Fluid & smoke
Splashing liquids made of points, and billowing smoke made of a grid, both running live on the GPU.
There are two ways to make things flow. Fluid is particle-based: water, splashes, goo, liquids that pool and spray. Smoke is grid-based: rising plumes, fog, turbulent gas. Both run on the same solver as the rest of the physics, so they collide and mingle with cloth, hair, rigid bodies and colliders.
Liquid with Fluid Sim
Feed a cloud of points into Fluid Sim and they are treated as a body of liquid. The points pull together, resist being squashed and slosh under gravity: pooling at the bottom of a container, spraying off an edge, dripping into a bowl.
The quickest start is a Points node filling a shape with points, wired straight in. Each point becomes a droplet, and more points give smoother, denser liquid at more cost to compute.
Two per-point attributes are written as the solver runs. @density is how tightly packed the liquid is at each point, useful for driving colour, thickness or splash effects. @lambda is the solver's internal pressure correction, for advanced looks.
To see the liquid as a surface rather than a swarm of dots, render the points as a Volume, or use sprites for a quick spray look. The raw points remain useful as a guide while dialling things in.
Smoke with Smoke Sim
Smoke Sim works differently. Rather than tracking individual particles, it simulates gas on an invisible 3D grid filling a region of space, the same approach used for large plumes and fog in film and games. The grid is what lets smoke swirl, diffuse and curl in ways loose points cannot.
Smoke and particles act on each other in both directions: particles push the smoke around, and moving smoke carries particles along with it. Fling sparks through a plume and both react.
Smoke is the one part of the physics system that runs on a grid rather than on shared particles, which is why it carries its own region and resolution settings. A finer grid looks better and costs more to cook. Start coarse and refine.
Mixing with other sims
All the physics shares one solver, so fluid and smoke are not islands. Drop a Rigid Body Sim into the same scene and the liquid splashes around it. Let cloth from Cloth Sim be pushed by a smoke plume. Nothing special is wired: sims that share a world interact.
Colliders shape the flow. Add a Volume as a collision boundary, or use solid shapes, and the liquid pools and the smoke deflects against them. Force fields are the other large lever, and a swirl of turbulence turns a calm pour into chaos.
Simulations build up over time. Scrubbing the timeline backwards re-runs the sim from the start to stay accurate, so jumping around pauses while it catches up. Playing forward is smoothest. See Time & playback.