[Gmsh] Volume within a volume

[Dominic North]

Dear Gmsh community,

Please could you help? I'm trying to define a volume within a volume in gmsh, but get an 'overlapping facets' error message (because I'm trying to extrude a surface directly into an existing volume, I think). The aim is to model heat travelling through a main volume: small regions defined within this volume will generate heat. The 'heat-generating sub-volumes' are otherwise unimportant to the system - they just need to define where the heat is generated and be easily moveable around the main volume.

Please see below for a representative model and the error message. 1D and 2D meshing is fine; the overlapping facets message occurs on 3D meshing.

Is there any way you can think of to achieve this? Many thanks for any help you can offer.

Best regards,
Dominic North

------------------------
//Make the  main volume:
p1 = newp;
Point(p1) = {0,0,0};
p2 = newp;
Point(p2) = {2,0,0};
p3 = newp;
Point(p3) = {2,2,0};
p4 = newp;
Point(p4) = {0,2,0};

l1 = newl;
Line(l1) = {p1,p2};
l2 = newl;
Line(l2) = {p2,p3};
l3 = newl;
Line(l3) = {p3,p4};
l4 = newl;
Line(l4) = {p4,p1};

ll1 = newll;
Line Loop(ll1) = {l1,l2,l3,l4};

s1 = news;
Plane Surface(s1) = {ll1};

mainExt[] = Extrude{0,0,2}{Surface{s1};};

//Make a sub volume
p5 = newp;
Point(p5) = {2,0,0};
p6 = newp;
Point(p6) = {2,1,0};
p7 = newp;
Point(p7) = {0,1,0};

l5 = newl;
Line(l5) = {p1,p5};
l6 = newl;
Line(l6) = {p5,p6};
l7 = newl;
Line(l7) = {p6,p7};
l8 = newl;
Line(l8) = {p7,p1};

ll2 = newll;
Line Loop(ll2) = {l5,l6,l7,l8};

s2 = news;
Plane Surface(s2) = {ll2};

subExt[] = Extrude{0,0,2}{Surface{s2};};


--------
Info    : Done meshing 2D (0.046875 s)
Info    : Meshing 3D...
Info    : 3D Meshing 2 volumes with 1 connected components
Info    : Tetrahedrization of 447 points in 0 seconds
Info    : Reconstructing mesh ...
Info    : Creating surface mesh...
Info    : Found two exactly self-intersecting facets.
Info    :   1st: [15, 16, 124] #6
Info    :   2nd: [15, 16, 338] #34
.Error   : Invalid boundary mesh (overlapping facets) on surface 6 surface 34
.Error   : No tetrahedra in region 1 2
Info    : Done meshing 3D (0.03125 s)
Info    : 439 vertices 1016 elements
.Error   : ------------------------------
.Error   : Mesh generation error summary
.Error   :     0 warnings
.Error   :     2 errors
.Error   : Check the full log for details
.Error   : ------------------------------
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