Month: June 2008

Clouds play an important role for creating realistic images of outdoor scenes. In order to generate realistic clouds, many methods have been developed for modeling and animating clouds. One of the most effective approaches for synthesizing realistic clouds is to simulate cloud formation processes based on the atmospheric fluid dynamics. Although this approach can create realistic clouds, the resulting shapes and motion depend on many simulation parameters and the initial status. Therefore, it is very difficult to adjust those parameters so that the clouds form the desired shapes. This paper addresses this problem and presents a method for controlling the simulation of cloud formation. In this paper, we focus on controlling cumuliform cloud formation. The user specifies the overall shape of the clouds. Then, our method automatically adjusts parameters during the simulation in order to generate clouds forming the specified shape. Our method can generate realistic clouds while their shapes closely match to the desired shape.

Feeback Control of Cumuliform Cloud Formation Based on Computational Fluid Dynamics

We present a combination of techniques to simulate turbulent fluid flows in 3D. Flow in a complex domain is modeled using a regular rectilinear grid with a finite-difference solution to the incompressible Navier-Stokes equations. We propose the use of the QUICK advection algorithm over a globally high resolution grid. To calculate pressure over the grid, we introduce the Iterated Orthogonal Projection (IOP) framework. In IOP a series of orthogonal projections ensures that multiple conditions such as non-divergence and boundary conditions arising through complex domains shapes or moving objects will be satisfied simultaneously to specified accuracy. This framework allows us to use a simple and highly efficient multigrid method to enforce non-divergence in combination with complex domain boundary conditions. IOP is amenable to GPU implementation, resulting in over an order of magnitude improvement over a CPU-based solver. We analyze the impact of these algorithms on the turbulent energy cascade in simulated fluid flows and the resulting visual quality.

Low Viscosity Flow Simulations for Animation

To simulate multiple fluids realistically many important interaction effects have to be captured accurately.
Smoothed Particle Hydrodynamics (SPH) has shown to be a simple, yet flexible method to cope with many fluid simulation problems in a robust way. Unfortunately, the results obtained when using SPH to simulate miscible fluids are severely affected, especially if density ratios become large. The undesirable effects reach from unphysical density and pressure variations to spurious and unnatural interface tensions, as well as severe numerical instabilities. In this work, we present a formulation based on SPH which can handle density discontinuities at interfaces between multiple fluids correctly without increasing the computational costs compared to standard SPH. The basic idea is to replace the density computation in SPH by a measure of particle densities and consequently derive new formulations for pressure and viscous forces. The new method enables the user to select the desired amount of interface tension according to the simulation problem at hand. We succeed to stably simulate multiple fluids with high density contrasts without the above described artifacts apparent in standard SPH simulations.

Density Contrast SPH Interfaces

We propose a novel solid/fluid coupling method that treats the coupled system in a fully implicit manner making it stable for arbitrary time steps, large density ratios, etc. In contrast to previous work in computer graphics, we derive our method using a simple back-of-the-envelope approach which lumps the solid and fluid momenta together, and which we show exactly conserves the momentum of the coupled system. Notably, our method uses the standard Cartesian fluid discretization and does not require (moving) conforming tetrahedral meshes or ALE frameworks. Furthermore, we use a standard Lagrangian framework for the solid, thus supporting arbitrary solid constitutive models, both implicit and explicit time integration, etc. The method is quite general, working for smoke, water, and multiphase fluids as well as both rigid and deformable solids, and both volumes and thin shells. Rigid shells and cloth are handled automatically without special treatment, and we support fully one-sided discretizations without leaking. Our equations are fully symmetric, allowing for the use of fast solvers, which is a natural result of properly conserving momentum. Finally, for simple explicit time integration of rigid bodies, we show that our equations reduce to a form similar to previous work via a single block Gaussian elimination operation, but that this approach scales poorly, i.e. as though in four spatial dimensions rather than three.

Two-Way Coupling of Fluids to Rigid and Deformable Solids and Shells