Flow through small cylinder recirculated inside larger cylinder.
 

Hello,

I'm currently working on a problem that requires some sort of a recirculation, that is, the fluid flows through a set of smaller cylinders where it undergoes temperature and velocity changes, flows into a larger pool of the same fluid before it gets re-introduced into the same set of smaller cylinders undergoing the same processes.

For initial testing, I decided to create a very simple geometry: a small cylinder placed inside a much larger cylinder. I used GAMBIT to create this very simple geometry and to mesh it, but I'm having troubles selecting the proper boundary conditions (either inside Fluent or GAMBIT). Ideally, I'd like to set a fixed velocity at the inlet of the interior cylinder as I want to fix my mass-flow-rate in my eventual geometry. But that doesn't work, as Fluent seems to see this type of BC more as a "flow-through" BC instead of a recirculation BC. Also, the recirculation BCs don't really work, as I don't want to connect the outlet of the smaller cylinder with the inlet of that same cylinder, thereby basically by-passing the larger cylinder. So what BCs should/can I use?

I looked into interface, but that seems more used for connecting meshing of different volumes, and I'm not sure I could specify any sort of flow condition on such a mesh.

If anyone can give me some pointers that'd ge great!

Just posting this in case anyone else runs into a similar problem later on but I more or less solved the problem myself. You need to specify two different continuum types in GAMBIT, one for the inner cylinder and another one for the external cylinder, and use walls as BCs. Then in Fluent, turn them into interfaces and create mesh interfaces. This allows flow through the whole domain. It won't allow you to set a mass flow rate though, so I'm using a volumetric momentum source. It's doing the trick more or less for me.

If anyone else has any better suggestions though, please post them!