Couple two meshes
 

How to couple two meshes if there is a small gap exist? (please see figure http://www.whlam.com/e6.jpg )

:confused:

You can either change the default tolerance for couple creation or remove the gap.

The first way is more simple but this can result in wrong cells assigned to couples (in your case, for example, from the second layer of fine mesh instead of the first one).

The second way is more complicated - you'll need to create shells on top of coarse mesh (mesh 2), select the vertices at the bottom surface of fine mesh (mesh 1) and project the to newly created shells with VPROJECT command.

I use the VPROJECTION to eliminate the gap. It is working.

What commands should I use to couple the fine mesh to the coarse mesh?

:)

CPCREATE, to create couples
And don't forget to allow partial boundaries for your couple type.
Foreseeing your question about partial boundaries - please study Tutorial 14. Studying tutorials will really help you, trust me! :)