Internal pressure: This value affects the forces generated between nearby particles. This is a sensitive value and should be handled carefully. The volume filled by a fluid is affected by the internal pressure. More internal pressure will make the particles fill a greater volume. Set to 0.0 to disable the fluid and gas behavior.

External pressure: This is an overall force that affects all of the particles in the fluid. The external pressure will tend to keep the fluid compact avoiding the tendency to expand.




Hint: Using fluid or gas without Int or Ext Pressure will cause those particles to behave as dumb particles.
Simulations where particles have low pressures produce low forces which can be calculated with lower substeps.


Viscosity: The viscosity is an internal drag force. Fluids like water have a low viscosity (1-5). Use a higher value for oil, honey, etc.

Surface Tension: This increases the cohesion of the liquid skin. This property is useful for obtaining mercury-like effects and can also be used to simulate high viscous effects.

Interpolation: Interpolation in RF4 is done when we change the resolution of a fluid. In RF4 the fluid is represented using particles and the interpolation process will just generate more particles to match the new fluid resolution. Please note that this is not just interpolating particles positions because the new generated particles satisfy the fluid equations. This is why this process is complicated and might take some time to compute.
Something you have to keep in mind, no matter the interpolation method is that if the fluid is colliding with geometry or there are particles near an object where they will collide with, you might have interpolated particles created beyond the collision geometry. This effect is more visible with the "Global" method. You could use the "selection particle" tool to remove by hand all those particles or create a script for removing those particles afterwards.
In next versions of RF we will provide a method for avoiding the creation of particles on those areas but for the moment is a constraint that has to be contemplated.



Hint: It would be possible to make a script to remove the particles outside an object.

None: Settin'g interpolation to None disables this feature. "None" won't interpolate particles.
Local: Basically the "Local" method should be used when users want to keep the original fluid's shape as much as possible. By using this method RF4 will try to create particles while keeping the original fluid shape. "Local" means that interpolated particles will be generated near the existent particles. In the next picture you can see the original fluid (left) and the interpolated one (right). You can see how the interpolated fluid matches the original shape very much.


Global: The "Global" method should be used when users want a lot of new interpolated particles and there is not a worry about fluid shape. For instance, this method is recommended when there is a filled tank where the fluid is settled and the user just wants more particles in it. "Global" means that interpolated particles might be generated in fluid areas relatively far from existent particles. In the next pictures we show the original scene and the result of the global interpolation method.





Hint: Care should be taken when RF4 does not interpolate after a change in the resolution. The fluid might become instable and explosions are likely to happen. That is the reason why the default interpolation method is set to "Local", which is the method that will fulfill user's needs in most cases.




Hint: Interpolation is very useful to increase particle count in initial states, however another use would be the ability to increase the number of particles in a simulation as a post-process.
Once you are happy with your simulation at low resolution you just increase the number particles for the whole simulation. To tackle this task we provide a system script (Change Resolution) that will do this operation. Depending on the particles configuration the interpolation might take some time, even it eventually could take more time that the simulation itself, but with little effort the interpolation process can be done in parallel in different machines whereas the simulation process is always bounded to just one machine.
Please note that depending on the sequence, interpolation can cause flickering.



Compute vorticity: Vorticity of particles is not computed by default. You can enable vorticity if you need it. Vorticity can help you to create some effects at the time of shading.


Hint: It seems that most of the time vorticity is not needed so it isn't worth to spend time computing it, if you need it you can activate it from this control.
In the case you want to have vorticity once the simulation has been calculated you can run the system script "Compute vorticity" that is in your system scripts tool bar.



Max particles: Limits the particle emission for the current emitter. Maximum number of particles emitted by the current emitter.


Hint: To find out the number of particles, look at the Num particles at the statistics subpanel.