient Solver for Two-way Coupling Rigid Bodies with Incompressible Flow

Posted by christopherbatty on June 17th, 2018 | Comments Off on An Efficient Solver for Two-way Coupling Rigid Bodies with Incompressible Flow Posted in Fluids, Rigid bodies

Collision-Aware and Online Compression of Rigid Body Simulations via Integrated Error Minimization

Timothy Jeruzalski, John Kanji, Alec Jacobson, David I.W. Levin

Methods to compress simulation data are invaluable as they facilitate efficient transmission along the visual effects pipeline, fast and efficient replay of simulations for visualization and enable storage of scientific data. However, all current approaches to compressing simulation data require access to the entire dynamic simulation, leading to large memory requirements and additional computational burden. In this paper we perform compression of contact-dominated, rigid body simulations in an online, error-bounded fashion. This has the advantage of requiring access to only a narrow window of simulation data at a time while still achieving good agreement with the original simulation. Our approach is simulator agnostic allowing us to compress data from a variety of sources. We demonstrate the efficacy of our algorithm by compressing contact-dominated rigid body simulations from a number of sources, achieving compression rates of up to 360 times over raw data size.

Collision-Aware and Online Compression of Rigid Body Simulations via Integrated Error Minimization

Posted by christopherbatty on June 15th, 2018 | Comments Off on Collision-Aware and Online Compression of Rigid Body Simulations via Integrated Error Minimization Posted in Rigid bodies

Energized Rigid Body Fracture

Xiaokai Li, Sheldon Andrews, Ben Jones, Adam Bargteil

Compelling animation of fracture is a vital challenge for computer graphics. Methods based on continuum mechanics are physically accurate, but computationally expensive since they require computing elastic deformation. In many applications, this elastic deformation is imperceptible, so simulation methods based on rigid body dynamic with breakable constraints are popular in practice. Simply deleting constraints when thresholds on force or displacement are reached ignores the elastic energy that is stored just before fracture, which is captured by continuum mechanics based methods. Our approach computes the energy stored in these constraints when they are broken, and reintroduces it to the system as kinetic energy. As a result, our method is able to animate energetic fracture scenarios with results comparable to continuum mechanics approaches, but with the computational efficiency of rigid body simulation.

Energized Rigid Body Fracture

Posted by christopherbatty on June 15th, 2018 | Comments Off on Energized Rigid Body Fracture Posted in Fracture, Rigid bodies
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