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Help needed: magnetostatic force calculation using Maxwell stress tensor

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This problem has been bothering me for weeks. I read previous posts on this forum on topic of Maxwell stress tensor but still could not find what is wrong.

I'm trying to calculate the repelling force between two magnet rings with one levitated above the other. I defined a force variable for the top ring. COMSOL (AC/DC 3.3) gives "topring_nTz_emnc" as the surface force density at the boundaries of the top ring for calculating z-component of the force.

On the other hand, I manually defined elements of the Maxwell stress tensor as Global Expressions

T =(1/mu0)* [ Bx^2-0.5*|B|^2, BxBy, BxBz ;
ByBx, By^2-0.5*|B|^2, ByBz ;
BzBx , BzBy, Bz^2-0.5*|B|^2 ]
= [Txx, Txy, Txz;
Tyx, Tyy, Tyz;
Tzx, Tzy, Tzz];

Since I need to compute the z-component of the force, I used the Boundary Integration Coupling variable to calculate

Fz = int{nx*Tzx+ny*Tzy+nz*Tzz} dA

for each boundary of top ring. [nx ny nz] is the outward surface normal.

Here, nx*Tzx+ny*Tzy+nz*Tzz is also surface force density, which should agree with "topring_nTz_emnc" that is automatically generated by COMSOL.

However, I am getting different results. For some of the boundary surfaces (e.g. the inner and outer cylindrical faces of the top ring),

nx*Tzx+ny*Tzy+nz*Tzz

does not agree with

topring_nTz_emnc.

Any hints on what may have gone wrong?

Thanks,
Zhiyong

3 Replies Last Post Jun 20, 2013, 8:07 a.m. EDT

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Posted: 1 decade ago Jul 19, 2010, 2:40 p.m. EDT
The model is attached here.

The geometry is very simple. Due to symmetry, only a quarter of the geometry is modeled.
The model is attached here. The geometry is very simple. Due to symmetry, only a quarter of the geometry is modeled.


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Posted: 1 decade ago Jul 23, 2010, 1:56 p.m. EDT
Some update on this. I found that if the integration surface is completely removed from magnetic material boundaries (that is, completely in air/vacuum), two methods (see my original post or the summary below) gave very close results. However, they don't agree with each other when integrating right on the material surfaces.

So I guess the issue is related to how COMSOL implements the force density "domainname_nTz_emnc" on magnetic material surfaces?

summary of two methods:

Method 1:
Calculating total force components by integrating COMSOL built-in force density "domainname_nTz_emnc" over all external material surfaces

Method 2:
Calculating total force components by literally integrating Maxwell stress tensor "Tzx*nx+Tzy*ny+Tzz*nz" over some closed envelope containing the magnetic material
Some update on this. I found that if the integration surface is completely removed from magnetic material boundaries (that is, completely in air/vacuum), two methods (see my original post or the summary below) gave very close results. However, they don't agree with each other when integrating right on the material surfaces. So I guess the issue is related to how COMSOL implements the force density "domainname_nTz_emnc" on magnetic material surfaces? summary of two methods: Method 1: Calculating total force components by integrating COMSOL built-in force density "domainname_nTz_emnc" over all external material surfaces Method 2: Calculating total force components by literally integrating Maxwell stress tensor "Tzx*nx+Tzy*ny+Tzz*nz" over some closed envelope containing the magnetic material

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Posted: 1 decade ago Jun 20, 2013, 8:07 a.m. EDT
Dear Mr. wang

please i have a work may be similar to your discussion, do mean the integration over the boundaries for example :


Fx : i should integrate both faces normal to X direction or just one face ?


me also i have a different results between the manual calculation for the Maxwell formula and in comsol formula,

so please if you solved this just tell me.


Best Regards


hassanain
Dear Mr. wang please i have a work may be similar to your discussion, do mean the integration over the boundaries for example : Fx : i should integrate both faces normal to X direction or just one face ? me also i have a different results between the manual calculation for the Maxwell formula and in comsol formula, so please if you solved this just tell me. Best Regards hassanain

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