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# Re: [femm] Force Calculus Methods

- To: femm@yahoogroups.com
- Subject: Re: [femm] Force Calculus Methods
- From: Robert Strand <rstrand@ihug.com.au>
- Date: Sun, 21 Jul 2002 15:21:49 +1000
- References: <ah49p3+u0tv@eGroups.com>
- User-agent: Mozilla/5.0 (Windows; U; Win98; en-US; rv:0.9.4.1) Gecko/20020314 Netscape6/6.2.2

`laurent_vp_fr wrote:`

> `Hello,
> I'm trying to evaluate the force acting on a solid in a magnetic field and I don't obtain similar results with the Maxwell tensor and the energy (or coenergy) variation.
> Is there someone who can explain that or who have meet this problem ?
> Thanks,
> Laurent VP.`

`
The basic problem is force computation is sensitive to small changes in the numerical results. There is a section in the manual which gives some recommendations.`
`I suggest you try doubling your mesh density in the appropriate areas to see if the results change significantly. If either of the results change you should try doubling it again until you get a stable result.`
`For the co-energy method try using larger or smaller pertubations to see if your result is stable. You can also try pertubing the system in both a positive and a negative direction then compute:`
`F = (Wc(p+dp) - Wc(p-dp))/(2 dp)`
`This method is likely to be more accurate and less susceptable to numeric problems (you can also use larger dp's).`
`For the Maxwell tensor method follow the guide-lines in the manual. If you still have problems try defining three close contours (similar to Fig 27 in the manual). Define a fine mesh in the two regions enclosed by the coutours and use the central contour for the integration. Double and halve the mesh densities in these regions to check stability of the results. If you are still having problems try to moving the three contours away from surfaces which have rapid changes in the field magnitude - I've found cases where increasing the mesh had little effect and only this produced a good result.`

    Regards
    Rob

``
``

- References:
  - Force Calculus Methods
    - From: "laurent_vp_fr" <laurent_vp@orange.fr>

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