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# Re: [femm] boundary conditions

- To: femm@egroups.com
- Subject: Re: [femm] boundary conditions
- From: David Meeker <dmeeker@foster-miller.com>
- Date: Thu, 13 Apr 2000 09:40:23 -0400
- Organization: Foster-Miller, Inc.
- References: <38F4AE1E.596EF8EC@cybertours.com>

    Steve Ryder wrote:

    > Hello,
    > I've been trying to model an alnico V slug (.187" dia. by .750" long)
    > for a guitar pickup and keep getting an unlikely field pattern. The
    > missing component could be boundary conditions but I'm at a loss as to
    > how to define them. Any hints would be helpful.

    Eckart's suggestion ("add a closed rectangular frame all around the
    geometry, with sufficient distance for leakage fields, and set A=0 on this")
    ought to work fine. However, I have a sneaky suspicion that boundary
    conditions are not the issue with your problem. You have to define the
    orientation of the magnet as part of the material properties. If you just
    pull the entry for Alinco 5 out of the materials library, the default is
    that the magnetization direction is 0 degrees. You need to change the
    direction to 90 degrees (or -90 degrees) to get the right results for your
    problem (so that the magnetization is pointing along the magnet's axis).

    Since it's only about 600 bytes, I've attached a zipped sample model of your
    magnet. As far as the boundary conditions, I have made the edge of the
    domain a half-circle, and applied a "mixed" boundary condition to this
    edge. The parameters for this boundary condition are specified in eqs. (42)
    and (43) on p. 49 of the manual. This boundary condition is meant to mimic
    the influence of the air outside the solution domain so that the solution
    that you get looks like the field of a magnet in unbounded free space (an
    asymptotic boundary condition). Along the axis, A=0 is automatically
    applied by the program (so you don't have to define a boundary condition
    explicitly here).

    Dave.

Attachment: `zip00000.zip`
Description: Zip compressed data

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    org:Foster-Miller, Inc.;Electrical and Electronic Systems Group
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- References:
  - boundary conditions
    - From: Steve Ryder <strydah@cybertours.com>

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