# outward normal

 Register Blogs Members List Search Today's Posts Mark Forums Read

 July 22, 2004, 00:08 outward normal #1 Prem Venugopal Guest   Posts: n/a Hello, I have a boundary surface discretized using triangular elements. I would like to compute the outward normal to each triangular element. Although, I was able to compute the normal I am not able to check whether the normal points inwards or outwards. How does one check this normally ? Thanks, Prem

 July 22, 2004, 04:07 Re: outward normal #2 Rami Guest   Posts: n/a This is determined by the order of the vertices of the volumetric cell to which this trinagular face belongs. To make it more concrete, suppose you deal with a tetrahedron, with its boundary surface defined by vertices labeled a, b, c and the off-boundary vertex labeled q. Let us define Vab as the vector from a to b, etc. Also assume that the order of vertices is such that q is located at the same direction of face a-b-c as the vector product Vab x Vbc (which is also twice the face area). The the outward normal is therefore the opposite direction, i.e., -Vab x Vbc (and you may wish to normilize it by its magnitude). The same argument holds for other cell types (e.g., prisms). I hope this helps.

 July 22, 2004, 10:37 Re: outward normal #3 Prem Venugopal Guest   Posts: n/a Thanks for the information, Rami. So without the off boundary node I have no-chance of determining whether the normal points inwards or outwards ? The data that I have contains vertex information for ONLY the boundary nodes. Also for a closed surface, shouldn't the off-boundary node be located inside the surface ? Thanks, Prem

 July 22, 2004, 15:12 Re: outward normal #4 Abhijit Guest   Posts: n/a Hi, I guess you could also use centroid of the triangle /tetrahedron. In my case what I did was to check the sign of dot product of unit normal and unit vector along line joining centroid and mid pt. of the boundary face. It works in all, but one case... both the vectors could be perpendicular....does'nt happen in 2D but maybe in 3D. Regards, Abhijit

 July 25, 2004, 02:35 Re: outward normal #5 Rami Guest   Posts: n/a Unless the surface is closed, I think it is undetermined what is "inside" and "outside" when only surface verices are given. If you have only surface vertices and the surface is closed, you may find algorithms for finding the outwards normal. I never did it myself, though, so I can just think of a very simple and inefficient solution. If you are that desparate, I'll put it forward at your request. I hope you'll come up with better ideas.

 July 25, 2004, 19:35 Re: outward normal #6 Prem Venugopal Guest   Posts: n/a Rami, Could you tell me your solution, please ? I am desperate.... Thanks, Prem

 July 26, 2004, 04:20 Re: outward normal #7 Rami Guest   Posts: n/a OK, if you are that desperate... Take the candidate normal you wish to examine. Find all its intersections with all other facets of the boundary surface and count them. If the number of intersections is even (including 0), the normal is outwards. As I mentioned, this is meaningful only for closed surface and probably very inefficient. Maybe someone else has a better idea?

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post g_niro Main CFD Forum 0 February 2, 2011 18:24 xiaoyoyo FLUENT 1 January 31, 2011 09:46 rcastilla OpenFOAM Meshing & Mesh Conversion 2 January 6, 2010 02:30 Franny Main CFD Forum 13 July 7, 2007 15:57 Prem Venugopal FLUENT 0 July 21, 2004 23:55

All times are GMT -4. The time now is 07:14.