Smoke
Plumes, wisps, steam and drifting clouds: particles as the source, the gas set in the Smoke and Fire panel, cell size and cost, obstacles, and the volume it renders as.
Smoke is two parts: particles that are the source, filling the air round them with smoke and heat; and the gas itself, which rises, cools, swirls and drifts, set once for the simulation. The bake saves the gas as OpenVDB frames and plays them as a volume object.
Start from
The Smoke starter: a plume rising and drifting in a breeze. It's a flow of one event:
The particles live a fraction of a second and aren't shown: they only put gas into the air.
The source: Smoke Source
Smoke Source fills the gas within Radius of each particle (times its scale) up to its Smoke and Heat. Push drags the gas round each particle along with it, so a fast source stirs the air.
How the particles move is how the source moves: a slow Speed up for a chimney, a Position on a surface for smoke off an object, particles trailing a rocket for its exhaust.
The gas: the Smoke and Fire panel
The panel shows when the flow has a Smoke Source. Its main settings:
| Setting | What it does |
|---|---|
| Cell Size | How big the gas's little cubes are: the detail, and the cost |
| Sharpness | How sharply the gas carries smoke, flame and its own swirls: Fast, Sharp or Extreme |
| Buoyancy | How hard heat lifts the gas. Negative sinks it |
| Weight | How hard smoke weighs the gas down: heavy smoke sinks as it cools |
| Cooling | The share of its heat lost each second: higher stops it rising sooner, so it hangs |
| Fade | The share of smoke that thins away each second. 0 keeps it all; more for wisps |
| Swirl | Keeps up the small curls the grid would smooth away |
Under Wind and Turbulence: Wind is the air's own velocity, and Drag how fast the gas is drawn to it (0: no wind at all). Turbulence stirs the gas everywhere, at its Size and Change. In Heat, In Fuel and In Smoke stir it more where those are, so with Turbulence at 0 only the hot gas, say, is stirred. Noise is what it stirs with:
- Curl, the default, is swirls that don't squeeze the gas, so none of them are lost.
- Value pushes this way and that, and the gas takes part of it back out.
- Cellular gathers its swirls in cells, sheared where two meet. It's billowy, and slower.
Detail adds smaller, weaker swirls under it, down to a few cells, so finer cells get finer detail.
Disturbance is small swirls only where it burns, to shred a flame into licks and tendrils. Disturbance Size sets how big they are in cells, not metres, so they stay just above what the cells can show at any cell size.
Swirl
Swirl keeps the gas's own curls up everywhere. Under Swirl, In Heat, In Fuel and In Smoke keep them up more where those are, and In Flame where it burns. Hot gas curling while the cold smoke above it drifts calmly is Swirl low and In Heat up.
Front and Back multiply the swirl at the plume's edge. The front is where hot gas pushes into cooler air, like the top of a rising plume or the leading edge of a fireball. The back is where it trails away. Front above 1 rolls the leading edge into curls, and Back below 1 lets the trail go smooth.
Edge Lift lifts the plume's edges harder than its middle, by how far their heat stands out from the heat beside them. That strengthens the shear at the edge, which rolls it up into small curls, more so at finer cells. It's off at 0; try 0.5. It mixes the smoke into the air a little faster, so thin smoke fades a little sooner.
Edge Swirl keeps the swirl to the gas's outer edges, where its smoke and heat end against clear air. The edges roll over into big billows while the inside churns less, the look of a rolling fireball or a smoke column boiling up. At 1 the swirl is only at the edges, so raise Swirl with it. It's off at 0; try 0.6.
Fine Mixing is mixing finer than a cell. Neighbouring gas that slides past itself is evened out, so rising fire doesn't pull into thin vertical lines, while broad, smooth motion is left alone. It's off at 0; try 0.17. Much higher and the small curls go too.
A look sets most of these at once: Thick Smoke, Steam and Wisps are for smoke.
Cell size and cost
Cell Size is the size of the gas's cubes. Half the size is eight times the cells and about twice the substeps: finer detail, much slower. Only cubes near smoke are worked, so a plume costs what it fills, not the size of the scene. Work at a coarse size, with Draft for a first look, and refine for the final.
Sharpness is the other half of detail. Sharp, the default, keeps thin sheets of smoke and flame crisp and the small swirls alive, at about twice Fast's time; Extreme is crisper still, and slower again. Draft bakes always use Fast. A finer cell shows new detail only where something makes it: keep Turbulence's Detail up, and some Swirl.
Obstacles
Under Obstacles, name a collection in Colliders: the gas goes round every object in it with a surface, and is pushed by them as they move. Closed meshes work best: inside them is solid. Ground gives the gas a floor at Ground Height.
These are the gas's own colliders, apart from the flow's Collide, which is for particles.
The volume and its look
The bake adds a volume object, “name Smoke”, with the VecFlow Smoke material. Its Render Look changes at once, with no new bake:
- Smoke Colour and Density: the smoke's colour and how thick it is.
- Opacity Exponent: the smoke's thickness is raised to this power. Above 1, thin edges fade and the thick folds stay, so the smoke looks crisper. 1 is even.
- Fuel Shadow: how much the unburnt fuel shades too, so the fuel-rich parts of a fire look dark.
The volume holds density, temperature, fuel, flame, crease and
velocity grids, for a material of your own. Cycles blurs it along its
velocity.
Solver Quality
Step Size (the most cells the fastest gas may go in one substep), Most Substeps, Most Rounds and Accuracy trade speed for steadiness. The defaults suit most shots; lower Step Size for fast, sharp gas. See Quality and seeds.
What goes wrong
- The smoke is blocky. The cell size is too big for the shot: halve it, or add Swirl.
- It's soft and smeared. Sharpness on Fast: set it to Sharp or Extreme.
- A finer cell adds nothing. The detail isn't being made: raise Turbulence (with Detail at 1) or Swirl.
- The edge of the plume is smooth. Raise Swirl's Front, and Swirl or Turbulence In Heat.
- It doesn't rise. Heat too low on the source, Buoyancy at 0, or Cooling too high.
- It's gone too soon. Fade too high, or Density too low in the Render Look.
- It passes through a wall. The wall isn't in the Obstacles collection, or it's an open plane thinner than a cell: make it a closed box.
- Nothing renders. The volume is “name Smoke”, not the particles object, which the starters hide from renders.
Related
Whitewater
Spray, foam and bubbles where a liquid crashes and crests: making them, sending each to its own event, and showing them as studios render them.
Fire and explosions
Flames off a burner or a mesh, and fireballs that swell and rise: fuel and how it burns, the flame's look, and the Fire and Explosion starters taken apart.