[Getdp] Magnetic force

[Christophe Geuzaine]

Matthias A. Fenner wrote:
> Dear Christophe,
> 
> I am trying to calculate the force on a paramagnetic body by f = chi/mu_0 
> (B * nabla) B. Is it possible to evaluate something like Grad[CompX[{d 
> a}]] in the post processing of a magnetostatic problem? My first straight 
> forward attempts to do so failed. Is there a hack to get it? 

Not directly in getdp: there is no "general" (numerical) implementation
of differential operators, only support for those which are implemented
as basis functions.


> If not: is 
> there an alternative approach to calculate the force (-density)?

Yes, there are nice ways to compute the force (using virtual works or 
Maxwell's stress tensor). I'm not sure if there are examples on the 
list. If not, we should post an example on the wiki. Francois and 
Patrick have probably ready-to-use .pro files for this.

If you want to use the formula you mentioned above, you could also use 
some external tools (like Gmsh) to directly evaluate the expression 
numerically.

Christophe



> 
> Best Regards
> 
> Matthias
> 
> 
> ---------------------------------------------------------------------
>        Matthias A. Fenner
> 
>        email: m.fenner at gmx.net                     Hugo-Eckener-Str. 94
>        fon: +49 6131 622 10 60                     D-55122 Mainz
> 
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-- 
Christophe Geuzaine
Applied and Computational Mathematics, Caltech
geuzaine at acm.caltech.edu - http://geuz.org