Problem with interior faces
 

Hello everyone,

I'm using Gambit 2.4.6 to create mesh to simulate centrifugal pump. I successfully made mesh, but it consists of several volumes (both stator and rotor). I will simulate it using fluent 12.0.16.
Now it's becoming clear that that approach is bad, because now i have faces which define those volumes that are inside the mesh. Gambit let's me define them as "interior" boundaries.
However, since faces actually have mesh only from one side (my domain is just divided into several volumes) fluent says that those boundaries can't bee interior, and that they will be wall.
So, right now i have a centrifugal pump with a several walls which don't exist.

I used this approach so it would be easier to make good mesh. Unfortunately, i didn't take into account that volumes must share a face for this to work. Off course, I don't want to change my mesh. Help me :)

note that this work is part of my school assignment, and I'm not cheating, my prof. is not available right now.

Many thanks kind stranger
- miro

P.S. in one case, those faces are not identical (on both volumes) since there are three volumes.

edit: picture 6

Questions: What approach are you using? Dynamic mesh, sliding mesh, moving frame of reference or mixing plane method.

Also some pictures from your mesh would help us.

Dear Mr Vasava,

thank you for your intrest. I see now that my description wasn't that great.

I'm using moving mesh. My domain is split in half, in which half is moving (rotor) and half is stationary. I'm simulating only 1/7th of a pump, and using periodic b.c. to compensate.

Pictures are in the attachment. I have selected stator in one picture, and problem faces in the other. Note that in that picture, one face in upper volume is split in two faces in lower volume.

I think problem with fluent happens because my volumes are defined by diferent faces that are on the same geometric position, although mesh is identical...

You don't have to define interior faces as interior.
If your splits are well done (understand volumes are connected), then gambit treat automatically internal surfaces as interior.
So delete those interior BC, and re-try
Then moving mesh is quite vague: are you using sliding mesh, remeshing, layering...? But this is another problem

Dear Max,

what you are writing is correct. It seems that said faces are not connected. It seems that other faces that are not making problems are connected. Is there a way to connect them now? There is a problem of that face that's split in two, I guess it would be a problem to connect them... Also, i have hard-linked (but disabled periodic option) front and back of the pump (walls) because Cooper wouldn't work otherwise.
I really don't want to make the whole thing from scratch. :)

Yes, again, I didn't clarify. I'm using dynamic mesh in which the upper region (stator) is stationary and the lower region (rotor) is rotating around Z axis. I'm using interface to connect them. The interface is the lowest marked face in the picture where stator is selected.
Please ask if you have further questions regarding this.

Thank you :),
- miro

*try faces/connect/All
But it should delete one or more volume mesh.

*If you are using an interface between rotor and stator, then you are using sliding mesh (rotor mesh will move as rigid body --> no deforming mesh)

I would work step by step: compute first a steady case, to be sure that your mesh is ok (is your fluid flowing through interface for instance)
If everything is ok, switch to unsteady case, and activate dynamic mesh (rotor as rigid body with angular velocity)

This sounds description of 'sliding mesh' or 'mixing plane model'. A dynamic mesh is approach where your mesh is updated every time-step. When you visualize dynamic mesh you should feel as if the impeller is moving through the mesh. See this animation http://www.youtube.com/watch?v=nWX6On8H3Mg.

OK, thank you for your advice, I find them very useful, especially about steady case.

@ mr. Vasava; Yes, you are correct, I didn't describe the problem properly.

At this point it seems that the most painless solution is to modify the volumes to remove duplicate faces and make them connected. Different ideas are also welcome.

tnx,
- miro

Hello again!

Well, I fixed the mesh and ran a steady simulation and everything works fine.

However, when I try to run transient, fluent says mesh has negative volumes. This makes me wanna cry, because it SHOULD work, especially because steady simulation works fine. :mad::(:mad::(

For some reason, mesh check fails in Fluent, but passes in Gambit. It says:
And has different view on mesh quality than gambit which is very happy with created mesh:
All these things aside, in case of a sliding mesh (interface), if steady simulation works, so should transient! Or? Am I wrong?
Because mesh doesn't change?

Well, anyway, thank you for your help so far,
sincerely,
- miro

P.S. I did scale the mesh in Fluent. But that also shouldn't make problems.

P.P.S. Also, fluent adds two b.c. called wall, but when I try to visualize them, there's nothing there. Also, when I import .msh into gambit again, nothing is wrong, and those b.c. don't exist...

Well, it seems my time-step was a bit too big.
But other questions still remain..

Thank you,
- miro

negative cells warning sounds like you are handling dynamic mesh.
But as I said you have to work with sliding mesh, and only the rotor domain should rotate. As the rotor is not connected to stator (interface), then you shouldn't get any "dynamic" update of your mesh (but just its rotation which shouldn't generate negative cells ---> except you have negative volume already with your initial mesh)

Hi,

It took me awhile, but I got it working in the end. The problem was that my interface - faces were assigned to the wrong volume. So, when the rotor region moved, it moved the interface - surface of the stator. That actually created dynamic mesh, and that's why it blew up.

There are still some questions regarding mesh check in Fluent, however, these are not important for my assignment.

Anyway, you have been a huge help,
thank you,
- miro