Hi everyone,

I'm working on a FLIP simulation and I'm running into a collision issue that I can't quite figure out.

I have a character mesh (a puma) that I want to fill with liquid. The idea is that the liquid should stay inside the mesh and react to the motion of the character.

The FLIP simulation itself works fine. If I pour liquid on top of the mesh, the collider works correctly and the liquid interacts with the surface as expected.

The problem happens when the liquid starts inside the mesh. It looks like the collision only works on the outer surface, as if the mesh is interpreted as a solid object instead of a container. So the inner walls don't seem to block the liquid.

My setup is roughly:

FBX Character Import

Bone Deform

Convert to VDB for the collider

Static Object in DOPs (Use Deforming Geometry enabled)

FLIP solver

The collision guide looks correct and follows the mesh shape.

I already tried a few things:

converting the mesh to VDB SDF

using VDB Reshape

closing the mesh with PolyFill after clipping

testing Reverse in case normals were wrong

But the behaviour is still the same.

I've also attached the .hip file in case someone wants to take a look at the setup directly.

If anyone has an idea of what might be causing this, I'd really appreciate the help.

Thanks!

I haven't got Houdini access at the moment, so can't check the file right now or tell you specific instructions, but the key thing with VDBs is, they have their own inner and outer sign sense, that may be independent of the mesh you used to generate it.

"Inside" is negative numbers and "Outside" is positive numbers... so to have an enclosed object be a container rather than a collider, you need to flip the signs of the entire volume. Outside needs to be negative and inside positive - ie. just multiply the whole thing by -1.