Guides

Wind and obstacles

Buoyancy lifts the hot gas, and turbulence and vorticity confinement keep it curling. Wind bends it, and solid spheres and boxes deflect it. A moving obstacle pushes the gas out of its way.

campfire with params.wind ramped from 0.35 up to about 3 m/s and back. The flame leans harder near the top, because the wind grows with height.

#Wind

fire.setWind(2);                      // m/s along params.windDirection
fire.setWind(1.2, [0, 0, -1]);        // strength + direction
fire.params.wind = 0.5;               // or set the params directly

Param	Default
`wind`	`0`	Strength in m/s. Negative values blow the other way (the demo slider runs from −4 to 4)
`windDirection`	`[1, 0, 0.18]`	Direction. Only x and z are used

Wind is horizontal and grows with height: the target speed runs from 0.35× wind at the domain floor to 1.35× at the top. The gas is dragged toward it gradually rather than set to it, so gusts ease in.

`campfire` after `fire.setWind(2.4)`: the flame and smoke lean downwind.

#Turbulence and vorticity

params.turbulence (default 1) scales two things together:

Noise forcing. Two octaves of scrolling 3D noise push the gas. It only acts where there is heat or smoke, so still air stays still.
Vorticity confinement. The solver finds the curl of the velocity field and pushes it back into small vortices that numerical diffusion would otherwise smooth away. Its gain is 1.2 + 2.4 × turbulence.

params.buoyancy scales the lift of hot gas, and smoke adds a little weight. Keep turbulence within about 0–2. The confinement gain is in per-cell units, and much larger gains run away.

#Obstacles

const rock = fire.addObstacle({ type: 'sphere', position: [0.35, 3, 0.1], radius: 0.6 });
const beam = fire.addObstacle({ type: 'box', position: [0, 4, 0], size: [3, 0.3, 0.4] });

rock.position.x = 1;        // move it: the gas is pushed out of the way
rock.enabled = false;
beam.remove();              // or fire.removeObstacle(beam)

Obstacles are rasterised into a solid mask once per frame. Inside one, the fields are empty and the velocity is the obstacle's own measured velocity (clamped to 6 m/s). A moving sphere drags and pushes gas like a paddle.

A 0.6 m sphere obstacle swept side to side above the campfire. The flame parts around it and gets pushed along with it.

Up to 4 obstacles are simulated. addObstacle() warns when you add a fifth, and the extras are ignored.
They're fully solid, with no partial occupancy. An obstacle thinner than one grid cell (about 6 cm on high) is lost.
The obstacle is only a boundary for the gas. Draw a mesh for it yourself: the demo copies fire.obstacles[0].position onto its metal sphere every frame.
Positions are world metres. In a preset file, they're relative to the domain floor centre.

The demo's obstacle sphere (the O key) above the campfire. The flame and smoke split around it.

#The domain box

The fire only exists inside domain, which is set at creation (default: centre (0, 3, 0), size (4, 6, 4)). The grid has cubic cells of size.x / n, where n is the tier's cell count across x. The floor is closed. The sides and the top are open: fields fade in the last few cells, so gas leaves the box instead of piling up against the walls.

fire.domain;   // { min, size, center } in world units
fire.grid;     // { nx, ny, nz, cell }, e.g. 64 × 96 × 64 with 0.0625 m cells on high