SW 2018 SP5.0

Hi,

1. I have Assembly 1 with 3 layers of flow channels for an internal simulation.
2. I put lids on all 6 entrances and exits.
3. I check the geometry of Assembly 1, and the fluid volume is recognized correctly.
4. I have Assembly 2 that is a manifold for 2 of the channels.
5. I put lids on the entrance and exit of the manifold (Assembly 2).
6. I check the geometry of Assembly 2, and the fluid volume is recognized correctly.
7. I suppress the exit lid of Assembly 2 and save it.
8. I insert Assembly 2 into Assembly 1 and mate it such that the face used to make exit lid in Assembly 2 is coincident with the face used to make the entrance lids for the 2 channels the manifold will be feeding.
9. I check the volume of Assembly 1 again , and the 2 channels the manifold is feeding are not recognized unless I use the "Improved geometry handling" option. (But apparently SW won't let you define simulation parameters - BCs, goals, etc. - using any faces in the regions recognized with this option enabled, but not with it disabled.)
10. I suppress the entrance lid on the manifold and check the geometry again, and the fluid volume is the same as before I added the manifold. This makes no sense.
11. I unsuppress the entrance lid on the manifold and suppress the entrance lids on the channels.
12. I check the geometry and get the same results as (10) - only the other channel  is recognized.

So, to sum up, 2 watertight assemblies that pass geometry tests cannot be combined together. And don't waste your time with the "Improved geometry handling" option. All it does is confirm that you have a model with well-defined fluid volumes that SW is saying isn't there.

I've already wasted 2 days trying to set up a simple simulation that should take no more than an hour to set up. I guess I will just get rid of the manifold and assume that flow is uniform coming into the channels. I was hoping to see how well the manifold distributed flow, but I guess that's just too hard for SW to do. I wonder if this has anything to do with later versions of Solidworks really having problems evaluating degrees of freedom (fully defined sketches going over-defined after removing relationships, mates in assemblies being OK - then overdefined next time the file is opened - then OK if you suppress and unsuppres particular mates, etc).

Any ideas as to how to get SW to recognize fluid volumes in a predictable manner (I've given up on it doing so correctly)?

Thanks,

Ken