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Karl Kevala February 7, 2001 15:23

Gambit preprocessing for periodic geometry
I am meshing several 3-d geometries that are rotationally periodic (similar to turbomachinery). It seems to be impossible to make one sector (say 30 degrees), mesh it, and then make copies and rotate it to get the entire 360 degree domain. The problem is in connecting the copied volumes together when they have previously been meshed -- it is impossible without deleting the mesh on all volumes.

It takes too long to create the entire 360 degree domain and then mesh it. Does anyone know how I can create a single sector and rotate it in gambit. Any comments appreciated.

Karl Kevala

Jonas Larsson February 7, 2001 17:12

Re: Gambit preprocessing for periodic geometry
Why don't you create one sector, mesh it, and save it, rotate it and re-save it and so on... then you can merge the different meshes with tfilter tmerge. I had a similar problem a couple of years ago with pcube/tgrid and then I ended up writing my own small program to rotate a the grid. With Gambit you can do this directly though.

Angus Duncan February 8, 2001 04:53

Re: Gambit preprocessing for periodic geometry
Due to bizarre limitations of Gambit i.e. if you have used the top-down modelling approach, tgrid allows the copying of geometry and meshes without 'disconnection' of interior faces, however, Gambit does not (arghh! stress!!), I use tgrid in conjunction with Gambit.

Build your 30 degree sector in Gambit, define each of the 'periodic boundaries' as named or numbered walls. Read into tgrid 3d then use >Mesh >Zone to copy and rotate the original mesh. Write out this mesh file and read into Fluent. Now use the >Grid >Fuse command N.B. shared faces must match exactly! You'll have to go through the grid display to identify which walls should be fused (Fusing merges two walls and creates an interior).

I realise that I'm assuming you've got tgrid 3d.

Also, is it not possible just to run your model as a periodic case?

Hope this is of use,


Karl Kevala February 8, 2001 10:38

Re: Gambit preprocessing for periodic geometry
Thank everyone for their answer. I have used the method of rotating the mesh in Gambit, saving, and then fusing them with tmerge. This seems to work ok but is time consuming to fuse the face zones back together once the mesh is read into Fluent. I did not know that tgrid would allow volumes to be copied without losing connectivity. Hopefully newer versions of Gambit will fix the many problems associated with copying portions of geometry. It would be nice to be able to copy a sector and be able to mesh different sectors differently. As far as simulating a single sector with periodic boundaries, this is impossible due to assymetry in some of the sectors near the outlet.

Martin February 8, 2001 15:40

Re: Gambit preprocessing for periodic geometry
Just a note: Why rotating the sector mesh in Gambit and saving in different files if you can do the same business with just one sector mesh, reading it several times in tfilter tmerge and everytime specifying different angle of rotation... Martin.

Karl Kevala February 8, 2001 15:54

Re: Gambit preprocessing for periodic geometry
Either way, rotating and saving in gambit or rotating in tfilter is about the same amount of work. If you rotate and save in Gambit then it is a little faster overall because you can use the GUI and don't have to type a new angle at the command prompt for each sector as you do with tfilter.

Shyam Kishor February 9, 2001 12:46

Re: Gambit preprocessing for periodic geometry
Upcoming release of Gambit (Gambit 2.0) will allow you to connect coincident meshed faces while preserving the mesh.

You can copy and rotate zones in Tgrid, however, will need to merge duplicate nodes at the interface (using Boundary -> Nodes -> Merge), to ensure proper connectivity for a conformal mesh.

Althea February 12, 2001 07:44

Re: Gambit preprocessing for periodic geometry
Can you use mesh face linking to help? I can't use copy and anything for many of my geometries - Gambit seem to take offence, but I'm looking forward to version 2. Good luck with your problem. Althea

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