Hi

im little bit newbie on Houdini but Its long time im using 3d software (3ds max , maya , zbrush . blender , nuke ,after effect, cinema 4d etc .. )

I have a issue with a simple scene I have a
geo1>sphere>blast(half sphere delete)>thicken>null and collision from shelf apply on it.
geo2>sphere> RBD grains apply from the shelf .
geo3>box>transform(get scaled and keyframed to do some rotation on him self)>null and get static object from shelf

geo1 x 0 y 0 z 1
geo2 x 0 y 0 z 3
geo3 (inside the geo1)

My Issue when I play the simulation is the RDB grains from geo2 don't go inside the geo1 its stay on top of it like a plane surface
thank in advance for helping me

I'd suggest checking the Bullet collision shape settings for the collider object - the default is convex hull, which may not be what you want for a concave object.