SPH - Particles: Gas

Owned by thomas (Unlicensed)
Last updated: 28 Oct, 2015
3 min read

RealFlow's gaseous particles are based on "Smoothed Particle Hydrodynamics" (SPH). With this fluid type is not possible to achieve the typical behaviour of swirling smoke clouds or fire. The fluids temperature plays an important role: high values increase the motion of the particles and lead to stronger expansion; cool gases fill a smaller volume.

 

 

Type

This menu allows you to define the fluid’s behaviour:

  • → “Gas” particles are used to simulate fluids like air.
  • → “Liquid” is RealFlow’s standard setting and provides parameters for all watery or high-viscous substances.
  • → “Dumb” particles are often used for spray or foam. These particles cannot react with each other and will not be affected by other emitters.
  • → “Elastics” can be used for effects like jelly-like fluids or expanding an contracting substances.
  • → “Script” lets you write your own fluid behaviour with Python scripting.
Resolution

With this setting you can change the amount of particles, “Resolution” mainly depends on scene scale and emitter scale, but it also affects the fluid’s mass and therefore depends on “Density”, too.

  • With “Resolution” set to 1.0, a volume of 1 m x 1 m x 1 m filled with 1,000 particles has a mass of exactly 1,000 kg. You can monitor the relationship between mass and density under Statistics > Particle mass.
  • There is also a relation between “Resolution” and a particle’s radius: radius = 1.0 / (1000.0 · Resolution1/3)
Density

This parameter is defined as mass per volume unit and is different for each substance. “Density” is only relevant for interacting fluids with different properties, e.g. water and oil, or water and foam. With single emitters, changes have no effect. The unit is kilograms per cubic metre. Avoid very small values close to 0, because they can lead to instabilities.

Int Pressure

“Int Pressure” simulates the forces between nearby particles and pushes them apart - this makes the fluid fill a greater volume. If “Int Pressure” and “Ext Pressure” are set to 0.0 the particles lose their fluid behaviour and they behave like “Dumb” particles.

Ext Pressure

“Ext Pressure tries to limit a fluid’s expansion tendency and can be compared to atmospheric pressure. With “Gas” particles you normally define very low “Int Pressure” and rather high “Ext Pressure” values to prevent the particles from flying away.

Viscosity

Substances with very high viscosity are honey, tar or syrup, for example. Fluids with low viscosity are alcohol, many solvents or liquid gases. The default value of 3.0 describes the viscosity of water. High settings (40-50 and higher) can lead to misbehaving particles.

Temperature

Here, the fluid's temperature is adjusted. Very hot gases have a very strong tendency to expand and they raise quickly. In RealFlow, temperature is measured in Kelvin. The Kelvin scale is based on absolute zero point at roughly -273 °C, which represents 0 K; a temperature of 0 °C is 274 K.

Ext temperature

This is the temperature of the fluid's surrounding environment. If this value is lower than "Temperature", the particles will cool down. A warmer environment will heat the fluid and accelerate the particles.

Heat capacity

With very low settings (0.2 - 1.5) the gas can be compressed effectively and with “Int | Ext Pressure” set to 1.0 you will see a natural diffusion. “Resolution” has a strong impact on this parameter.

Heat conductivity

Good thermal conductors are generally materials with many free electrons, for example metals. A poor conductor shows low conductivity and is called an insulator.

Compute Vorticity

Vorticity hardly contributes to a fluid simulation, but can be very important for the render process. Therefore it is possible to enable and disable this property on demand.

Max Particles

Please enter a value to stop emission at a certain amount of particles. "Max particles" is based on the total amount of emitted particles. Let's say you have entered "50,000". If there are currently 1,000 particles in your scene, but you have already deleted 49,000 particles RealFlow will stop the emission. Here you can read how to use this parameter to stop particle emission: → "Quick Start Workflows - Stopping Particle Emission".