[Gmsh] clockwise vs counterclockwise

[Bart Vandewoestyne]

Hello list,

I'm working with a 2D geometry and mesh and for my application,
all triangles of my mesh should have counterclockwise node
ordering.

My educated guess was that if the normals to the planes are in
the positive Z-direction, then all triangles in that plane will
have counterclockwise node ordering.  I therefore created the
following .geo file:

http://www.kuleuven-kortrijk.be/~bartv/pml_test_1layer.geo

Note that on line 28 I wrote

  Plane Surface(2) = {1,-2};

in order to get the normal vector for that plane right.

However, it now appears that all elements (triangles) from that
'border layer' have a clockwise ordering instead of
counterclockwise, see the resulting .msh file at

http://www.kuleuven-kortrijk.be/~bartv/pml_test_1layer.msh

(for example element 18 hase nodes 9, 17, 12 which is clockwise).

So apparently, the fact that the normal is in the positive
direction, doesn't seem to assure me that the node ordering of my
elements (triangles) will be counterclockwise???

How can I make sure that my elements (triangles) will *always*
have a counterclockwise node ordering?  Is it related to the
normals of the plane, or is there any other rule to follow?

Regards,
Bart

-- 
	"Share what you know.  Learn what you don't."