We propose a particle-based technique for simulating incompressible fluid that includes adaptive refinement of particle sampling. Each particle represents a mass of fluid in its local region. Particles are split into several particles for finer sampling in regions of complex flow. In regions of smooth flow, neighboring particles can be merged. Depth below the surface and Reynolds number are exploited as our criteria for determining whether splitting or merging should take place. For the fluid dynamics calculations, we use the hybrid FLIP method, which is computationally simple and efficient. Since the fluid is incompressible, each particle has a volume proportional to its mass. A kernel function, whose effective range is based on this volume, is used for transferring and updating the particle’s physical properties such as mass and velocity. Our adaptive particle-based simulation is demonstrated in several scenarios that show its effectiveness in capturing fine detail of the flow, where needed, while efficiently sampling regions where less detail is required.
Vortex Methods for Incompressible Flow Simulation on the GPU
We present a remeshed vortex particle method for incompressible flow simulations on GPUs. The particles are convected in a Lagrangian frame and are periodically reinitialized on a regular grid. The grid is used in addition to solve for the velocity–vorticity Poisson equation and for the computation of the diffusion operators. In the present GPU implementation of particle methods, the remeshing and the solution of the Poisson equation rely on fast and efficient mesh-particle interpolations. We demonstrate that particle remeshing introduces minimal artificial dissipation, enables a faster computation of differential operators on particles over grid-free techniques and can be efficiently implemented on GPUs. The results demonstrate that, contrary to common practice in particle simulations, it is necessary to remesh the (vortex) particle locations in order to solve accurately the equations they discretize, without compromising the speed of the method. The present method leads to simulations of incompressible vortical flows on GPUs with unprecedented accuracy and efficiency.
Vortex Methods for Incompressible Flow Simulation on the GPU
Real-time Animation of Sand-Water Interaction
Recent advances in physically-based simulations have made it possible to generate realistic animations. However, in the case of solid-fluid coupling, wetting effects have rarely been noticed despite their visual importance especially in interactions between fluids and granular materials. This paper presents a simple particle-based method to model the physical mechanism of wetness propagating through granular materials; Fluid particles are absorbed in the spaces between the granular particles and these wetted granular particles then stick together due to liquid bridges that are caused by surface tension and which will subsequently disappear when over-wetting occurs. Our method can handle these phenomena by introducing a wetness value for each granular particle and by integrating those aspects of behavior that are dependent on wetness into the simulation framework. Using this method, a GPU-based simulator can achieve highly dynamic animations that include wetting effects in real time.
Optimizing Cubature for Efficient Integration of Subspace Deformations
We propose an efficient scheme for evaluating nonlinear subspace forces (and Jacobians) associated with subspace deformations. The core problem we address is efficient integration of the subspace force density over the 3D spatial domain. Similar to Gaussian quadrature schemes that efficiently integrate functions that lie in particular polynomial subspaces, we propose cubature schemes (multi-dimensional quadrature) optimized for efficient integration of force densities associated with particular subspace deformations, particular materials, and particular geometric domains. We support generic subspace deformation kinematics, and nonlinear hyperelastic materials. For an r-dimensional deformation subspace with O(r) cubature points, our method is able to evaluate subspace forces at O(r^2) cost. We also describe composite cubature rules for runtime error estimation. Results are provided for various subspace deformation models, several hyperelastic materials (St.Venant-Kirchhoff, Mooney-Rivlin, Arruda-Boyce), and multimodal (graphics, haptics, sound) applications. We show dramatically better efficiency than traditional Monte Carlo integration.
Optimizing Cubature for Efficient Integration of Subspace Deformations
Magnets in Motion
We introduce magnetic interaction for rigid body simulation. Our approach is based on an equivalent dipole method and as such it is discrete from the ground up. Our approach is symmetric as we base both field and force computations on dipole interactions.
Enriching rigid body simulation with magnetism allows for many new and interesting possibilities in computer animation and special effects. Our method also allows the accurate computation of magnetic fields for arbitrarily shaped objects, which is especially interesting for pedagogy as it allows the user to visually discover properties of magnetism which would otherwise be difficult to grasp.
We demonstrate our method on a variety of problems and our results reflect intuitive as well as surprising effects. Our method is fast and can be coupled with any rigid body solver to simulate dozens of magnetic objects at interactive rates.
Algoryx and Phun, CMLabs
There seem to be quite a few companies in the business of physics simulation these days.
Graham Fyffe pointed out this one to me: Algoryx focuses on 3D multi-physics simulations. They are also responsible for the Phun demo I posted a while ago, that has also been floating around YouTube.
Another company that does rigid-body physics simulations is CMLabs.
Thesis: Controlling Multibody Dynamics via Browsing and Time Reversal
Christopher Twigg’s thesis from CMU:
“Animation techniques for controlling passive simulation are commonly based on an optimization paradigm: the user provides goals a priori, and sophisticated numerical methods minimize a cost function that represents these goals. Unfortunately, for multibody systems with discontinuous contact events these optimization problems can be highly nontrivial to solve, and many-hour offline optimizations, unintuitive parameters, and convergence failures can frustrate end-users and limit usage. On the other hand, users are quite adaptable, and systems which provide interactive feedback via an intuitive interface can leverage the user’s own abilities to quickly produce interesting animations. However, the online computation necessary for interactivity limits scene complexity in practice. This thesis presents two methods for controlling the rigid body simulations.
The first is Many-Worlds Browsing, a method which exploits the speed of multibody simulators to compute numerous simulations in parallel (offline and online), and allow the user to browse and modify them interactively. By bolting responsive, powerful, intuitive interfaces onto relatively simple sampling techniques we get a method that enables animators to produce compelling results with a minimum of effort. The second method is time-reversed simulation: we provide only the final resting configuration of the system and run the simulator backwards in time. During the development of this method we encountered a number of surprisingly counter-intuitive results, which can be elucidated using a combination of numerical simulation and thought experiments.”
Controlling Multibody Dynamics via Browsing and Time Reversal
Cosserat Nets
Cosserat nets are networks of elastic rods that are linked by elastic joints. They allow to represent a large variety of objects such as elastic rings, coarse nets, or truss structures. In this paper, we propose a novel approach to model and dynamically simulate such Cosserat nets. We first derive the static equilibrium of the elastic rod model that supports both bending and twisting deformation modes. We further propose a
dynamic model that allows for the efficient simulation of elastic rods. We then focus on the simulation of the Cosserat nets by extending the elastic rod deformation model to branched and looped topologies.
To round out the discussion, we evaluate our deformation model. By comparing our deformation model to a reference model, we illustrate both the physical plausibility and the conceptual advantages of the proposed approach.
PixeLux's DMM
I added Pixelux Entertainment’s link on the side. They have developed a piece of software known as DMM (for Digital Molecular Matter), that “is a real-time finite element system that is being used in the “Force Unleashed”, an upcoming video game by LucasArts. [They] also have a plug-in that allows people to utilize FEA-based deformation and fracture within Maya as well as for [their] real-time engine.”
Book: Fluid simulation for computer graphics
“This book is designed to give the reader a practical introduction to fluid simulation for graphics. The field of fluid dynamics, even just in animation, is vast and so not every topic will be covered, and many wonderful papers will sadly be passed over in the hope of distilling the essentials; this is far from a thorough survey. The focus of this book is animating fully three-dimensional incompressible flow—from understanding the math and the algorithms to actual implementation. However, there is also a small amount of material on height field simplifications which are important for efficiently modeling large bodies of water.”