Houdini 19.0 Nodes Dynamics nodes

Labs Gas Flamefront geometry node

A simple flamefront combustion model.

This microsolver can be used to augment a pyro network with a flamefront based combustion model. Instead of relying on temperature diffusion to ignite fuel, this will use an explicit flamefront. This allows the speed of the flamefront to be tuned with greater control.

Several additional solver fields are created and used by this microsolver.

fuel

The amount of fuel in a voxel. This should be a mass-fraction, so be a 0..1 range. 1 implies the voxel has 100% fuel and no other constituent. Usually this is sourced by the pyrosolver using maximum.

waste

The result of burning fuel and ambient air; this is the waste products. Tracking this allows for oxygen limited burning. Like fuel, this should be a 0..1 range.

flamefront

A signed distance field specifying the flame front. Anything inside this field is considered burning so will combust if sufficient air and fuel is present.

burn

This is an output field that records how much combustion occurred in a voxel.

ignite

An input field used to trigger combustion wherever it is greater than 0.5 and fuel exists. This is reset after applying, so can be sourced in the pyrosolver using maximum.

It is important to setup the pyrosolver to process these extra fields. Rather than performing advection internally to this solver, it is expected the fields are added to the extra advection fields of the pyrosolver.

The PyroSolver’s Advanced::Resizing::Extra Fields should have fuel waste flamefront added to it. The Advanced::Resizing::Reference Fields should have fuel added to it. The Advanced::Advection::Extra Fields should have fuel waste flamefront added.

Parameters

Properties

Voxel Size

Should be linked to the division size of the pyrosolver. Used to slowly combust fuel as it crosses the flamefront boundary in a sub-voxel step.

Flame Front

Ignition Temperature

If a voxel’s temperature exceeds this point it will add itself to the flame front and ignite. Note too low of an ignition temperature will cause the fire to spread primarily from heat diffusion, making the flame speed irrelevant.

Flame Speed

How far the flame front moves in one second. If units are meters, a value of 0.35 will correspond to natural gas. 0.55 would be methan, and 7.7 hydrogen.

Min Speed Ratio

The speed of flame is dependent on how far off the stoichiometric conditions the fuel/air ratio is. The fuel is treated as an ideal substance with a 1:1 ideal ratio, with full speed at that ratio and falling to this percentage of that speed at the extremes. A min speed ratio of 0.2 means the flame will travel at 20% of its ideal speed when in the least advantageous conditions.

Bandwidth

The flamefront has to be re-initialized to ensure the signed distance field can be updated in a single step. Sufficient bandwidth is needed or the entire object will burst into flame in a single step.

(flamespeed / (FPS * substeps)) < bandwidth * voxelsize

Burning

Expansion on Combust

When fuel burns it directly adds to the divergence field by this amount. This covers instanteous heating and molar changes due to chemical reactions.

Temp Gain on Combust

The amount to add to the temperure field when a unit of fuel burns. Since it is oxygen limited, this often is greater than one as an entire unit of fuel can’t burn as it lacks air.

Diffusion Rate

Without fresh oxygen the fuel will burn out and be replaced by waste products, quenching any further reaction. Diffusion allows the fuel and waste to transport and mix into air, creating a diffusion flame.

Visualize

Simple visualization options are included for the main fields. They can also be exported from the pyrosolver and processed farther.

Use Timestep

Determines if the current solver timestep will be used to apply this node.

If set, the current timestep size will be multiplied by the scale and used for the time increment for this operation. Otherwise, the time scale will specify an absolute fictitious time to integrate by.

By disabling the link between the actual real time and the microsolver time, you can perform operations in a separate, fictitious, time.

Time Scale

The timestep used for this microsolver will be scaled by this amount. This allows one to achieve non-realistic effects, such as parts of the simulation operating at different speeds than other parts.

Similarly, it is useful if a solver needs to be evaluated independently of the main timestep.

Dynamics nodes