Could someone tell me what boundary condition you apply when you have seperate flow domains with in the main domain, I.E - when you split the domain up so you can create a finer mesh in certian places. What boundary condtions do you apply to the joining edges/faces??

Many Thanks

It depends... when you split the domain, one of two things happen:

1. you split the geometry with the "connected" option on... in this case both volumes will share the same face (or both faces will share the same edge in 2D) and therefore will share the same face mesh. Do not apply any boundary conditions to these edges/faces. When gambit exports the mesh it will ignore internal faces of connected volumes (well, not really ignore, but it won't write them to the mesh file) .

2. you split the geometry with the "connected" option off, or you disconnected the geometry after the split... in this case the split function will create two faces, one for each volume, at the intersection. This allows you to have different mesh sizes or even meshing schemes in each volume. Then you apply the interface boundary condition in gambit (on each of the interface faces separately) and in Fluent you define the interface zones (you pick the two interface faces that are coincident). This gives you a non-conformal mesh interface where the flow can pass through the face and the mesh nodes do not have to line up.

Typically, stick with option 1. Its usually best to avoid adding anymore complexity to the model than you have to, so don't use a non-conformal interface unless you have to.

Hope this helps, and goodluck, Jason

thanks for your help, How do you make sure the connected option is on??

Also, with option 1, can you still apply different mesh spacing or schemes, as this is what im looking at doing.

The default should be that connected option is on. When you go to the split volume command, there is a few check boxes toward the bottom of the command window... one of which is "connected"... as long as this is checked (the select box in front of "connected" is highlighted in red) then the connected option is on.

It depends what you are trying to accomplish... often I will use different sizing functions on different volumes... one example would be a slow growth rate in a volume close to a bluff body, then a faster growth rate on the larger volume around this localized volume. Both volumes will share the mesh on the common face though! Therefore you should make sure the mesh sizes have a smooth transition from volume to volume. You can combine quad and tet meshes, but on the tet mesh Gambit will use pyramidal elements to make the transition from the tet volume mesh to the quad surface mesh. You have to mesh the volume with the structured mesh first, or at least do a quad face mesh on the shared face before doing the tet mesh. This tells Gambit that pyramidal elements are required to make the transition.

If sharing this face mesh is not going to get you what you want, then you'll be using a non-conformal mesh and you have to set the BCs as interface, and then define the interfaces in FLUENT.

Hope this helps, and goodluck, Jason