[Getdp] Re-2: Magnetic force

m.fenner at gmx.net m.fenner at gmx.net
Mon Aug 1 10:29:43 CEST 2005


Dear Christophe,

thank you for the hints. I used an older version of gmsh which had no gradient plugin. Could I ask you again for an example for the Maxwell's stress tensor or virtual works approach?

I am trying to solve the magnetic force problem in a split coil geometry (see the attached files). Unfortunately the body in the gap is not axisymmetric. So I have to go 3D.

Even the basic problem without a body is not solved, using default parameters in solver.par. When I reduece the characteristic length (not touching anything else) I get different error messages from getDP:

lc = 6*mm

P r o c e s s i n g . . .
Operation : Generate[A]
Solver    : Loading parameter file 'solver.par'
Info      : Setting System {A,b} to zero
Resources : cpu 7.801000 s / mem 20180 kb
Operation : Solve[A]
Solver    : No scaling of system of equations
Solver    : RCMK algebraic renumbering
Solver    : N: 18322, NZ: 236746, BW max/avg: 38/12, SW max: 18247
Resources : cpu 8.71000 s / mem 22056 kb
Solver    : ILUTP (Float, fill-in = 20)
Solver    : N: 18322, NZ: 709271, BW max/avg: 40/38, SW max: 2938
Resources : cpu 22.993000 s / mem 27624 kb
Solver    : Generalized Minimum RESidual (GMRES)
    1  Inf  NaN
Solver    : 1 Iterations / Residual: NaN
Resources : cpu 23.43000 s / mem 20264 kb
Operation : SaveSolution[A]
Resources : cpu 23.543000 s / mem 20272 kb
E n d   P r o c e s s i n g


lc = 4*mm

P r o c e s s i n g . . .
Operation : Generate[A]
Solver    : Loading parameter file 'solver.par'
Info      : Setting System {A,b} to zero
Resources : cpu 38.305000 s / mem 66580 kb
Operation : Solve[A]
Solver    : No scaling of system of equations
Solver    : RCMK algebraic renumbering
Solver    : N: 76863, NZ: 988519, BW max/avg: 48/12, SW max: 76756
Resources : cpu 39.616000 s / mem 78216 kb
Solver    : ILUTP (Float, fill-in = 20)
Error     : Zero row encountered in ILU
Stop Fri Jul 29 11:00:26 2005
Runtime: 227 Seconds


Do you have an idea where to search for a remedy?

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
                                   
---------------------------------------------------------------------








-------- Original Message --------
Subject: Re: [Getdp] Magnetic force (08-Jul-2005 22:32)
From:    geuzaine at gmail.com
To:      m.fenner at omicron.de

> Matthias A. Fenner wrote:
> > Am Thu, 07 Jul 2005 22:31:30 +0200 hat Christophe Geuzaine 
> > <geuzaine at gmail.com> geschrieben:
> > 
> >> 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.
> 
> > 
> > 
> > I fear there are not, please correct me if I am wrong.
> 
> I did a quick search and did not find any hits either. I'll try to post
> an example on the wiki if Patrick of Francois don't.
> 
> > 
> >> 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.
> >>
> > I generally use gmsh for pre and post processing. I also tried to use 
> > the Extract and Evaluate plugins to calculate this expression, but 
> > failed to implement the derivative. Could you give a more detailed hint 
> > on how to do this?
> 
> You can use Plugin(Extract) to extract the x-component of B, then apply
> Plugin(Gradient) to compute the gradient.
> 
> Christophe
> 
> > 
> >> Christophe
> >>
> >>
> >>
> >>>
> > Thanks in advance and 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
> > ---------------------------------------------------------------------
> > 
> > 
> 
> 
> -- 
> Christophe Geuzaine
> Applied and Computational Mathematics, Caltech
> geuzaine at acm.caltech.edu - http://geuz.org
> 

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