Difficulty when using gmsh to create a structured mesh.
 

Hi!

I am trying to create a structured 3D mesh from an annular duct, using gmsh. I begin by creating a 2D mesh. I would then like to extrude the mesh to create the 3D mesh, but it is at this stage that the problems occur... Below is my .geo:

// element spacing
Nz = 50; //136
innerh = Pi/2*0.13/10;
outerh = Pi/2*0.424/10;
// centre point
Point(1) = { 0, 0, 0.4};
// inner circle points
Point(2) = { 0.13, 0, 0.4, innerh};
Point(3) = { 0, 0.13, 0.4, innerh};
Point(4) = { -0.13, 0, 0.4, innerh};
Point(5) = { 0, -0.13, 0.4, innerh};
// outer circle points
Point(6) = { 0.424, 0, 0.4, outerh};
Point(7) = { 0, 0.424, 0.4, outerh};
Point(8) = {-0.424, 0, 0.4, outerh};
Point(9) = { 0, -0.424, 0.4, outerh};
// lines
Line(1) = {2, 6};
Line(2) = {8, 4};
Transfinite Line{1, 2} = (0.424-0.13)/0.01;
// circle arcs
Circle(3) = {2, 1, 4};
Circle(4) = {4, 1, 2};
Circle(5) = {6, 1, 8};
Circle(6) = {8, 1, 6};
// surfaces
Line Loop(10) = {1, 5, 2, -3};
Plane Surface(10) = {10};
Transfinite Surface{10};
Recombine Surface {10};
Line Loop(11) = {1, -6, 2, 4};
Plane Surface(11) = {11};
Transfinite Surface{11};
Recombine Surface {11};
// extrusions
// Extrude{0,0,-0.2}{Surface{10}; Layers{Nz*0.2}; Recombine;}
Extrude{0,0,-0.2}{Surface{11}; Layers{Nz*0.2}; Recombine;}

This gives me a strange dislocated geometry, but I am not sure why. If I comment the last line, and uncomment the second last, then the bottom half of the duct disappears.. Can anyone see my error?

Cheers,
Al