Houdini 19.0 Pyro

Simulating a burning tire

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This setup is the second of a series of three turorials for SOP-based pyro simulations. Please go through this lesson first, before you continue here.

  1. Creating smoke

  2. Simulating a burning tire

  3. Pyro Burst Source: creating a fireball

Note

The scene is based on the network from the SOP smoke turorial. You can load the network from a previously saved version or recreate the nodes manually.

Adding the burn attribute

Select the Pyro Source SOP node to see its parameters. Right now there are a density and a temperature attribute. Click the + button to add a third attribute:

  • From the Attribute drop-down menu choose Burn.

You have to rasterize this attribute as well:

  1. Select the Volume Rasterize Attributes SOP node.

  2. Complete the Attributes list by adding burn.

Open at the Pyro Solver SOP node’s Sourcing tab. There you can see a Volume 3 entry with the burn attribute again as a Source Volume. The solver uses the burn volume to create a flame field. All this is done by the solver automatically.

Visualizing the flames

The Pyro Solver has everything prepared you need to visualize flames and you can start the simulation. Right now, the result is basically nothing more than smoke with different colours. The simulation lacks the typical behavior of fire, but you can already see a transition from flames to smoke.

You can adjust the flames' appearance in the solver’s Look tab. There, the Fire option is turned on by default:

  1. Intensity Scale defines the flames' brightness. Set this value to 40.

  2. The Fire Intensity Ramp controls how the values are mapped to represent intensity. For example, select the first point, expand the ramp and set Value to 0.5. You will notice that the fire in areas of flame-smoke transition become brighter. If you set the second point’s Value to 0.3 the newly created flames look cooler. This way you can define hotter and cooler zones.

Another, very important parameter can be found under Fields ▸ Flame ▸ Flame Lifespan. With smaller values, the transition from fire to smoke happens faster. In conjunction with Look ▸ Smoke ▸ Smoke Color this is a very effective method to simulate different types of fire from “clean” to sooty oil or burning rubber.

This video shows a fire with almost black smoke and Flame Lifespan set to 1.3.

Making fire

To create a more flame-like look, it’s necessary to get rid of the smoke plumes and billowing structures. An obvious method is to tweak the parameters in the solver’s Shape tab:

  1. Increase Disturbance to 30 to break up the smoke plumes.

  2. Turn on Shredding. This noise type acts on the flame field and adds more high-frquency turbulence without changing the fluid’s general motion. Set the value to 0.6.

With the new settings the simulation starts to look like fire. There’s an initial ignition and small-scale noise, but the flames don’t vanish fast enough:

  • In the solver, set Fields ▸ Flame Lifespan to 0.6.

Another crucial parameter is the solver’s Fields ▸ Dissipation. It controls, how fast the smoke vanishes. It takes several test to find a good value, and small changes my have a big impact. Therefore you should change the parameter in small steps:

  • For this simulation, set Dissipation to 0.085.

Below you can see the result of the various changes.

Enhancements

The network already contains a Attribute Noise SOP node to apply noise on the temperature attribute. Another idea is to displace the seed points' positions directly to add some flicker to the fire. Add one more Attribute Noise SOP and place it after the first noise node.The second noise re-introduces a moderate billowing effect to the smoke, and adds more randomness to the flames.

  1. Under Attribute Names remove the Cd entry and enter P.

  2. Set Noise Value ▸ Amplitude to 0.25. The change should be moderate, because otherwise the simulation changes too much.

  3. Turn on Animate Noise.

Finally, set all Particle Separation and Voxel Size parameters to 0.03 to get more seed particles and a higher resolution.

Shading and rendering

For detailed information on how to apply the Pyro Bake Volume SOP shader, please visit the Shading and rendering page.

Pyro

Sparse Pyro

Legacy Pyro