# [Gmsh] Specify boundary points or lines within a surface

Jose Paulo Moitinho de Almeida moitinho at civil.ist.utl.pt
Fri Jun 8 12:56:52 CEST 2007

```Hello, this has been answered before, but ....

"How is it possible to specify boundary points or lines within a surface (in
the same way of a hole)?". The answer is "the point must be connected to a
line, itself connected to your surface"

The solution is simple, but places an artificial constraint on the resulting
mesh - the extra lines connecting the isolated point or line to the rest of
the boundary.

Would it be to complicated to use one isolated line as an inner loop? Or
two which are in fact the the same? In fact this corresponds to two different
usages:

- Define an edge along which we want to enforce the existence of nodes
(implicitly line elements), so that values can be measured or boundary
conditions  imposed (1). In this case the nodes/line elements on both sides
of the edge would be the same.

- Define an edge that divides the domain (the simple example is a crack). In
this case the nodes/line elements on both sides of the edge would be
different, except at the extremes.

I know that this has implications on the topological definition of the domain,
so, before starting anything, I would like to know i you think that, using
the current structures within gmsh, this is feasible.

Thanks

ZP

Notes:

(1) This is the case I am personally interested in: to have lines along which
we can impose bc's that will indirectly provide integrals of some variables
(and afterwards of their errors).

(2) What is written for lines in 2D could be written for lines or planes in
3D. In fact the general idea is to allow the definition within a nD domain to
be meshed of mD domains (m<n) which will become part of the mesh. In my text
n=2 and m=1.

```