[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

# Re: [femm] Re: discontinuities in stress tensor mask in thin air

- To: femm@yahoogroups.com
- Subject: Re: [femm] Re: discontinuities in stress tensor mask in thin air
- From: David Meeker <dmeeker@ieee.org>
- Date: Fri, 14 Mar 2003 19:38:52 -0500
- In-reply-to: <b4rf33+gev8@eGroups.com>
- References: <b4rf33+gev8@eGroups.com>
- User-agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.3b; MultiZilla v1.1.34 (d)) Gecko/20030210

mikeshonle wrote:
> Hi, Dave, thanks for your detailed analysis and response. I have
> been doing some more experimenting, and have come up with the
> following:
> First, I tried to replicate your results for the axisymmetric case,
> and I got exactly the same number using the line integral method but
> doing the WST method, I got .65 instead of .59.
I tried this again and still got 0.59. I suspect that you might have used a different mesh size than I did in the magnets. What this highlights is that the force results are, with the weighting function from the 3.3a3 version, pretty sensitive to how the magnets are meshed.
> Also, I went back and used the line integral in my test cases, and
> got much more consistant (vs. changing mesh spacing) results (within
> about 2% instead of 30% !). I also re-ran my moving magnet
> experiment and it was completely monotonic and almost completely
> monotonic in the derivative as well.
In this case, there is a clearly "good" place to put the contour for the line integral, but this is often not the case. It's fairly easy to get bad results via the stress tensor line integral in some other problems. It's also a pain to manually draw the integration contour.
The idea of the "weighted stress tensor" was to simplify the mechanics of performing the computation and to give better accuracy. I think (or at least hope) that the weighting method from the 3.3a3 version is easier to use than the line integral, and it has given good results in all of the motor and solenoid cases that I had tried (e.g. the cogging example that I had posted a few days ago). However, it doesn't seem to work that well on your "open-circuit" problem because it is incorporating less accurate stress tensor results from near the sides of the of the magnet.
I did a little head-scratching about this, and I've come up with a different way to do the weighting that seems to avoid problems in the open-circuit case, while giving at least as good results in the motor and solenoid cases that I've considered. One basically wants to avoid all non-air regions as much as is possible, while staying in the most densely meshed air region. What seems like a good way to do this is to take the mask that I was computing and perform an additional post-processing step in which all of the nodal values of the mask that are greater than 1/2 are set to 1, and all other values to zero. This limits the integration area to a single layer of elements that tends to run down the middle of any air gaps, automatically staying as far away as it can from any non-air regions.
Anyhow, if you want to check out this approach, I've changed the version of 3.3 that is on the website at http://femm.berlios.de/femm33bin.exe and http://femm.berlios.de/femm33src.zip to include this modification to the way that the stress tensor mask is computed. On the axisymmetric example problem that we'd been considering previously, I get the following force results when I change the mesh size in all blocks uniformly:
Mesh size, number of nodes, wst force on upper magnet, time required to run the solver on a 2.4Ghz machine:
0.5 682 -0.6774159 <1s
0.25 2555 -0.6214242 <1s
0.125 10151 -0.5984702 ~1s
0.0625 40115 -0.5940128 ~8s
0.03125 160069 -0.5930197 ~59s
0.015625 638902 -0.5925622 ~7:17
The wst results on the cogging example from http://groups.yahoo.com/group/femm/message/1804 are about the same with this revised wst scheme as with the previous one.
Unless anyone turns up any particularly egregious bugs, I think that this one will be the 3.3 "release" version. The only other difference from the 3.3a3 version is that I added the ability to display the name of the material applied to each block label on the geometry--this is one of those "gee, why didn't I do that a long time ago" features that was suggested to me recently. You can turn it on and off via View|Show Block Names selection or set a default for it in the Preferences.
> Also, B.T.W, would you happen to know what the density of NIB 37 is?
How about 0.269 lb/in3 for Crumax 3714 from Crucible Magnetics.
Dave.

    --
    David Meeker
    dmeeker@ieee.org
    http://femm.berlios.de/dmeeker

- References:
  - Re: discontinuities in stress tensor mask in thin air
    - From: "mikeshonle" <mike@psychonic.net>

- Prev by Date: Re: [femm] non linear stainless steel meterial
- Next by Date: Re: calulation of the magnetic flux
- Previous by thread: Re: discontinuities in stress tensor mask in thin air
- Next by thread: non linear stainless steel meterial
- Index(es):
  - Date
  - Thread