Fluids: Liquids and Gases - Computer Animation: Algorithms and Techniques - page 269

FIGURE 8.16
Images from a particle system simulation of fire.
modeled to look like explosions. The particles in each one of these particle systems are oriented to fly
up and away from the surface of the planet. The initial position for a particle is randomly chosen from
the circular base of the particle system. The initial direction of travel for each particle is constrained to
deviate less than the ejection angle away from the surface normal. Figure 8.16 (Color Plate 4) shows a
simple particle system fire.
Other approaches
Various other approaches have been used in animations with varying levels of success. Two-
dimensional animated texture maps have been used to create the effect of the upward movement of
burning gas, but such models are effective only when viewed from a specific direction. Using a
two-dimensional multiple planes approach adds some depth to the fire, but viewing directions are still
limited. Stam and Fiume [ 18 ] present advection-diffusion equations to evolve both density and tem-
perature fields. The user controls the simulation by specifying a wind field. The results are effective,
but the foundation mathematics are complicated and the model is difficult to control.
8.1.4 Summary
Modeling and animating amorphous phenomena is difficult. Gases are constantly changing shape and
lack even a definable surface. Volume graphics hold the most promise for modeling and animating gas,
but currently they have computational drawbacks that make such approaches of limited use for anima-
tion. A useful and visually accurate model of fire remains the subject of research.
8.2 Computational fluid dynamics
Gases and liquids are referred to collectively as fluids . The study of methods to compute their behavior
is called CFD.
Fluids are composed of molecules that are constantly moving. These molecules are constantly col-
liding with one another and with other objects. The molecular motion transports measurable amounts of
fluid properties throughout the fluid. These properties include such things as density, temperature, and
momentum. In CFD, the assumption is that the fluid is a continuous medium in the sense that these
properties are well-defined at infinitely small points of the fluid and that the property values are
smoothly varying throughout the fluid. This is called the continuity assumption .