[GiDlist] Re: GiDlist digest, Vol 1 #406 - 1 msg

[Jeramy Ashlock]

Hi Philippe, I have found the same problem with the circle command. I
needed to create a cylinder as well, and the arc formed by
rotation-extrusion of the vertical sides was different from that obtained
by the circle command. I have learned to use the exact same lines to
generate surfaces that need to be connected.

Furthermore, I have found that a perfectly symmetric geometry with
symmetric surface divisions will not result in a symmetric mesh. I then
tried to create a single "slice" of the cylinder geometry, which I then
meshed and copied by rotation. I have found that there is unacceptable
round off error in rotating the mesh like this, so I now create 1/4 of the
cylinder, mesh it, and copy it by mirroring twice so that the resulting
mesh is actually symmetric. I have asked if there was an option to
increase the numerical precision of the rotation operation, but was told
that generating 1/4 of the mesh and copying by mirroring was the only way
to get a truly symmetric mesh.

I do see this and your problem as small limitations, although I can't
complain because I am using a great program for free. Had I paid for the
license, then I would not be happy, but hopefully this will be addresed in
future versions.


On Fri, 20 Jun 2003 gidlist-request at gatxan.cimne.upc.es wrote:
Hello:

When I use the Geometry | Create | Object | Circle function in GiD, I
obtain an almost closed circle defined by a line and a single point. A
surface is also automatically generated.

I found out that if I remove the surface and split the line in slices (just
like a pizza pie), then the radius of each slice changes. The variations
are actually sufficient to produce spurious results in my FE solvers. So,
obviously, that is a problem.

I found out that if I create my circle using 2 arcs (2 lines and 2 points),
then I can slice these arcs and the numerical errors on the radius become
very small. The same thing for circles generated by rotation extrusion.

Also note that the disc used as the base of the Cylinder object is actually
constituted of two arcs of circle, and does not present the strange
behavior of the single-point-circle.

I want to ask the following:

- Do you agree with my remarks, i.e. the Circle object in GiD is defined in
such a way that it is not a very good representation of a true circle? So
this object should be used carefully if numerical analysis is to be
performed. Maybe you can give me some recommendations about the usage of
that circle?

- Would it be possible to have a new object, a "true circle" that would be
defined with two arcs, just like the base of a cylinder object?

- I believe GiD uses single precision for the definition of objects. Is
this true? Do you see that as a limitation?

Thanks.

Philippe