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# Re: [femm] Problem in force calculations

- To: femm@egroups.com
- Subject: Re: [femm] Problem in force calculations
- From: Dave Squires <djsquires@plix.com>
- Date: Fri, 26 May 2000 07:43:13 -0700
- References: <8gk94e+dvle@eGroups.com> <392E7CC9.4793D060@foster-miller.com>

    David Meeker wrote:

    > Dave Squires wrote:
    > \Anyhow, there are several things that you might look at that could be
    > influencing things. A lot of these issues are summarized in section 4.10 of
    > the manual. The first is the line over which you perform the integration.
    > Generally, it's a lot more accurate to calculate forces along a contour
    > through the air that surrounds a body, rather than on a line at the surface
    > of the body. The reason is that there can be discontinuities in B and H at
    > the surface of a ferromagnetic or PM body. These sudden changes aren't
    > resolved that well by a first-order triangle formulation. In theory, the
    > integration path doesn't matter so long as it encloses of interest, so more
    > accurate results are obtained by drawing a path through the air, where
    > things are better behaved. The second tip is to always use a relatively
    > fine mesh in the area where perform the integration.

    OK, I did do both of these things, fine mesh in air.

    > Lastly, be careful
    > that you have located your boundaries adequately far away from the magnets.
    > For example, the "default" boundary condition (normal derivative of A=0)
    > functionally behaves like the surface of an iron object.

    This is one thing I wondered about. I did not know how the boundary proximity
    affected things. I suspected it might. If it behaves like iron then I need to
    keep
    it quite long way away from a neodymium magnet. Problem is that the simulation
    time goes up substantially if the mesh needs to be fine all the way out.
    Can I keep the fine mesh in close proximity, say 10 to 20 elements away and
    go to a coarse mesh farther away. Will this affect the force results much?
    I would think it might introduce some small error.

    > Permanent magnets
    > are attracted to the boundary in just the same way as if you'd put a big
    > metal plate there. As long as the boundaries are "far enough" away, there's
    > no problem--the forces from the boundary as small enough that they don't
    > muck things up too much. If the boundaries are too close, they influence
    > the forces of the object of interest in a significant way. (See appendix A3
    > for more stuff about approximating "open" domains).

    BTW, I was using simple rectangular cross sections of equal thickness for
    the magnets and the iron section. I can send you the file if you like.
    I would like to know the conditions that get the right answer so I can
    have a good "feel" for what is the right way to do it when doing other
    simulations.

    Dave Squires

- References:
  - Problem in force calculations
    - From: "Dave Squires" <djsquires@plix.com>
  - Re: [femm] Problem in force calculations
    - From: David Meeker <dmeeker@foster-miller.com>

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