We propose a hybrid method for simulating multiphase fluids such as bubbly water. The appearance of subgrid visual details is improved by incorporating a new bubble model based on smoothed particle hydrodynamics (SPH) into an Eulerian grid-based simulation that handles background flows of large bodies of water and air. To overcome the difficulty in simulating small bubbles in the context of the multiphase flows on a coarse grid, we heuristically model the interphase properties of water and air by means of the interactions between bubble particles. As a result, we can animate lively motion of bubbly water with small scale details efficiently.
Month: May 2008
Spline Joints for Multibody Dynamics
Spline joints are a novel class of joints that can model general scleronomic constraints for multibody dynamics based on the minimalcoordinates formulation. The main idea is to introduce spline curves and surfaces in the modeling of joints: We model 1-DOF joints using splines on SE(3), and construct multi-DOF joints as the product of exponentials of splines in Euclidean space. We present efficient recursive algorithms to compute the derivatives of the spline joint, as well as geometric algorithms to determine optimal parameters in order to achieve the desired joint motion. Our spline joints can be used to create interesting new simulated mechanisms for computer animation and they can more accurately model complex biomechanical joints such as the knee and shoulder.
Porous Flow in Particle-Based Fluid Simulations
This paper presents the simulation of fluid flowing through a porous deformable material. We introduce the physical principles governing porous flow, expressed by the Law of Darcy, into the Smoothed Particle Hydrodynamics (SPH) framework for simulating fluids and deformable objects. Contrary to previous SPH approaches, we simulate porous flow at a macroscopic scale, making abstraction of individual pores or cavities inside the material. Thus, the number of computational elements is kept low, while at the same time realistic simulations can be achieved. Our algorithm models the changing behavior of the wet material as well as the full two-way coupling between the fluid and the porous material. This enables various new effects, such as the simulation of sponge-like elastic bodies and water-absorbing sticky cloth.
Discrete Elastic Rods
We present a discrete treatment of adapted framed curves, parallel transport, and holonomy, thus establishing the language for a discrete geometric model of thin flexible rods with arbitrary cross section and undeformed configuration. Our approach differs from existing simulation techniques in the graphics and mechanics literature both in the kinematic description — we represent the material frame by its angular deviation from the natural Bishop frame — as well as in the dynamical treatment — we treat the centerline as dynamic and the material frame as quasistatic. Additionally, we describe a manifold projection method for coupling rods to rigid-bodies and simultaneously enforcing rod inextensibility. The use of quasistatics and constraints provides an efficient treatment for stiff twisting and stretching modes; at the same time, we retain the dynamic bending of the centerline and accurately reproduce the coupling between bending and twisting modes. We validate the discrete rod model via quantitative buckling, stability, and coupled-mode experiments, and via qualitative knot-tying comparisons.
Fast Viscoelastic Behavior with Thin Features
We introduce a method for efficiently animating a wide range of deformable materials. We combine a high resolution surface mesh with a tetrahedral finite element simulator that makes use of frequent re-meshing. This combination allows for fast and detailed simulations of complex elastic and plastic behavior. We significantly expand the range of physical parameters that can be simulated with a single technique, and the results are free from common artifacts such as volume-loss, smoothing, popping, and the absence of thin features like strands and sheets. Our decision to couple a high resolution surface with low-resolution physics leads to efficient simulation and detailed surface features, and our approach to creating the tetrahedral mesh leads to an order-of-magnitude speedup over previous techniques in the time spent re-meshing. We compute masses, collisions, and surface tension forces on the scale
of the fine mesh, which helps avoid visual artifacts due to the differing mesh resolutions. The result is a method that can simulate a large array of different material behaviors with high resolution
features in a short amount of time.