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# Re: [femm] "Small skin depth" Impedance Boundary Condition

- To: femm@yahoogroups.com
- Subject: Re: [femm] "Small skin depth" Impedance Boundary Condition
- From: David Meeker <dmeeker@foster-miller.com>
- Date: Wed, 04 Apr 2001 10:40:08 -0400
- Organization: Foster-Miller, Inc.
- References: <3AC45B25.538223D1@iai.csic.es> <3AC4BCE4.A4F81827@foster-miller.com> <3ACAE591.EEABE695@iai.csic.es>

    Fernando Seco wrote:

    > Hi David & FEMM people,
    >
    > When you discuss Boundary Properties in the FEMM User's
    > Manual and you come to the Small Skin Depth, it is stated that:
    >
    > dA/dn + (1+j)/delta * A = 0 (eq 19)
    >
    > I feel that it should be:
    >
    > dA/dn + (1+j)/(mur*delta) * A = 0,
    >
    > with mur = mu(metal)/mu0 being the relative permeability of the
    > metal. This comes from the continuity of the tangential component
    > of the magnetic intensity, Ht.
    >
    > How do you feel about it? Greetings from Spain,

    Yes, you are right--thank you for catching this. Fortunately, this appears
    to be just a typo in the manual. What I've actually implemented in the code
    is the boundary condition

    (1/mu) dA/dn + (1+j)/(1/mu_ssd*delta) * A = 0

    where mu is the permeability of the material inside the boundary, and mu_ssd
    is the permeability of the small skin dept material. If the material inside
    the boundary is air, mu = permeability of free space, and the BC becomes just
    as you had said. The way that it is implemented now also gives a consistent
    result if, say, the material bordering on the small skin depth BC were
    laminated iron where permeability is high but the eddy currents might be
    negligible.

    Anyhow, thanks again for catching this--I will make the correction to the
    manual.

    Dave.

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    title:Senior Engineer
    adr;quoted-printable:;;350 Second Avenue=0D=0A;Waltham;MA;02451-1196;USA
    fn:David Meeker, Ph.D.
    end:vcard

- References:
  - Electric circuits vs. material current density
    - From: Fernando Seco <fseco@iai.csic.es>
  - Re: [femm] Electric circuits vs. material current density
    - From: David Meeker <dmeeker@foster-miller.com>
  - "Small skin depth" Impedance Boundary Condition
    - From: Fernando Seco <fseco@iai.csic.es>

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