Re: [femm] coenergy vs. stress tensor calculations of magnetic force

Dear Jim, I just noticed your work and hadsome problem running your code due to intense mesh. Any way in your stress tensor simulation how are you choosing stable equilibrium position. The excel fileyou attached does not give much detail. I am also working on same problem and apply the amperian current approximation (J.App.Phys.volume:86,page:6396,year:1999) to the diamagnetic system. Thanks for any information you provide.

best regards

Dr.Ahmet Cansiz

----- Original Message -----

From: James Rabchuk

To: Femm

Sent: Thursday, January 30, 2003 9:02 PM

Subject: [femm] coenergy vs. stress tensor calculations of magnetic force

Hi, I've written before about making a simulation of a levitation device with diamagnetic materials in FEMM. I was interested in determing the equilibrium position of the levitating magnet by calculating the force for a number of positions between the diamagnetic plates.

Following the advice of the FEMM manual, I used the coenergy method for calculating the field energy, and then looking for the force from that. But what I found was that the Coenergy displayed no observable trend with height of the magnet. On a lark, I tried the same simulation and calculated the Force from the stress tensor, using a fixed contour. Surprisingly, I got very believable results that showed the stable and unstable equilibrium points for the levitating magnet quite nicely. Iwas content with that, but I'm still intrigued why the coenergy method doesn't seem to be working in this case. We tried moving the outer boundary to increase the area for the problem. I tried increasing the mesh density inthe region where the force on the magnet is being calculated. But none of those things seems to be able to get the coenergy method to provide meaningful results. I might be tempted to say that my stress tensor calculations arejust misleading, but they mirror so nicely the behavior of the actual system that I find that hard to believe.

Would anybody be willing to take a look at the simulation I've put together and make some suggestions as to where the coenergy calculation might be messing up, or where I could improve the model to get the "right" behavior?

I've attached the femm file, the associated pre and post lua files, and an excel file with the coenergy numbers, and graphs of both the coenergy and stress tensor calculations of the force with height (gravity is included manually).

If you look at the Excel file, you can't possibly discern a pattern of behavior from the coenergy graph, while the stress tensor graph is pretty good, though not as smooth as you might like.

Any help would be appreciated.

Thanks,

Jim

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