I want to export a surface mesh from gridgen to Tgrid. The STL format works, but all surfaces are imported in Tgrid as a single zone. I need separate zones. How can I do this?

One way to do this would be to write a Glyph script that would loop over all your domains (surface grids) and export each one to a separate STL file. While that would be relatively easy to write, I don't know how laborious that would be to import into Tgrid - depends on how many files it creates.

Can Tgrid read a Fluent .cas file? You might (depending on the surface grid topology) be able to tell Gridgen that your surfaces are a 2D mesh. Then give each mesh (block in 2D) a unique Volume Condition (VC). Each VC is written to the .cas file as a separate zone.

Can Tgrid read VRML 1.0? Unlike STL, VRML files can have topology so all your meshes would remain separate.

Also, the success of all this depends on whether Tgrid does any post-processing after import where it tries to connect everything in one zone.

Hope this helps.