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# Re: [femm] Force calculation on an axisymmetric model

- To: femm@egroups.com
- Subject: Re: [femm] Force calculation on an axisymmetric model
- From: David Meeker <dmeeker@foster-miller.com>
- Date: Mon, 09 Oct 2000 11:28:11 -0400
- Organization: Foster-Miller, Inc.
- References: <3.0.5.32.20000929182911.00881400@sun.isec.pt> <3.0.5.32.20001006164352.008e5220@sun.isec.pt> <39DE046F.E1F4051D@foster-miller.com> <200010061834.QAA19402@if.ufrj.br> <39E1CDE2.2B958851@foster-miller.com> <200010091408.MAA12224@if.ufrj.br>

    Agnaldo Souza Pereira wrote:

    > The greater forces arised from femm calculations, with
    > a greater number of nodes.

    There's probably no way to tell a priori whether the predicted force result
    is too high or too low, compared to an exact solution.

    Anyhow, the example in my previous message was for two magnets in
    attraction, rather than in repulsion as in your problem. That is, the
    forces quoted are in a direction that tends to pull the magnets together.
    To treat the problem where the magnet are in repulsion, you just flip one of
    the magnets. The analytical solution predicts exactly the same magnitude of
    force, but with the force acting in repulsion rather than attraction..

    To see what happens, I re-ran the same case as the previous message with the
    magnets in repulsion. In this case, the force (in a direction so that the
    magnets tend to repel each other) is:

    Analytical solution: 0.02486 N (same as before)
    Force from femm 3.0 using a mesh of about 6000 nodes: 0.02479 N
    Force from Quickfield 3.4a using a 499 node mesh: 0.011737 N

    The same integration contours were used as in the "magnets in attraction"
    case, to keep the comparison consistent. With the magnets changed so as to
    be in repulsion, the quickfield result is about 53% low, as compared to the
    analytical solution, whereas the femm solution (with a much higher mesh
    density) is still within 1% of the analytical result. This seems consistent
    with your result.

    Again, I guess the point is that if you want accurate force calculations,
    use a lot of elements. You can run multiple cases with increasingly fine
    mesh densities to check for convergence of the force result.

    Dave.

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    x-mozilla-html:TRUE
    url:http://members.aol.com/dcm3c
    org:Foster-Miller, Inc.;Electrical and Electronic Systems Group
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    email;internet:dmeeker@foster-miller.com
    title:Senior Engineer
    adr;quoted-printable:;;350 Second Avenue=0D=0A;Waltham;MA;02451-1196;USA
    fn:David Meeker, Ph.D.
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- References:
  - Force calculation on an axisymmetric model
    - From: Paulo José Gameiro Pereirinha <ppereiri@sun.isec.pt>
  - Re: [femm] Re: Force calculation on an axisymmetric model
    - From: Paulo José Gameiro Pereirinha <ppereiri@sun.isec.pt>
  - Re: [femm] Re: Force calculation on an axisymmetric model
    - From: David Meeker <dmeeker@foster-miller.com>
  - Re: [femm] Re: Force calculation on an axisymmetric model
    - From: Agnaldo Souza Pereira <agnaldo@if.ufrj.br>
  - Re: [femm] Force calculation on an axisymmetric model
    - From: David Meeker <dmeeker@foster-miller.com>
  - Re: [femm] Force calculation on an axisymmetric model
    - From: Agnaldo Souza Pereira <agnaldo@if.ufrj.br>

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