[Gmsh] Ghost cells

[nikki dakotam]

I am trying to understand the ghost cells from gmsh partitioning.
Basically I am interested in getting one layer of ghost cells in each
partition that belong to a neighboring partition.

In the following code, I am trying a very simple example, a square domain
that i am partitioning into 4x4 quads. I would output 2 partitions from
this. What I endup is each partition has 8 elements with some elements
belonging to the other partition which a negative id. No element is
repeated with a negative id on the other partition.

Insteead, what I expected is to find some elements (interior boundary)
belonging to both partitions, but the id is negative on partition that is
not owned by the partition.


==================== GhostInput.geo ===================

element_size = 0.5;
Point(1) = {0.0, 0.0, 0.0, element_size };
Point(2) = {2.0, 0.0, 0.0, element_size };
Point(3) = {2.0, 2.0, 0.0, element_size };
Point(4) = {0.0, 2.0, 0.0, element_size };

Line(7) = {1,2};
Line(8) = {2,3};
Line(9) = {3,4};
Line(10) = {4,1};

Line Loop(13) = {7,8,9,10};
Plane Surface(14) = {13};
//Transfinite surface:
Transfinite Surface {14};
Recombine Surface {14};

Physical Surface("front") = {14};

Mesh.Partitioner = 2;
Mesh.NbPartitions=2;
Mesh.MshFilePartitioned=1;

==========================================================


What I get as output is:

=============== GhostInput..msh_000001 =======================
$MeshFormat
2.2 0 8
$EndMeshFormat
$PhysicalNames
1
2 1 "front"
$EndPhysicalNames
$Nodes
15
3 2 2 0
4 0 2 0
9 2 1.0 0
10 2 1.5 0
11 1.5 2 0
12 1.0 2 0
13 0.5 2 0
14 0 1.5 0
15 0 1.0 0
18 0.5 1.0 0
19 0.5 1.5 0
21 1.0 1.0 0
22 1.0 1.5 0
24 1.5 1.0 0
25 1.5 1.5 0
$EndNodes
$Elements
8
1 3 5 1 14 2 1 -2 15 18 19 14
2 3 4 1 14 1 1 14 19 13 4
3 3 5 1 14 2 1 -2 18 21 22 19
4 3 4 1 14 1 1 19 22 12 13
5 3 5 1 14 2 1 -2 21 24 25 22
6 3 4 1 14 1 1 22 25 11 12
7 3 5 1 14 2 1 -2 24 9 10 25
8 3 4 1 14 1 1 25 10 3 11
$EndElements
==========================================================

=============== GhostInput..msh_000002 =======================
$MeshFormat
2.2 0 8
$EndMeshFormat
$PhysicalNames
1
2 1 "front"
$EndPhysicalNames
$Nodes
15
1 0 0 0
2 2 0 0
5 0.5 0 0
6 1.0 0 0
7 1.5 0 0
8 2 0.5 0
9 2 1.0 0
15 0 1.0 0
16 0 0.5 0
17 0.5 0.5 0
18 0.5 1.0 0
20 1.0 0.5 0
21 1.0 1.0 0
23 1.5 0.5 0
24 1.5 1.0 0
$EndNodes
$Elements
8
9 3 4 1 14 1 2 1 5 17 16
10 3 5 1 14 2 2 -1 16 17 18 15
11 3 4 1 14 1 2 5 6 20 17
12 3 5 1 14 2 2 -1 17 20 21 18
13 3 4 1 14 1 2 6 7 23 20
14 3 5 1 14 2 2 -1 20 23 24 21
15 3 4 1 14 1 2 7 2 8 23
16 3 5 1 14 2 2 -1 23 8 9 24
$EndElements
==========================================================
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