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## RE: [EXT] Re: compute node normal on exterior surface of mesh

 From: Lesage,Anne Cecile J Subject: RE: [EXT] Re: compute node normal on exterior surface of mesh Date: Fri, 4 Feb 2022 20:46:23 +0000

Dear Konsta

Thank you

Indeed I will go with the faces solution

Just to clarify 0.9 is the minimum criterium on the scalar product of the norm with the direction in bracket

The think the outer faces minus that region aligned with positive x

I can use region_substract right?

Or  m.set_region(99,m.outer_faces_with_direction([-1,0],-0.7))?

Regards

AC

From: Konstantinos Poulios <logari81@googlemail.com>
Sent: Friday, February 4, 2022 2:37 AM
To: Lesage,Anne Cecile J <AJLesage@mdanderson.org>
Cc: getfem-users@nongnu.org
Subject: [EXT] Re: compute node normal on exterior surface of mesh

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Dear Anne-Cecile

It is easy to get averaged normals on a mesh_fem:

m=gf.Mesh("load", "ladder.mesh")
md=gf.Model("real")
m.set_region(99,m.outer_faces())
mf=gf.MeshFem(m,3)
mf.set_classical_fem(1)
U=md.interpolation("Normal",mf,99)
mf.export_to_vtu("normals.vtu", mf, U, "Normals")

you can then interpolate U on some given points with computer_interpolate_on.

However, I would rather recommend a solution with faces rather than points. Averaging unit vectors leads in general to non-unit vectors. Moreover, points in general do not make sense in a physical world. Surfaces and volumes do. I would recommend you to use the outer_faces_with_direction function. For example with

m.set_region(99,m.outer_faces_with_direction([0,1,0],0.9))

you can define a region with outer normals pointing approximately in the positive Y direction.

Best regards

Kostas

On Thu, Feb 3, 2022 at 12:22 AM Lesage,Anne Cecile J <AJLesage@mdanderson.org> wrote:

Dear all

I see in the python documentation that  the mesh class has a function normal_of_face

I would like to compute the mean normal at node points (mean of the faces normal to which this point belong) on the exterior surface of the mesh

I have the list of those points id

This is to do a direction test for setting boundary conditions

Thank you

Regards

Anne-Cecile

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