Using Gambitt, is it possible to model infinitely thin solid plate inside a domain e.g. partition inside office, hanging floor inside building? If yes, how? thank you all.

Yes... Create the volume for the buliding, then create any surfaces that represent the partitions or hanging floors. Then split the volume with these surfaces (make sure the "connect" option is on... this makes meshing, assigning BC's and outputing your mesh easier). When you're defining your BC's just define these surfaces as a wall. You'll also get a 'shadow-zone', but I don't rememeber if this shows up in Gambit, or if it doesn't show up until you import the model into Fluent... what happens is a wall is a single-sided BC (unlike pressure-jump or interior) so in Fluent, the "wall-1" becomes one side of the wall and then Fluent creates "wall-1-shadow" which is the other side of the wall.

Hope this helps, and good luck, Jason

thank you, jason.