Mark Tucker

The issue is that nowhere in this setup (that I can see) have you done anything to actually make the smoke "loop". The most obvious place to do this would be in Geometry Clip Sequence LOP which has controls for looping. In USD, even when using value clips, looping never happens "automatically". You must always explicitly author a long animation with looping built in for the maximum duration that you may want to loop over.

That said, another common mistake you've made here is to use a value clips to loop a volume simulation. Volumes are the absolute simplest primitive in all of USD, and derive no benefits from being embedded in a value clip (I've ranted about this in a couple other threads so I won't go into it again). But I'd recommend just using a Volume LOP instead of a Geometry Clip Sequence LOP. Set the volume LOP's volume file path to a simple expression that will do the looping for you, like 'foo.`$F % 71 + 1`.bgeo.sc', and author a nice long 2000 frame loop like this. This should cook quickly enough that you probably don't need to write this out to disk as a USD file as you've done with the value clip, but of course you can if you'd like.

Looks like you're inspecting the flattened stage rather than the active layer? When USD flattens a stage, all implicit instance prototypes become "explicit" and show up on the stage as regular prims. In the scene graph tree you can turn on the display of implicit instance prototypes and you'll see this same /Flattened_Prototye_1 prim appear. I'm not sure exactly why flattening a stage saves out these implicit prototypes, but I'm sure there's a good reason for it.

Yep - UsdStagePrimMenu.xml.