Getting element face based on face nodes (quadratic tetrahedron)

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awa5114
Posts: 43
Joined: Thu Jan 26, 2017 5:10 pm

Getting element face based on face nodes (quadratic tetrahedron)

Postby awa5114 » Mon Jul 23, 2018 3:43 pm

Hello

I know from previous posts that the local numeration of element faces for a 4-noded tetrahedron are as follows:

1-2-3 2-4-3 3-4-1 4-2-1

I would like to ask what is the local numeration for a quadratic (10-noded) tetrahedron? I have been looking throught the documentation for this but cannot find it.

PS: My goal is to write a script that gets the element face based on 11 inputs which are as follows:

proc face_elements {ElementNum FaceNode1 FaceNode2 FaceNode3 FaceNode4 FaceNode5 FaceNode6 FaceNode7 FaceNode 8 FaceNode9 FaceNode10}
awa5114
Posts: 43
Joined: Thu Jan 26, 2017 5:10 pm

Re: Getting element face based on face nodes (quadratic tetrahedron)

Postby awa5114 » Tue Jul 24, 2018 7:51 am

I just found them by making 1 quadratic element in GiD and then assigning various face_element conditions to it to determine the local element enumeration. For quadratic elements the faces are numbered as follows:

Face 1 --> nodes 1 2 3 5 6 7
Face 2 --> nodes 2 3 4 6 9 10
Face 3 --> nodes 1 3 4 7 8 10
Face 4 --> nodes 1 2 4 5 8 9

It would be nice if this kind of information was either 1) provided in the GiD documentation or 2) accessible via some sort of tcl command
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escolano
Posts: 1568
Joined: Thu Sep 18, 2014 3:31 pm

Re: Getting element face based on face nodes (quadratic tetrahedron)

Postby escolano » Tue Jul 24, 2018 3:30 pm

The faces of linear elements are described with a picture at the 'GiD customization manual', Appendix B (Classic problemtype)
(find the words GlobalNodes or LocalNodes)
https://www.gidhome.com/documents/customizationmanual/APPENDIX%20B%20(classic%20problemtype%20system)/PROBLEMTYPE%20'CLASSIC'/Template%20files/Commands%20used%20in%20the%20.bas%20file/Multiple%20values%20return%20commands

About the quadratic case: you can consider it to identify faces exactly like the linear case, consider only the 'n fist corner nodes'
e.g. for the quadratic tetrahedra consider only its first 4 nodes, (the local numeration of GiD is hierarchical, first appear the corner nodes, and after the quadratic ones (and for its faces consider only the first 3 nodes, ignore the last 3 quadratic nodes)

the order of local face nodes provided by GiD .bas or Tcl related commands is this for a tetrahedron (see pictures of manual)
face 1: 1-2-3
face 2: 2-4-3
face 3: 3-4-1
face 4: 4-2-1
traversing a face in this order the normal vector (counter-clock wise rule) will point inside the element

Do you want: knowing the list of local nodes of a face obtain the local face index? (e.g a face index from 1 4 for a tetrahedron)
If nodes are provided ordered (with GiD order) then is a little easier,
for example if the face nodes are
2-4-3
then checking only that the first node is ==2 do you know that is the face 2 (the only face starting by this node)
if nodes are provided unordered (without GiD order) then it could be the face 1, 2 or 4, because only the face 3 doesn't has the node 2

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