export 2D mesh

Hi all,

I am trying to export the 2D mesh from acusolve. As far as I know, in acusolve, we always create a 3D mesh. Acusolve only works with 3d mesh. The following is the domain I am working with.

The domain is 1m x 1m x 0.05m. I tried to export the mesh to Nastran to Gmsh to Matlab for my work. In gmsh, it shows 20602 elements (not 10100). I am thinking only surface meshes from all the faces have been exported and treated as 2D mesh. 

How can I export only 2D mesh from top surface only (as in 2D simulation; mesh as seen from the top)?