Houdini 19.0 Vellum

Fluid tips and troubleshooting

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How to

Vellum fluids seamlessly integrate into the Vellum solver. You can simulate interactions between Vellum fluids and Vellum grains or Vellum cloth, for example. Vellum hair is currently not supported. For best results it can be necessary to change specific attributes and parameters.

To...Do this

Optimize solver settings

Substeps play an important role for the fluid’s quality and stability. You can find them in the Vellum Solver DOP node’s Common tab under Iterations.

We recommend at least 5 Substeps, while more substeps give you better results. For high Viscosity and/or Surface Tension value we recommend values between 10 and 20. Note that higher substeps will also increase cooking time.

To optimize pure fluid simulations you can decrease Constraint Iterations to values around 20, and Smoothing Iterations can usually be set to 0.

Achieve continuous fluid emission

  1. Create an object to emit from. You can source fluids and grain in two different ways:

  2. Select the object and click Vellum Grains tool on the Vellum tab.

  3. Dive into the DOP Network and click the Vellum Source DOP node. In the node’s parameters pane tab, look for Emission Type.

    Alternatively, create your own node network. Dive into the network, and go to the Vellum Source DOP node. There, look for Emission Type.

  4. The standard selection is Once Only. For a continuous stream of particles, select Each Frame or Each Substep. With Each substep, emission relies on the number of Substeps in the Vellum solver node. For better results, we recommend around 10 for Vellum fluids, and 5 substeps for Vellum grains. Higher substeps create a more natural emission and more particles. Note that higher substeps will also increase simulation times.

Fix disappearing particles in fluid-grains simulations

When you simulate grains and fluids, and particles disappear, go the Vellum Solver DOP node and turn off Advanced ▸ OpenCL Neighbor Search.

Improve crown splashes

Crown splashes occur, for example, when a water drop is hitting a calm fluid surface. In the video above the Vellum Solver DOP node’s Time Scale is 0.5 to enhance visibility.

If the splash turns out too small and shallow, consider to go to:

  1. Vellum Configure Grains SOP. Increase Surface Tension (around 250) and Viscosity (try values between 0.1 and 1)

  2. Vellum Solver’s Advanced ▸ Integration, and set it to First Order

  3. Vellum Solver’s Common ▸ Iterations, and increase Substeps.

Work with friction

The impact of Friction and Dynamic Friction on Vellum fluids is limited, but can be useful for fluids, interacting with Vellum cloth. You can also increase Friction values to slow down fluids and prevent them from spreading out too far.

Improve fluid-grains interaction

Low Repulsion Weight values (< 0.1) will help to improve interaction between fluids and Vellum grains. With high Repulsion Weight values, Vellum grains will hardly mix with fluid particles.

To use Minimal Solver

The Minimal Solver is used by the Vellum Brush SOP and isn’t recommended for end users.

Simulate multi-phase fluids

Multi-phase simulations are set up using the Vellum Configure Grains SOP node’s Phase attribute, which you can use to simulate mixing fluids like water and oil. Fluids with

  • equal Phase values, but different Viscosity and/or Surface Tension settings often stick together in touching areas.

  • different Phase values, but no Viscosity difference will interact, but behave like two fluids with no friction between them

  • different Phase values, and Viscosity and/or Surface Tension will separate more and hardly stick together.

For a description of how to set up a scene with two fluids and the Phase attribute, please go to the Vellum fluid phases page.

Fix problems with low density fluids

In Vellum fluids, simulations tend to become unstable when Density is very low (< 10). You might also see exploding particles in simulations, where fluids have a higher variance in Density values, for example 100 and 20000. Even high Substeps might not able to fix this issue. We recommend equalizing the Density values until the simulation becomes stable.

Decrease simulation time with high-viscosity fluids

Simulations can become slow with very high Viscosity values. To speed things up, you can add a vector attribute called dvisc to the points before the simulation. This attribute will warm start the viscosity solver.

Avoid terracing

Sometimes you can see layered or terraced fluid particles. To avoid this, go to the Vellum Configure Grains SOP node and turn on Dither Surface.

Get splashier Vellum fluids

By default, acceleration is limited to avoid exploding particles. For splashier fluids, this default might be too low. To change this on the Vellum Solver DOP node, go to Advanced ▸ Motion and increase Max Acceleration. This attribute is, on the other hand, also suited to decelerate splashes.

If the amount of splashes is not sufficient, you can also increase the Vellum Configure Grains SOP node Particle Scale to create more particles.

Make particles match the sourcing object

Fluid particles can be “over-packed” with the Vellum Configure Grain SOP’s Packing Density. Higher values will displace particle positions to create a better representation of the sourcing object. Normally, values between 1.5and 2.0 are enough.

With grains, Packing Density should not be greater than 1.

Example files

Vellum Fluid Crown Splash

This Vellum fluids example demonstrates how to create a typical crown splash from a drop of water falling into a pool.

Vellum Fluid Phase Attribute

This example demonstrates how to create two interacting Vellum fluids with different viscosity and surface tension settings through the Phase attribute.