Re: [femm] Force Calculus Methods

[Robert Strand <rstrand@xxxxxxxxxxx>]

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