[Getdp] calculate magnetic forces?

[Kubicek Bernhard]

Hello!
In "On Movements of Small Ferromagnetic Particles in Inhomogeneous Magnetic Fields",Friedrich Blaha, 1950 Proc. Phys. Soc. B 63 12-14, the force is given proportional to grad(H^2) (in case this is valid, probably this is a _bit_ outdated).
I would expect that the force is equal to the gradient of the total energy of the magnetic field, that the polarised ferromagnetic sphere creates due to the locally homogenous field, in addition to the causing field.

Your force is definitely a force density in Newton/m^3 (if everything is in SI). However, if you calculate the via magnetic scalar potential, it is not possible to calculate curl(B) in GetDP (and most other FEM codes), as it is a second order differential of the potential, and only first order is allowed, because the second differential of a picewise linear function is either zero or singular in the volume, and thus quite useless.

You could e.g. import h^2 as a scalar field into your CFD software as cell values, and perform numerical differentiation there. Or, also some postprocessors have a grad plugin.
BTW: We have inhouse-codes, that couple GetDP and Fluent and can do mesh and data exchange, however I am (sadly) not allowed to give them away royalty free. In case you use Fluent, and want some further information, feel free to contact me privately...

very nice greetings, 
 bernhard

-----Ursprüngliche Nachricht-----
Von: getdp-bounces at geuz.org [mailto:getdp-bounces at geuz.org] Im Auftrag von Pei-Ying Hsieh
Gesendet: Donnerstag, 22. März 2007 02:42
An: getdp at geuz.org
Betreff: [Getdp] calculate magnetic forces?


Dear GetDP Group:

I searched the GetDP mailing archive (from 1998 - 2007) and saw several discussions about how to calculating magnetic forces under permanent magnets with no current.  But, I did not see any examples of how this should be done.  It will be appreciated if someone can post an example.

My current project involves Computational Fluid Dynamics.  Iron particles were suspended in fluid.  Each particle is about 2 micron-meter in diameter.  At some point in time, the iron particles were exposed to magnetic field induced by permanent magnets (no electric current).  The particles were then attracted toward the magnets. I would like to calculate the time needed for the particles to reach the magnets.  After receiving help from several kind people on the GetDP forum, I have computed the magnetic flux density and H using scalar potential (3-dimensional).  Does the magnetic forces also depend on the permeability of the particles too?

Someone has suggested Force = 1/mu0 * B x (curl B).  Does this has a unit of Newton?  How can this be done in post-processing?

Thanks!

Pei-Ying



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