I wish to model a vertical two-phase separator. I have prepared the geometry of the vessel: A vertical cylinder with on horizontal inlet no the side, a vapor out on top and liquid outlet on bottom.

There is to be liquid level in the bottom of the vessel. The remainder of the vessel is vapor space.

I created two volumes from the initial single volume using the Decompose Booleans operation. One volume for vapor and one for liquid.

How do I now unite these volumes into a single flow domain so I can create initial liquid level conditions and mesh the geometry? I'm stuck at this point...

My ultimate goal is to measure the shear on the face of liquid surface caused by the vapor entering the vessel.

Thanks!