# [Gmsh] boundary help

**Geordie McBain**
gdmcbain at freeshell.org

*Mon Jun 28 01:03:42 CEST 2010*

2010/6/26 Manan Patel <mananpatel11 at gmail.com>:
>* hi all!
*>* i am not using gmsh solver. my problem is that of heat transfer in a human
*>* tissue. i need boundary faces so that i can add the effects of convection.
*>* does gmsh provide the boundary faces? if yes, then please specify the link
*>* where the details are given, or explain what is the format of specifying the
*>* boundaries.
*>* if gmsh does not provide these boundaries, then please list any alternate
*>* meshing software for the required purpose.
*>* thank you!
*
Hello. I assume your problem is three-dimensional (analogous
considerations apply in two dimensions). Your boundary will consist
of polygonal elements, e.g. triangles; to save these boundary faces,
define the geometric surface they belong to as a Physical Surface.
See Section 6.2 `Elementary vs Physical Entities'.
http://geuz.org/gmsh/doc/texinfo/gmsh.html#Elementary-vs-physical-entities
You should also define the geometric volume the domain (or subdomains,
e.g. if you have different kinds of tissue and want to prescribe
different partial differential equations or coefficients in each)
belongs to as a Physical Volume.
The number you assign to the Physical Volumes and Surfaces should then
be stored in the mesh. Set the options Mesh.SaveElementTagType=2 to
ensure this. Exactly how this is stored might depend a bit on what
your output format is, and that I suppose will depend on what your
solver is expecting. I think the above should clarify what you need
to do, but if not, let us know what mesh output format you're using.