Explosia FX: Simulation

This page contains the reference manual for the Simulation tab in the Explosia FX object.

Interface

The tab interface looks like this:

Parameters

Sim Scale

Changing this value alters the resolution of the simulation so that reducing this value results in a larger and faster simulation (within the confines of the solver) but with lower resolution. A higher value causes a smaller simulation but a more detailed one.

Sim Speed

This is the speed with which the simulation progresses, so the higher this value, the faster it runs. If you set it too high though, you may see a jerky animation as the appearance seems to jump from one frame to the next. Compare these two videos with the basic setup video on the Explosia main page. In the first the Sim Speed is set to 250%, in the second it is 500%:

 

Buoyancy Settings

Gravity

Gravity is inherent in Explosia so a Gravity modifier is not needed. This is the gravity strength on the positive Y axis.

Note that increasing the gravity will make the smoke etc. rise faster. This may seem counter-intuitive but it is how the original Explosia FX worked and we have retained this method for compatibility. You can set negative values to make the smoke flow downwards (negative Y axis).

Smoke Buoyancy

Smoke rises due to the heat of burning. This value can reduce or increase the smoke buoyancy, so it will rise slower or faster.

Heat Buoyancy, Fuel Buoyancy

As for the smoke buoyancy but for heat and fuel respectively.

Vorticity

Increasing this value will increase the amount of curl within the simulation. For example, in this two videos, Vorticity is set to 35%:

 

Vorticity Radius

Increasing this value will increase the size of the curly areas in the simulation.

Max Pressure Iter

For the most part you can leave this setting alone. It is used internally as the number of sub-frames used when calculating pressure changes each frame. If you have a very rapidly changing simulation, such as an explosion, you may need to increase this value.

Diffusion Settings

Diffusion in Explosia can be thought of as the rate of movement of a parameter, such as smoke or heat, throughout the solver. The higher these values are, the greater the even distribution of the parameter and so the less contrast you will see in the result. Consider these videos in which the display is set to temperature. In the first, heat diffusion is zero (the default setting); in the second, it is set to 250:

Note the much smoother appearance and reduction in contrast, consequent on a more even distribution of heat, when the heat diffusion value is raised.

Smoke Diffusion, Heat Diffusion, Fuel Diffusion

The diffusion values for smoke, heat and fuel respectively.

Viscosity

This setting is labelled viscosity but it is actually another diffusion setting, this time for velocity. As you might expect, increasing this value smooths out movement and gives the impression of a thicker, more viscous medium. In this video, it has been set to 300:

 

Dissipation Settings

These values control the dissipation of a parameter, the effect being to reduce it over time. It can best be explained in this video, where smoke dissipation has been set to 0.9:

You can see how quickly, even with a small dissipation value, the smoke fades away with time.

Smoke Dissipation, Heat Dissipation, Fuel Dissipation

The dissipation values for smoke, heat and fuel respectively.

Fuel Ambient

This is the minimum fuel value in a given voxel which is subject to dissipation. Smaller values will not be affected by dissipation.

Velocity Damp

This setting acts to reduce velocity over time. You can see this in these videos, which have display set to speed. In the first, velocity damp is zero, in the second it is set to 3:

 

Burning Settings

Burning Model

This is a drop-down menu containing the various burning models used by Explosia FX. The model defines how heat, smoke and pressure is generated and dissipated. The options are:

Simple

This is the simplest model with linear dissipation of fuel and proportional generation of smoke (if enabled) and heat.

Chemical

This model simulates actual chemical processes inside the burning fuel. This is the default setting.

Custom

With this option you can design your own burning model (see below).

Burn Rate Var

This controls the rate of burning of fuel in each voxel.

Ignition Heat

This is the threshold value in a voxel above which burning occurs.

Max Heat

The maximum heat value which can be achieved at a given voxel.

UVW Update Frames

This is the duration over which the UVW fields recalculate UVW coordinates.

Custom Burning

If you select 'Custom' from the 'Burning Model' drop-down menu, the interface changes slightly. You can then use this to design your own burning model. Experimentation is the key here.

From here you can set the required parameters for the model. Note that each parameter has a small black arrow next to it. Click that to reveal a spline giving further control over the parameter:

Burning

How much fuel consumed in the voxel during burning.

Heat Production

How much heat generated in the voxel during burning.

Smoke from Heat, Smoke from Fuel

How much smoke is generated in the voxel during burning. Both values are multiplied together before the actual smoke generation,

Gas Expansion

How much pressure is generated in the voxel during burning.