# Re: [Gmsh] boundary layer disconnects at the corner of geometry(stl)

```Hi Christophe,

Thanks for the prompt reply. I have tried the Extrude option for boundary
layer. It works perfect. But the normal direction is the issue.```
```
I am working on a project where my input geometry is always in .stl format
and I create compound surfaces to generate surface mesh. During this
process, the normal direction is disturbed or random, even if I provide the
input stl with normal facing inward. I can add a -ve sign for the outward
facing surfaces, while extruding boundary layer.

But I want to write a generic code that works fine for all the stl
geometries I import. Is there a way to re-orient all the surface normals
inwards?

Best regards
Praveen

On Thu, Sep 15, 2016 at 1:13 PM, Christophe Geuzaine <cgeuza...@ulg.ac.be>
wrote:

>
> Dear Praveen,
>
> The Boundary Layer field was an experiment: the 3D version was not
> functional and has been removed in recent versions. The 2D version is
> fairly usable. New implementation of boundary layers are forthcoming. In
> 3D, I currently advise simply using the Extrude command: it will not create
> fans, but for simple geometries like yours it should be sufficient.
>
> Here's an example:
>
> // create a square
> lc = 0.3;
> Point(1) = {0.0,0.0,0.0,lc};
> Point(2) = {1,0.0,0.0,lc};
> Point(3) = {1,1,0.0,lc};
> Point(4) = {0,1,0.0,lc};
> Line(1) = {4,3};
> Line(2) = {3,2};
> Line(3) = {2,1};
> Line(4) = {1,4};
> Line Loop(1) = {2,3,4,1};
> Plane Surface(1) = {1};
>
> // create a cube
> Extrude {0,0.0,1} { Surface{1}; }
>
> // remove the volume
> Delete { Volume{1}; }
>
> // extrude the boundary of the cube inwards by 0.05, with 5 layers of
> elements
> Extrude { Surface{26, -1, 13, 17, 21, 25}; Layers{5, 0.05}; }
>
> // create the inner volume
> Surface Loop(159) = {92, 158, 70, 48, 136, 114};
> Volume(160) = {159};
>
> General.ExpertMode = 1; // disable warning about mixing Delaunay and
> extrusion
>
>
>
> > On 14 Sep 2016, at 10:52, Praveen Kumar <prav...@cctech.co.in> wrote:
> >
> > Hi,
> >
> > I am trying to generate boundary layers on all the surfaces of a simple
> rectangular duct geometry (imported stl). The prism layers are not getting
> connected on the edges/corners between each surfaces.
> >
> > <BL-issue.JPG>
> > ​
> >
> > Can someone help me understand what is missing here? Please Note, I am
> using 2.11.0 version of gmsh as this is not working at all in the latest
> version
> >
> > --------------------------------------
> > Merge "duct.stl";
> > CreateTopology;
> >
> > ll[] = Line "*";
> > l = newl;
> > For j In {0 : #ll[]-1}
> >     Compound Line(l+j) = ll[j];
> > EndFor
> >
> > ss[] = Surface "*";
> > s = news;
> > For i In {0 : #ss[]-1}
> >     Compound Surface(s+i) = ss[i];
> > EndFor
> >
> > bl = newl;
> > bs = news;
> >
> > Field[1] = MathEval;
> > Field[1].F = "0.1";
> >
> > Background Field = 1;
> >
> > Mesh.CharacteristicLengthExtendFromBoundary = 0;
> >
> > Physical Surface("Side-01") = {s};
> > Physical Surface("Top") = {s+1};
> > Physical Surface("Side-02") = {s+2};
> > Physical Surface("Outlet") = {s+3};
> > Physical Surface("Bottom") = {s+4};
> > Physical Surface("Inlet") = {s+5};
> >
> > sl = newsl; Surface Loop(sl) = {s:s+5};
> > vo = newv; Volume(vo) = {sl};
> >
> > //Physical Volume("V_Fluid") = {vo};
> >
> > pp[] = Point "*";
> >
> > Field[2] = BoundaryLayer;
> > //Field[2].AnisoMax = 60;
> > Field[2].FacesList = {s:bs-1};
> > Field[2].EdgesList = {l:bl-1};
> > Field[2].NodesList = {pp[0]:pp[7]};
> > //Field[2].FanNodesList = {pp[0]:pp[7]};
> > //Field[2].IntersectMetrics = 0;
> > //Field[2].Quads = 0;
> > //Field[2].hfar = 0.1;
> > Field[2].hwall_n = 0.025;
> > //Field[2].hwall_t = 0.1;
> > Field[2].ratio = 1.1;
> > Field[2].thickness = 0.05;
> > BoundaryLayer Field = 2;
> > //Background Field = 2;
> >
> > ----------------------------------------------
> >
> > --
> > Praveen Kumar R | Technical Manager
> > Centre for Computational Technologies - CCTech
> > 1 Akshay Residancy | 50 Anand Park | Aundh | Pune | 411007 | India
> > Ph: +91 20 40098381/82 | M: +91-9923406816 | www.cctech.co.in
> > CCTech – "Transforming human life by democratization of technology"
> >
> >
> > <duct.geo><duct.stl><BL-issue.JPG>__________________________
> _____________________
> > gmsh mailing list
> > gmsh@onelab.info
> > http://onelab.info/mailman/listinfo/gmsh
>
> --
> Prof. Christophe Geuzaine
> University of Liege, Electrical Engineering and Computer Science
> http://www.montefiore.ulg.ac.be/~geuzaine
>
> Free software: http://gmsh.info | http://getdp.info | http://onelab.info
>
>

--
Praveen Kumar R | Technical Manager
*Centre for Computational Technologies - CCTech *
1 Akshay Residancy | 50 Anand Park | Aundh | Pune | 411007 | India
Ph: +91 20 40098381/82 | M: +91-9923406816 | www.cctech.co.in
*CCTech – "*

*Transforming human life by democratization of technology"*
```
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