hi daniel, in the flow simulation pull down go to tools > check geometry and run a check geometry test
you'll see there are many invalid contacts and invalid surface pairs
some sources of problems that i can see:
1. if you cut your model down the middle, you'll see a lot of sliver cuts. flow simulation is likely having trouble figuring out which volumes to ignore.
2. if you drop your part into an assembly and run an interference detection (include multibody parts), you'll see many interferences. those will also cause problems.
for your external analysis, I would get your model so that it is 1 solid body. if you're going to have multiple bodies, make sure that they touch face to face and don't generate any gaps between them. (you can check this with interference detection using the "treat coincidence as interference" option)
a technique we would normally use to fix this is to build a big solid cube over your model at the assembly level and then use the cavity function to "fill" the gaps and then merge them to the rest of the bodies. however with the interferences..etc, it might be difficult. try using the combine command at the part level first and see if you can eliminate some of the interferences..etc.