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Subject: Re: PowerGraphics
Author: Scott Reiss
Date: 1999-08-09 14:20:00

John, in general I wholeheartedly agree with your assessment of power
graphics....however there is one issue about power graphics which bothers
me. I commented on this previously and got no feedback either because no one
felt it was a concern or because no one understood what I was talking about.
I don't mean to belabor the point, but my objection is that power graphics
results (both solid and shell) are tied to the angle settings on the /edge
command. I use the angle field of the /edge command to remove edges from
displays for clarity especially when looking at and plotting results. I'm
sure many of you do this. Unfortunately, power graphics uses this angle
value to decide whether element faces are coplanar and then averages results
only on those faces which it considers coplanar. The net result is that by
turning edges on or off (using a different angle setting in /edge) one can
inadvertantly vary results depending on the face angles. The ability to vary
averaging based on edge angle would be a desirable feature if it wasn't
"tied" to a graphics command like /edge. Second, I understand why it affects
shell model results, but I don't understand why it is correct to affect
solid model results. Third, I am not particularly confident in the algorithm
used in /edge. My experience has been that on occassion certain edges will
plot which should not according to the angle I have set. Other times the
opposite is true. Perhaps it is a graphics glitch or something, but in any
case it gives me an unsettled feeling knowing that the graphical results
values depend upon this algorithm. This is my personal opinion and not meant
to influence others. However, I would be interested to hear what other folks
think.

-----Original Message-----
Sent: Monday, August 09, 1999 6:02 AM
To: ANSYS Users List

Why are some people unhappy with PowerGraphics? My own experience has
been that PowerGraphics is more accurate than Full Graphics for any case
where the highest stress is on the exterior of the body, and it's faster
too. PowerGraphics also displays top and bottom stresses on shell
elements at the same time, which is kind of nice. My only complaint is
that PowerGraphics doesn't give the SBMN and SBMX values on the contour
displays. When doing postprocessing of tet element models, I'll
occasionally switch from PowerGraphics to Full Graphics as a way of
evaluating how adequate my mesh is. If the answers are signficantly
different I will either refine the mesh, recognize that the discrepency
is due to a singularity and ignore the results there, or I'll document
that there is significant uncertaintly regarding the accuracy of the
results obtained in this region. Hopefully, by switching between
PowerGraphics and Full Graphics, plotting the SDSG (or TDSG) values,
comparing PLNS and PLES results, and reviewing SBMN and SBMX (when Full
Graphics is on) I can ascertain how much meshing error is present in my
results. It's not enough for me to provide an answer. I also have to
provide an estimate of how accurate I think these answers are.

Which brings me back to PowerGraphics. Are people upset about
PowerGraphics because it frequently yields higher anwers for tet
elements? If so, is this a legitimate complaint or is it a case of
shooting the messenger? I've heard one or two comments regarding
turning PowerGraphics off which make perfectly good sense, such as when
initially reviewing subsurface stresses like those found in Hertz
contact problems. But for the vast majority of cases I've found
PowerGraphics to be faster and more accurate than Full Graphics.

There are special cases when I'll use Full Graphics, such as comparing
new results to prior results which were postprocessed using Full
Graphics. But if speed and accuracy are of interest to me, I use
PowerGraphics.

Maybe I've missed something. Would anybody care to comment why they
prefer Full Graphics over PowerGraphics? Perhaps someone can share an
insight that will make us all a bit better at our chosen craft.

Thanks.

John Crawford
AlliedSignal Engines
Phoenix, AZ
john.crawford@a...

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