# [Gmsh] Extrusion of one element that is split into multiple regions

Christophe Geuzaine cgeuzaine at uliege.be
Tue Mar 31 12:08:05 CEST 2020

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> On 31 Mar 2020, at 00:15, Bucksot, Jesse Edward <Jesse.Bucksot at utdallas.edu> wrote:
>
> Hi all,
>
> I’m pretty new here. I’ve had good success with gmsh so far, but I just ran into an issue when extruding surfaces that have been split into multiple separate regions due to boolean operations. When I just do one extrude, it seems to come out right, but then if I try to label the extrusion (e.g. “Physical Volume("part") = {ex1[]};”) or try to extrude again by extruding ex1[0], it only does one single part of the surface.
> Basically, if I take a rectangle and cut it into two smaller rectangles using BooleanDifference and then extrude it, the created extrusion (ex1[0],ex1[1]) only contains one of the smaller rectangles and not both of them.
>
> Does anyone have any solutions or workarounds to this?
> Also, my real geometry is pretty complicated, so I’m not sure how feasible it is for me to manually check for any instances of surfaces being split apart before extruding.
>

To get something conformal you indeed need to extrude the different parts at the same time. Here's how I would do it:

s2858 = news;
Rectangle(s2858) = {-1, -1, 0, 2, 2};
s2859 = news;
Rectangle(s2859) = {-0.1, -1, 0, 0.2, 2};
bo1[] = BooleanDifference{ Surface{s2858}; Delete; } { Surface{s2859}; };

ex1[] = Extrude {0,0,1} {Surface{s2859}; Layers{1}; };
ex2[] = Extrude {0,0,1} {Surface{bo1[]}; Layers{1}; };
ex3[] = Extrude {0,0,1} {Surface{ex1[0]}; Layers{1}; };
ex4[] = Extrude {0,0,1} {Surface{ex2[0]}; Layers{1}; };
Physical Volume("tool") = {ex1[1], ex3[1]};
Physical Volume("cut") = {ex2[1], ex4[1]};
Else // new way
ex1[] = Extrude {0,0,1} {Surface{s2859,bo1[]}; Layers{10}; };
ex2[] = Extrude {0,0,1} {Surface{ex1[0],ex1[6]}; Layers{10}; };
Physical Volume("tool") = {ex1[1], ex2[1]};
Physical Volume("cut") = {ex1[7], ex2[7]};
EndIf

Christophe

>
> Jesse
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—
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science
http://www.montefiore.ulg.ac.be/~geuzaine

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