# [Gmsh] discrete-continuum mesh

**Christophe Geuzaine**
geuzaine at acm.caltech.edu

*Sun Jul 3 09:56:43 CEST 2005*

Kovacs, Attila wrote:
>*
*>* Christophe,
*>*
*>* I attached the files I am trying to deal with.
*>* I generated a 2D mesh with our discrete mesh generator, and also generated a 3D mesh with gmsh.
*>* After that I used "merge" to combine the two meshes in gmsh.
*>* The 2D element edges don't seem to fit the 3D element edges on the surface of the resulting combined mesh.
*>* An alternative way could be to import the fractures as surfaces, and to generate both the 2D and 3D element within gmsh.
*>* This I could not achieve so far.
*>* Probably I did something wrong. Please explain how I could generate a combined 2D-3D mesh in which
*>* the neighbouring element edges and nodes fit each-other.
*
Hi Attila - You cannot just merge the two surface meshes: you need to
define the surfaces as (part of) the boundary of a volume. From the
documentation:
"The mesh generation is performed in the same bottom-up flow as the
geometry creation: lines are discretized first; the mesh of the lines is
then used to mesh the surfaces; then the mesh of the surfaces is used to
mesh the volumes. In this process, the mesh of an entity is only
constrained by the mesh of its boundary(1). This automatically assures
the conformity of the mesh when, for example, two surfaces share a
common line. But this also implies that the discretization of an
"isolated" (n-1)-th dimensional entity inside an n-th dimensional entity
does not constrain the n-th dimensional mesh."
Christophe
--
Christophe Geuzaine
Applied and Computational Mathematics, Caltech
geuzaine at acm.caltech.edu - http://geuz.org