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The Vellum fluid solver is a particle-based fluid simulation framework. Vellum fluids are fully integrated into Houdini’s Vellum dynamics system: fluid particles can interact with grains, cloth, and soft bodies. Hair-fluid interaction is currently not supported. This allows for multi-material simulations, e.g. sand being washed away by water, or cloth reacting on rain drops.
In contrast to FLIP fluids, Vellum fluids are not limited through grid and domains. The particles can move freely and they're connected through constraints.
Physical attributes like Density, Viscosity, and Surface Tension can be used to simulate different types of materials such as water, oil, or honey. Surface tension controls a fluid’s tendency to contract and create drops or tendrils.
For a complete Vellum fluid project, visit the SIDEFX SNACKS V2 Mojito Magic tutorial. In 21 hours and 14 lessons, Tim Van Helsdingen talks you through Houdini 19's new Vellum fluid solver and its endless possibilities. Finally you’ll learn how to render your scene in Karma and do final the final compositing in Blackmagic’s Fusion.
This tutorial requires Houdini 19 and comes packed with downloadable project files (10.3 GB).
Vellum fluids can be set up through shelf tools, but require a few manual settings:
Create some geometry on
objlevel, for example a Sphere.
Open the Vellum shelf and choose Vellum Grains. Houdini creates the entire network structure for you.
Houdini dives into the newly created DOP Network.
Select the Vellum Solver node.
Set Substeps to
5, the recommended value for Vellum fluids.
Set Constraint Iterations to
10and Smoothing Iterations to
Now, adjust source object and particles:
objlevel and dive into the Sphere node.
Click the sphere and set Center.Y to
Change Uniform Scale to
Click the Vellum Configure Grain node to open its parameters.
Set Type to Fluid.
Decrease Particle Size to create a sufficient amount of particles, e.g.
If the particles don’t match the source object, consider to increase Packing Density.
The steps under Fluid Setup represent a complete Vellum fluid network. For interaction with objects do the following:
objlevel, create a collision object, for example a Cube.
Dive into the cube, and Center it below the sphere. You can also rotate and scale the object.
objlevel, and select the cube/box.
From the Collisions shelf, choose Static Object.
This action creates a a Static Solver inside the DOP Network.
Simulate the scene for a first impression.
Fine-tuning the fluid
It is very likely that you will see lots of tiny droplets instead of a watery fluid. You can this by adjusting the fluid’s parameters.
Dive into the geometry node again and select the Vellum Configure Grain node.
Under Physical Attributes go to the Mass dropdown menu. Choose Calculate Uniform.
For Viscosity add
3- the physical value of water at room temperature.
Surface Tension creates a more natural look. Use values between
Friction and Dynamic Friction are important for fluid-object interaction. With values of
2and more, the particles will stick on the collision object’s surface, for with viscous fluids like honey or oil.
The particles might still not behave as expected. To get a more fluid-like look, return to the DOP Network and dive into it:
Add a POP Drag node and wire it into the Vellum Solver’s 2nd input.
Drag has an effect similar to air resistance and it’s a very effective way to control the fluid’s flow. Higher values slow down the fluid, and keep the particles together. This way you’ll reduce splashes and droplets. Below you can see the fluid without and with a POP Drag force applied:
If you simulate viscous fluids with Viscosity values of 50 and more, increase the Vellum Solver’s Substeps to values between