I have this geo that I modeled in C4D then brought into Houdini as an alembic but I'm having issues with the fluids sliding around all over the surface. I think it has to do with Houdini not knowing where the boundaries of the geo are or something like that but I'm at a total loss for how to fix it.

I tried using a Polycap to close off the geo but that didn't seem to make any difference.

When I use this same particle setup in the same scene with a houdini primitive the fluid reacts just like I want: it falls, stays pretty much in place on the surface but starts to slowly spread outwards like honey. But when I use my c4d geo the particles hit and bounce (no clue why it bounces) then they immediately start sliding to the edge of the geo.

Is there anyway to fix this?

Particle simulations can collide against volumes or geometry.
FLIP colliders need to come in as a proper collision volume.

Your edge extruded logo is just a surface with walls and no volume. When you try to construct a volume from this, you will get a solid filled in logo, not the channel carved in to a block that FLIP needs. Your logo is concave and not convex.

I took a box and cookied it with your logo to create a carved channel in the surface.


Have a look at the attached scene file.

Thanks so much for the explanation and the scene file, Jeff!

So if I'm understanding correctly the box is basically taking on the shape of the logo with the cookie so that the particles are colliding with the box instead of my logo. Almost like the box is booling out the logo?

Thanks again for your help.

Yes.

FLIP needs to see a collision volume for the fluid to interact with.

I just took a box and carved out your logo from the top building a large manifold object which can create a good collision volume to feed in to the FLIP solver as a good working collider.

You can render the initial geometry if you wish. The carved out box is just there for the fluid to collide against. It just exists for the simulation to work.

Switch and bait is a very common Houdini workflow.

Ok that makes sense.

If you were doing a similar simulation would you set it up in a similar way? I want a glob of particles to fall and then gradually spread out.

So I made a basic sphere, emitted particles from it for 3 frames and that was basically it. I wasn't sure if there was a better way to set up the sim.

Sounds good to me for specifying an emitter.

The shape of the emitter has some bearing on the sim but only for the first second or so, if that.

One other choice you have is do you want to add some curl noise velocity to your emitter to add more of an initial chaotic movement. That is done with the Fluid Source SOP appended to your emitter geometry.

Great, thanks for the tip!

Yeah I'd love to add a little bit of wobble, like stuff is moving inside the sphere as it falls so it's not so uniform. The curl noise sounds like it could add that for me.