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Ahmet,
Sorry
for the intense mesh. I was getting a little frustrated trying to smooth things
out and especially to get the coenergy method to work. Never did, for the
reasons Dave pointed out. (Here I just want to express my appreciation again to
Dave for the amazing help and consultation given with this
program!!!)
However, Dave recently posted a revised version of FEMM, v. 3.3a, with a
weighted stress tensor calculation. What an improvement! What was a choppy but
noticeable trend in the force with height of the lower magnet (even with an
intense mesh density in the region of interest) became a smooth and
absolutely clear force vs. height curve. Absolutely on a par with the best of
the coenergy results. I'll post the results in a bit. We're still generating the
graphs.
(I do have a question for Dave, here. I shouldn't be lazy,
and just look at the code, but I'll ask the source. In the implementation of the
weighted stress tensor, are you doing a volume integral over the entire air
region of the model? That seems to be what's implied from your description and
the reference you posted. Also, you mentioned that you weight elements in more
tightly meshed regions more than you do elsewhere. I wasn't quite sure how that
would work. If that applies over a single contour, then don't you need to keep
the integration over the entire contour at the same weight, in order to get
the proper "sum" of momentum flux across the
boundary?)
As far
as your question goes, the graph shows force with height. I've accounted for the
constant force of gravity by subtracting it from the Femm result in my post-lua
script before writing it to my output file. Equilibrium means the force is zero.
Stable equilibrium means the force below the equilibrium point is up, the force
above the equilibrium point is down (negative). For the diamagnetic materials
available, the plate separation needs to be quite small for the equilibrium
point to become stable. But the program works beautifully and, having matched
the material properties as closely as I could to the experimental setup, the
agreement between prediction and observation is excellent.
Thanks
again for a great tool and great support for it.
Dear Jim, I just noticed your work and had some
problem running your code due to intense mesh. Any way in your stress tensor
simulation how are you choosing stable equilibrium position. The excel file
you attached does not give much detail. I am also working on same problemand
apply the amperian current approximation
(J.App.Phys.volume:86,page:6396,year:1999) to the diamagnetic system. Thanks
for any information you provide.
best regards
Dr.Ahmet Cansiz
----- Original Message -----
Sent: Thursday, January 30, 20039:02
PM
Subject: [femm] coenergy vs. stress
tensor calculations of magnetic force
Hi, I've written before about making a simulation
of a levitation device with diamagnetic materials in FEMM. I
was interested in determing the equilibrium position of the levitating
magnet by calculating the force for a number of positions between the
diamagnetic plates.
Following the advice of the FEMM manual, I
used the coenergy method for calculating the field energy, and then looking
for the force from that. But what I found was that the Coenergy displayed no
observable trend with height of the magnet. On a lark, I tried the same
simulation and calculated the Force from the stress tensor, using a fixed
contour. Surprisingly, I got very believable results that showed the stable
and unstable equilibrium points for the levitating magnet quite nicely.I
was content with that, but I'm still intrigued why the coenergy method
doesn't seem to be working in this case. We tried moving the outer boundary
to increase the area for the problem. I tried increasing the mesh density in
the region where the force on the magnet is being calculated. But none of
those things seems to be able to get the coenergy method to provide
meaningful results. I might be tempted to say that my stress tensor
calculations are just misleading, but they mirror so nicely the behavior of
the actual system that I find that hard to believe.
Would anybody be willing to take a look at the
simulation I've put together and make some suggestions as to where the
coenergy calculation might be messing up, or where I could improve the model
to get the "right" behavior?
I've attached the femm file, the associated pre and
post lua files, and an excel file with the coenergy numbers, and graphsof
both the coenergy and stress tensor calculations of the force with height
(gravity is included manually).
If
you look at the Excel file, you can't possibly discern a pattern of behavior
from the coenergy graph, while the stress tensor graph is pretty good,
though not as smooth as you might like.
Any help would be appreciated.
Thanks,
Jim
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