I'm trying to make a static object (a cup) to interact with a particle fluid emitter. I used a closed tube, then blasted the top cover, but the fluid passes right through the bottom cover. How do I fix this?

do you have a hip file we can look at?

sounds like you might need to invert the normals.

When you create a tube, the normals face outwards, but in your case, you want them to face inwards…

Append a primitive sop, and from the face/hull menu select vertex->reverse. There's also a reverse sop, but I haven't quite figured out what it does.

Maybe, another way to fix it is activating 'Invert Sign' in Collisions->Volume options.

You need a volume to create the collision geometry. A regular tube with a blasted cap is a only a surface, so it wouldn't work.very well with the default volume-based collision.

To get a better collision geometry for a cup, keep the top cap and extrude it inward so that your cup actually has some thickness.

Also go into the AutoDopNetwork and look at the tube static RBD object; at the bottom of the Collisions tab you can check on “Show Collision Guide Geometry” to see what the geometry in the RBD sim actually look like. If it looks like nothing like a cup, increase the “Divisions” in X, Y, and Z for higher precision, and check off Laser Scan.

it seems that the best way to do this is boolean the surface with a cube
like you did.
Its a shame that collisions are so so cumbersome you actually have to spend more time thinking how to break up your object or tweaking it in order to make more accurate SDF's than making the actual particle animation.