Fluid and FX simulation is software that works out how water, fire, smoke or dust would really move, so that a computer-made flood, storm or blaze looks real on screen. In visual effects, FX means these moving effects, as opposed to creatures or buildings. An artist does not animate every wave by hand. Instead, the artist sets up the scene, and the software calculates the motion, using the physics of real fluids.
How does the software do this? It divides the space into a grid of tiny boxes, called cells or voxels. Each cell holds numbers for the water or smoke in it: how fast it is moving, in which direction, and at what pressure. For every frame, the software works out how each cell pushes on its neighbours, and moves the fluid forward by a fraction of a second. Splashes and spray are often added as millions of separate particles. A large simulation can use hundreds of millions of cells, and take hours for every second of film.
Take, for example, a scene in which a fishing boat is caught in a storm at sea, for a film made at a studio in Hyderabad. The shot is 10 seconds long, which is 240 frames at 24 frames per second. The artist builds a digital ocean around a 3D model of the boat, and sets the wind speed and the height of the waves. The software splits the sea around the boat into 300 million cells. Each frame takes about 15 minutes to calculate on one machine. So the 240 frames take about 60 hours, and the job is split across many machines. The director then asks for bigger waves on the left side of the boat. The artist changes the wave settings and runs the simulation again. Only when the motion is approved does the shot go on to lighting and rendering.
Why not just use a quick, approximate method? Water and fire are the effects that audiences notice most quickly when they look wrong. A wave that moves too slowly looks small, like a model in a bathtub. So VFX studios spend heavily on simulation. Some build their own simulation software, and some buy it. The studio's pipeline software tracks each simulation version, and extra computing power is often rented from a GPU cloud.
How is this different from engineering simulation (CAE)? Engineers use similar physics to test whether a pump or a dam will work. Their results must be accurate, because a real thing will be built. A film simulation only has to look right, so artists may bend the physics to get the shot the director wants. A game engine also simulates water and smoke, but in real time, 60 times a second, so its effects are much simpler.