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# Re: Re: discontinuities in stress tensor mask in thin air

- To: femm@yahoogroups.com
- Subject: Re: Re: discontinuities in stress tensor mask in thin air
- From: "mikeshonle" <mike@psychonic.net>
- Date: Mon, 10 Mar 2003 19:52:58 -0000
- In-reply-to: <3E6C04F1.1040107@ieee.org>
- User-agent: eGroups-EW/0.82

    Hi, Jim & Dave,
    Thanks for your responses. Sorry it's taken me a while to respond-
    - doing some of these tests takes all night, and then if you find a
    mistake...

    Anyway, I've been doing some more experiments with varying the mesh
    spacing, and it seems to make a relatively large difference in the
    calculated force.

    In one set of experiments, I varied the mesh size in the region just
    outside the inner region (which has both magnets), and depending on
    what values I used, it seems to let more of the force
    tensor "escape", which changes the force:

    Inner spacing outer spacing force
    .001 .005 1.208e-3
    .001 .003 0.951e-3
    .001 .002 0.902e-3

    I also varied the inside spacing while holding the outside constant:

    .0017 .005 1.125e-3
    .001 .005 1.208e-3

    Is this what should be expected, or could I have something setup
    incorrectly?

    Thanks again,

    -Mike

    --- In femm@yahoogroups.com, David Meeker <dmeeker@i...> wrote:
    > James Rabchuk wrote:
    >
    > > As I understand from Dave's description of the technique, the
    mesh
    > > density serves as a type of permeability in the solution of
    > > the differential (Laplaces?) equation which sets the level
    contours,
    > > the gradient of which becomes the weighting function for the
    volume
    > > integral carried out in the air over the divergence of the stress
    > > tensor. The abrupt change in permeability between regions of
    different
    > > mesh density leads to abrupt changes in the "mask". Regions of
    higher
    > > density will have a greater percentage of the level contours.
    > >
    > > But how about your force values? Were they more realistic,
    smoother, etc.?
    > >
    > > Jim
    > >
    > >
    > The way that things are currently set up, the program solves a
    Laplace
    > equation to get the "mask." It uses the inverse of the mesh size
    > specification sort of like a "permeability," so that more highly
    meshed
    > regions tend to contribute more to the force calculation than less
    > highly meshed regions. The lines can change directions at
    boundaries
    > between mesh densities--this is analogous to the change in
    direction of
    > flux lines at the interfaces between materials of different
    > permeabilities. I don't think that this should particularly cause
    any
    > problems with the integration.
    >
    > mikeshonle wrote:
    > > I'm using version 3.3a3. And what's a good practical upper
    > > limit on the number of mesh nodes?
    >
    > The "practical" upper limit depends on how much memory you have.
    If the
    > problem is so big that you start running off of virtual memory,
    things
    > can bog down a lot--that's really the upper limit.
    >
    > Dave.
    >
    > --
    > David Meeker
    > dmeeker@i...
    > http://femm.berlios.de/dmeeker

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