Turbulence and a vortex are formulas: given a position and a time they return a push, and they have no memory. The Fluid Field is a simulated grid instead — an incompressible flow that the emission stirs and the particles ride — and that memory is what makes smoke curl back on itself rather than shimmer in place. On top of it sit force emitters: jets, vortices, blasts and heat sources placed at a point in the box, with no particles of their own.
This page covers the FLUID / SMOKE and FORCE EMITTERS tabs of the Particle Solver. The plain per-box forces — gravity, wind, drag, turbulence, vortex, follow path — live in the solver’s own MOTION tab and are described there.
The Fluid Field
Model is the switch, and it is Off by default:
- Fluid — velocity only: an incompressible grid the emission stirs.
- Smoke — the same field plus a density and heat pair riding it, so buoyancy, weight, cooling and dissipation come into play. This is what makes a plume.
The panel says when nothing is driving the field, so a still grid is a stated condition rather than a mystery.
How particles ride it
- Fluid force — how fast a particle trades its own velocity for the field’s. High makes a tracer that goes exactly where the flow goes; low leaves it its inertia.
- Driving fluid / smoke by — Whole layer means emission drives the field and every particle in the box rides it (the field is the whole medium). Force emitters makes the sources local: only the particles inside a source’s radius move, and the rest keep their own motion — the way a Houdini source drives what is close to it and leaves the rest alone.
- Time factor — the field’s own clock. Particles keep real time.
The grid
- Fidelity — Low, Medium, High, Ultra, plus Flat HD and Volume HD. Cells are about 10 px on every axis at Volume HD; Flat HD spends the same cell budget in-plane, for thin-column looks.
- Pressure steps — iterations of the pressure solve. More is cleaner incompressibility and a slower bake.
- Viscosity — how much the flow drags on itself.
- Swirl and Swirl size — vorticity confinement, which feeds back the small eddies the grid would otherwise smear away. Swirl size is measured in cells: 1 reinforces cell-scale curls, larger values give fat, rolling eddies.
- Advection — the sharper MacCormack scheme for the velocity field.
- Bounds — Closed box gives the field walls, so a plume pools under the top; Open lets it flow out.
- Field fade — how fast the field calms with nothing stirring it.
Smoke
With the Smoke model on, five more numbers describe the material rather than the flow:
- Buoyancy — how hard heat lifts. This is what makes a plume rise.
- Smoke weight — how much the density itself pulls down, which is what gives a plume its head and its curl-back.
- Emit heat and Cooling — how hot the material arrives and how fast it gives that up, so a column rises and then spreads.
- Dissipate — how fast the density itself thins out.
Draft
A gusting wind on the field: Draft, Draft direction and Gustiness. Its variation is purely temporal — smooth sines of time — because any spatial pattern reads as a pattern.
Stirring from the emitter
The particle emitter stirs the field wherever it emits, with Stir (how hard) and Stir radius (over what area). Variance wobbles the emission’s direction, strength and heat over time: zero is a perfectly steady source, which reads as a uniform carpet of curls, and some variance gives puffs and billows at every scale.
Setting Stir to 0 leaves the force emitters as the only thing making smoke in the layer — which is what you want when the smoke should rise from a place rather than from everywhere the emitter is.
Force Emitters
Pushers with no particles of their own. A system can hold three, and each is drawn on the preview with a handle you can drag: Show on preview puts them on screen, where the ring is the force’s reach, a blower shows its push direction and a vortex its spin.
Forces
Every shape the global force offers is its own row here — the same force, placed at a point instead of on the whole box:
| Force | What it does |
|---|---|
| Vortex | Tangential pull with a slight inward bias — the one that gathers a cloud rather than flinging it outward. |
| Orbit | Tangential only. Particles keep their radius, so a ring stays a ring. |
| Twist | Tangential plus a lift along the axis: a funnel. Lift is what turns a carousel into a tornado. |
| Attract | Straight in toward the point. With a long radius it reads as gravity; with a short one, as a black hole. |
| Repel | Straight out from the point. Pair it with a blast for a shockwave. |
Each carries Strength, Radius (a falloff, not a boundary) and a spin direction.
Emitters
| Emitter | What it does |
|---|---|
| Blower | A fan. Everything inside its radius is driven toward its push speed, in its direction. Drag with is that speed, Push direction the angle (0° right, 90° up). |
| Blast force | A blast that shoves everything outward from its point, at its own times: First blast at, then Blast every n seconds. 0 fires once and is done. |
| Layer velocity | Another layer’s own animation becomes the force. Its keyframes, motion path and parenting are read straight off the timeline in 3D — travel, spin, scale and depth — so what it does on screen is what it drags here. Nothing is read from its pixels, and it has no position of its own: the layer it follows is where it is. |
| Thermal | A hot spot in the fluid. Smoke amount is how much material it puffs out a second and Heat how hot the puff is — heat is what buoyancy lifts, so it sets how fast the plume rises. Adding a thermal turns the Smoke model on for you, because with no field there is nothing for it to warm. |
Motion
A force emitter does not have to stay put. Motion is Hold, Orbit or Sweep, with Motion speed and Motion radius, so a blower can pan across a plume or a vortex can travel through it.
Follow path
Particles can be given a spline to seek instead of a field. Attract pulls toward the nearest point of the path, Flow pushes along its tangent (the “move along path” behaviour), Path swirl rotates them around that tangent to braid the stream into a helix, and Proximity is how far from the path the forces still reach before they fall off.
What re-simulates
Everything on this page is part of the simulation: changing the fluid model, a force emitter, the draft or the path re-bakes the system (or, on the GPU engine, restarts it). The look and colour sections never do — see Particle Solver for the split.
Where to go next
- Particle Solver
- Particle Emitters and Distribution
- Particle Collisions and Packing
- Camera Layer — the 3D world view the force handles are drawn in
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