This page contains information on how Phoenix simulations work internally. Steps for setting up a simulation can be found on the Getting Started with Phoenix page.

How does Phoenix's fluid solver work?

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Phoenix's fluid simulation calculates how a fluid would evolve during a period of time. While you might be familiar with the term fluid as meaning "liquid", in physics the term fluid refers to both liquids and gases. When this documentation refers to a fluid, it means liquid or fire/smoke. The simulation runs in sequential steps, and at each step the fluid is calculated a little further in time ahead of the previous step. This way each new step depends on the previous step, and a new step cannot start before the last one has already finished. This is one of the biggest differences between distributed rendering and distributed simulations - e.g. if you want to simulate 100 frames, you can not run frames 1-50 on one machine and frames 51-100 on another machine simultaneously.

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While the simulation runs, it's in the background, and the user interface of 3ds Max remains active. You can change many simulation parameters (except for a few initial core parameters) during the simulation and see how they affect it. Rendering is also enabled during simulation, so you don't have to wait until the end of the simulation to render images and see how the simulation looks and fits into the scene.

Particle and Grid-based Simulation

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There are two main approaches used in simulation systems:

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A third hybrid method between particle and grid simulations are the FLIP simulations, which Phoenix uses for liquids. FLIP simulations take the best from both worlds and produce much realistic liquid effects quickly. FLIP liquid simulations were first added in Phoenix 3.0. Before that, liquid were simulated using the grid solver. In Phoenix 3, you can still open and simulate older scenes saved from Phoenix 2 using the old grid liquid solver. Note that Phoenix uses pure particle simulation for the secondary particle systems such as Foam, Splash, WetMap and Mist as these are simpler and don't suffer from issues that liquids in pure particle or grid simulations have, such as having to maintain volume and not collapse on themselves if put in a large container.

Fire/Smoke vs. Liquid Simulation

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Simulations performed by Phoenix can be roughly divided into two major categories:

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These categories are for convenience only, and are not rigid. Once you've created a few basic simulations and have become more familiar with Phoenix, you will have a better grasp of how the tools work, and when creating an effect that is not strictly fire or liquid you'll know how to represent it most efficiently. For example, sparks or thin smoke might seem to fall into the "fire" category, but in practice these effects might be better served by particles and thus could use some of the tools designed for liquids.

Simulation and Rendering

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The results from Phoenix simulations are saved to cache files. After that, Phoenix can read this cache data for quick viewport preview, or for rendering. Fire/smoke simulations usually produce grid voxels and can additionally produce particles that accompany and enrich the fire/smoke effect. Liquid simulations mainly produce particles and Phoenix can also automatically convert them to voxels when saving cache files during simulation.

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