# [Getdp] Just a litle help ..

Nacho Andres de la Fuente nacho.andres at deos.tudelft.nl
Sat Feb 28 15:59:49 CET 2004

```Hi everybody, I am starting to seriously use gmsh + getdp and I am
pretty amazed about its capabilities. In principle everything looks
quite simple but nevertheless doubts always arise. I hope someone can
help me on this matter:

If you do not mind, I would like to frame the questions into this
example, since I have calculated the analytical solution:

"Given two concentric conducting spheres (a spacecraft) in outer space,
solve Div[sigma.Grad(Phi)]= 0 with given Neumann b.c. in the outer
surface of the external sphere."
(Conductivities of sphere 1 and sphere 2 are different => b.c. at the
discontinuities as well)

1) Are the b.c. at the discontinuities absorbed by the weak formulation?
Or do I have to separate all the domains and impose all the Neumann
b.c.?

2) My problem is an electro-quasistatic one with an equation which
includes the Neumann conditions like:

Formulation {
{ Name PhEl_v; Type FemEquation;
Quantity {
{ Name v; Type Local; NameOfSpace PhEl_v_Ele; }
}
Equation {
Galerkin { [ sigma[] * Dof{d v} , {d v} ]; In ConductingDomain;
Galerkin {
[IllPhotoFunc[{v},Colat[X[],YNew[X[],Y[]],ZNew[X[],Y[]]]] , {v} ];
In SurfIll; Jacobian JSur; Integration I1; }
Galerkin { [ DarkPhotoFunc[{v}] , {v} ]; In SurfDark;
Jacobian JSur; Integration I1; }
}
}
}

Do I miss something here? because I am just getting zeros as a solution
(and the analytical solution yields a different solution). In case that
you need it, I have attached both the *.geo and the *.pro files.

3) Concerning the axilsymmetry conditions in the problem? I am using
cartesian coordinates since the geometry of my problem is the real one
(i.e. the gmsh file is made of two concentric spheres). In principle the
conditions should be (in spherical formulation):

phi= floating potential at the center of the spheres.

Partial{phi}/Partial{azimuth} = 0
Partial{phi}/Partial{colatitude} = 0 at the axis of symmetry

Since I have created the right geometry (two concentric spheres), I
solve my problem in cartesian coordinates, but the above mentioned
conditions become thus something like this:?

Partial{phi}/Partial{azimuth} = Partial{phi}/Partial{x} *
Partial{x}/Partial{azimuth} + ...
foe every point ..
and the last one:
Partial{phi}/Partial{normal} = Partial{phi}/Partial{colatitude} =
= Partial{phi}/Partial{x}
*
Partial{x}/Partial{colatitude} + ...

It looks a little weird though, so I do not think I am doing the right
things .. some ideas?

Sorry for the long mail and the many thanks in advance!

Nacho
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```