how to turn off double (rear facing elements)

[abarrie]

When I export a FEM I end up with the elements I wanted, plus a copy of every element in the mesh with a reverse facing normal. How do I turn this behavior off and just export the forward facing elements?

thanks!

 

[potrero]

Can you explain a little more - is your mesh a 2D model? And when you say "export a FEM" do you mean File > Export > Simulation etc?

 

[abarrie]

Yes, I mean that I want to export the mesh for use in another program, such as with export->simulation.

THe problem is that if I have a cube, for example, I would expect taht cube to be comprised of 6 elements - 1 per face with normals facing outward. What I get though is a cube with 12 elements - 2 per face with opposing normals.

I suspect that it it some checkbox somewhere along the lines of 'turn off reverse facing elements' I just dont know where it is.

Thanks!

 

[potrero]

Ahh. I've seen a similar situation when trying to create surface meshes on a solid body that had been split (say, with planes or a surface) into multiple bodies. There can be ambiguity as to the surface normal in such situations. (However, I think that in all cases I've personally seen, where you truly have a stand-alone cube - ie: 6 faces, the surface normals should be unambiguous.)

What you can do to fix the reversed normals is the following:
Analysis > Finite Element Model Check > 2D Element Normals By Seed (select an element) > Apply > Reverse Seed Normal > Apply

Do the above procedure on the meshes which have reversed normals, and you should be able to manually fix your model, ending up with properly facing normals.

Hope this helps you.

 

[abarrie]

the problem is not that the normals are reversed - it is that i have the normal mesh with the correct normals, and then right after that in the output is a duplicate set of elements that have reverse facing normals. so it looks like it is trying to have a back side to the element or something. i dont want that second set of elements to be output at all. So with the cube example, I would be getting 12 elements instead of just 6. The first six would have outward normals and the 2nd 6 would have inward normals, but otherwise be the same elements.

Or are you saying that if there is an ambiguous normal it will duplicate the mesh? for example if the cube has 3 inward normals and 3 outward normals it will not know what to do and just make more elements? If that is the case is there an easy way to identify the problem area?

 

[abarrie]

here is an example of the sort of thing that happens in my mesh. I have an assembly with one central structure and then some parts sticking off of it. The parts sticking off the central structure are separate meshes that do not connect directly to the central mesh. The parts had a cutting plane so they do not intersect the central mesh, but they also do not have a rear element, the side of the small parts facing the central part is open, but the meshes do meet so there are no open spaces. The problem may be that there are some areas of the central mesh pointing into the parts. I tried as best as possible to explain it in the drawing. Does that make sense and might it cause this behavior?

 

[potrero]

I don't think I've ever seen a situation like this, or else (likely) I just can't really understand what you're describing. Can you just post a fem which displays the behavior you're describing?