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A magnet attached to an iron plate
Posted Dec 31, 2014, 3:49 p.m. EST Low-Frequency Electromagnetics, Modeling Tools & Definitions, Parameters, Variables, & Functions, Structural Mechanics Version 5.2a 3 Replies
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I am modeling a magnet attached to an iron plate in COMSOL. Enclosed is the picture of this simple model. I used two air boxes: one in top which includes the magnet and one in bottom which includes the plate. I used "Union 1" and "Union 2" to unite these air boxes with magnet and iron plate parts. Then used the "Form Assembly" command to simulate contact surfaces between these units. As the figure shows, it seems that the COMSOL calculates the flux density well. But, the problem is that COMSOL gives a very small value, almost close to zero, for the magnetic force applied on the Iron plate. This does not make sense. Because in another model, when I put the magnet close to the plate with a 1 mm gap, I get a force close to 160 N. So, it seems that the force in the first case, when they are in touch, should be some thing close to 200 N; however, I am not sure!
I would be thankful if someone help me to figure out where is my mistake in my FE model.
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The force calculation feature simply integrates the Maxwell stresses, evaluated just outside the selected domain (or domains), over the entire outer boundary of the domain selection which should be a group of domains moving together (a single mechanical body). But there is a limitation to this when the magnet is attached to the ferromagnetic domains, for example.
If you just want to calculate the magnetic force on the magnet that is attached to the ferromagnetic material, you must have a "thin low permeability gap" feature assigned at the common boundary touching magnet and the magnetic material. This is because the Maxwell Stress should be evaluated in air rather than in the ferromagnetic material.
Hope this will be useful.
Best Regards,
Nirmal Paudel
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Happy new year and thank you for your reply.
So, because the Maxwell's tensor is integrated over the surface of bodies by COMSOL, we must model these surfaces explicitly in COMSOL and one solution is using an infinitesimal air gap between these attached surfaces. I think 0.1 mm gap will be sufficient to get stable results, am I right? If so small what about meshing? Is there any advice for meshing this small air gap?
by the way, I think this paper is also helpful to understand the concept of this problem:
Computation of Magnetic Contact Forces
ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6749127
Thank you for your kind help,
Mohsen
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Now I try to move one step further from your problem.
I am modeling a magnet attached to a thin iron plate in COMSOL.
But I focus on how the magnets influence on the propagation of elastic waves inside the plate.
Do you have any idea how I can model this problem?
Thanks very much
Kun