[Gmsh] Small cylinder in huge domain

[Christophe Geuzaine]

Dear Songt,

Very thin (or very long) cylinders have very anisotropic parametric representations in OpenCASCADE. When the ratio is sufficiently large you run into floating point tolerance issues in the mesh generation process. 

With OpenCASCADE cylinders, the workaround is to segment the cylinders in several parts. Attached is a modified version of your file that shows one way of doing it:

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Christophe

PS: once the new hxt algorithm supports Fields (hopefully later this year), 3D meshing time for very refined meshes will be dramatically decreased.


> On 23 Feb 2020, at 17:52, tao song <songtao198801 at gmail.com> wrote:
> 
> Dear all,
> While i meshing a model with small cylinder in a huge domain, if the
> filed is not given, it gives me an error "No elements in volume x",
> where x denote the small cylinder volume, and if the field is used,
> the gmsh blocked and finally crashed.
> How can i get the right mesh of this kind of model (small cylinder in
> a huge domain)? I want the whole domain and the small cylinder both
> are meshed.
> 
> Kind regards,
> Songt.
> 
> The code i used:
> ---------------------------
> SetFactory("OpenCASCADE");
> 
> Point(1) = {0,0,0}; Point(2) = {0,0,-40};
> Line(1) = {1,2};
> 
> Box(1)={-50,-50,-50,100,100,100};
> Cylinder(2) = {0, 0, 0, 0, 0, -40, 0.01, 2*Pi};
> //Cylinder(2) = {0, 0, 0, 0, 0, -40, 1, 2*Pi};
> BooleanDifference(3) = { Volume{1}; Delete; }{ Volume{2}; };
> //Coherence;
> Physical Volume ( "c",11 ) = {1};
> Physical Volume ( "d",12 ) = {2};
> 
> len = 0.001;
> Field[1] = Distance;
> Field[1].NNodesByEdge = 100;
> Field[1].EdgesList = {1};
> 
> Field[2] = Threshold;
> Field[2].IField = 1;
> Field[2].LcMin = len*5;
> Field[2].LcMax = 20;
> Field[2].DistMin = 200*len;
> Field[2].DistMax = 30;
> Field[3] = Min;
> Field[3].FieldsList = {2};
> Background Field = 3;
> ---------------------------
> 
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? 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine