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There are several MPM Configure examples available through the tab menu. These are similar to shelf tools that put down networks of nodes for learning purposes. The MPM Configure Spinning Tire example illustrates the use of the “viscous” Mud material type. It puts down a simple network of nodes to simulate a tire spinning around in a mixture of water and mud. It is a good demonstration of two different source materials interacting together in a simulation. It also shows the stickiness you can get between materials and colliders, which can be tricky to achieve with other solvers in Houdini.
Important nodes ¶
water and mud
These are the 
MPM Sources that uses the Water and Mud material preset that are interacting in the scene. They are colored blue (color_water) and brown (color_mud), respectively, to better visualize the difference between them in the viewport.
timescale_slowmo
The expression in the Frame parameter slows down the animation to be the same rate as the 
MPM Solver. It takes the floating point frame and multiplies it by the Time Scale parameter on the MPM Solver, which is set at 1/10th of the speed (0.1). This gives the proper slow motion effect to the wheel.
wheel
This is the 
MPM Collider, which uses the Animated (Rigid) collider type. It allows for very precise interpolation of the collider by the solver, as it keeps only one VDB and interpolates between the transforms resulting in very accurate collisions and stickiness on the collider. The Voxel Size is also decreased in order to override the simulation resolution defined by the Particle Separation parameter on the 
MPM Container. This will allow you to have even more modeling details. If you zoom into the tire, you will be able to see the treads. Additionally, Friction is turned up to 10 and Sticky is turned up to 1, resulting in the mud sticking to and being picked up by the tire.
mpmcontainer
This setup uses a default MPM Container, which defines the resolution of the simulation using the Particle Separation parameter, and sets limits on whether particles should be deleted or bounce on contact with the boundary. The Boundaries are set to Closed, and the Friction and Sticky are turned up to 10. This makes sure the mud sticks to the walls as it’s thrown against it.
noise_viscosity
An 
Attribute Wrangle that adds some spatial variation to the viscosity of the mud.
mpmsolver
This setup uses a default MPM Solver, which does the work of solving the scene. The Material Condition is decreased slightly to fix some instability where the material requires more substeps than what is provided by default.
Learning from this example ¶
| To... | Do this |
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Visualize the noise pattern on the varied viscosity of the mud |
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Fix instability in the behavior of the material |
In some cases, there could be some instability in the simulation where the material requires more substeps than what you're providing, which could result in points that merge or collapse onto each other. The quickest way to solve this is to slightly decrease the Material Condition on the |
