nxFluids - SPH solver parameters

Overview Video

The SPH (Smoothed-Particle Hydrodynamics) solver provides small to large-scale fluid dynamics.

nxFluids with the Solver setting of SPH.

Fluid Type

Set at Liquid, by default.

The alternative is Granular.

Fluid Type set to Liquid.

Solver settings

Smoothing Radius

Set at 100%, by default, this controls the blending with neighbouring particles, to create a coherent simulation.

Lowering this setting will involve fewer particles in the smoothing, giving a more choppy look to the fluid.

Values over 100% will add more volume to the liquid, at the cost of detail.

Animation showing a reduction in the Smoothing Radius value, from 100% to 50%, then rising to 150%, to demonstrate the effect on liquid volume.

Substeps

The number of steps, per frame, being solved.

You can increase this, from the default setting of 3, in order to get a greater accuracy in fluid velocity calculations.

This may be necessary for scenes with very fast moving particles or animated collider objects.

Granular scenes also require very accurate velocity solves, and may require the maximum substep value of 30.

This animation demonstrates increasing the Substeps setting, from 1 to 5, to increase solving accuracy and therefore stabilize the simulation.

Min Density

Set at 3, by default, this is the number of iterations, per step, needed to work out the fluid density.

Max Density

Set at 7, by default.

Again, this is the number of iterations, per step, being used to work out the fluid density.

The more iterations, the more accurate, but the longer the calculation will take, with an effect on the simulation time.

Max Compression

The amount that the fluid is allowed to compress by, without having to use a further iteration to make it more accurate.

At higher percentages the solver will use fewer iterations.

At lower percentages the solver will use up towards the Max Density amount of iterations, again, with an effect on the simulation time.

Velocity Correction

Checking this box causes nxFluids to correct any divergence when the fluid starts to compress or expand.

Velocity Correction does an extra solve to change the velocity, which enables the fluid to flow around more naturally and not be compressed or smoothed out.

Note: Turning this on will slow the simulation, so it is off by default.

Check Density

Fluids are normally incompressible so have a constant density.

If this parameter is turned on, the density will be checked to ensure consistency in the particle stream, avoiding excess pressure levels, which can occur with emission.

The value used in the density check is found in the Max Density setting, set at 110%, by default.

Any particles found to have a fluid density above this setting will be killed before they are born.

This setting is off by default. When it is turned off, no check will be applied.

Check Density unchecked on the left and checked on the right.

Max Density

The value to use when Check Density is on.

Animation demonstrating the effect of increasing and decreasing the Max Density value.

Iterations

The number of times the maximum density is calculated.

By default, this is set to 3.

This parameter helps to check the density values and can be increased to give greater accuracy, but this will have impact on the scene simulation speeds.

Liquid settings

Viscosity

Controls the levels of viscosity in the fluid.

Note: Increasing the Substeps value will further encourage viscosity.

This animation shows an increase in the Viscosity value, from 3 to 100.

Viscosity Iter

The number of iterations, per step, used to calculate the viscosity.

Raise this value to get a thicker viscosity.

Vorticity (Small)

This setting controls the small-scale swirling levels in the fluid.

In this animation there is an increase in the Vorticity (Small) setting, from 0 (zero) to 100, adding small-scale vorticities. Note: This is most visible with a low Viscosity value.

Vorticity (Large)

This setting controls the large-scale swirling to help exaggerate the natural levels found in the fluid solve.

To increase the vorticity levels further, this works well in conjunction with the settings in the nxVorticity modifier.

Surface Tension

Surface tension occurs when particles attempt to inhabit the smallest possible surface area.

Increasing this value will encourage this tension even more, leading to beading.

At lower values the particles will be allowed to spread out, encouraging more flow.

Animation demonstrating that increasing the Surface Tension setting, over time, will create droplets from scattered particles.

Internal Pressure

Set at 100%, by default.

This separates individual particles out, as a force, which prevents them collapsing in on each other, maintaining the volume of the fluid body.

This image shows a comparison of the Internal Pressure settings, with an identical particle amount. On the left it is at 1% and on the right it is raised to 100%.

External Pressure

Ext Pressure is set at 30%, by default.

It acts as a binding agent, a cohesive force, which keeps the particles together as a fluid shape.

Granular settings

Friction

Sets the level of friction between particles in the simulation.

As with the viscosity, increasing the Substeps value will further encourage friction.

In this animation, the Friction setting is 10% on the left and 100% on the right.

Friction Iterations

The number of iterations, per step, used to calculate the friction.

Raise this value to get even stronger friction.

Animation to show a Friction Iteration comparison, with a value of 1% on the left and 20% on the right.

Stability

This is a relative speed setting.

It checks whether particles are moving, relative to each other, faster than this value.

If so, then they are removed from the effects of the friction and are therefore able to break away from each other.

If they are moving (relative to each other) slower than this value then the full effect of the friction force is applied to them, keeping them together.

This animation has a Stability value of 5 on the left and 60 on the right.

Cohesion

This setting determines how far apart from each other particles are allowed to get before they are bound back together.

Note: There is a spring effect to this setting, which will show itself at higher settings, pulling particles towards each other.

Animation demonstrating a Cohesion comparison, with a value of 1% on the left and 20% on the right.