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# RE: [femm] (Femm 3.3a works!) coenergy vs. stress tensor calculations of magnetic force

- To: <femm@yahoogroups.com>
- Subject: RE: [femm] (Femm 3.3a works!) coenergy vs. stress tensor calculations of magnetic force
- From: "James Rabchuk" <ja-rabchuk@wiu.edu>
- Date: Thu, 27 Feb 2003 09:08:40 -0600
- Importance: Normal
- In-reply-to: <005301c2de6c$68753060$1faa0b0a@AZAWZ161EXVMQM>
- Reply-to: <JA-Rabchuk@wiu.edu>

Ahmet, Sorry for the intense mesh. I was getting a little frustrated trying to smooth things out and especially to get the coenergy method to work. Never did, for the reasons Dave pointed out. (Here I just want to express my appreciation again to Dave for the amazing help and consultation given with this program!!!) However, Dave recently posted a revised version of FEMM, v. 3.3a, with a weighted stress tensor calculation. What an improvement! What was a choppy but noticeable trend in the force with height of the lower magnet (even with an intense mesh density in the region of interest) became a smooth and absolutely clear force vs. height curve. Absolutely on a par with the best of the coenergy results. I'll post the results in a bit. We're still generating the graphs. (I do have a question for Dave, here. I shouldn't be lazy, and just look at the code, but I'll ask the source. In the implementation of the weighted stress tensor, are you doing a volume integral over the entire air region of the model? That seems to be what's implied from your description and the reference you posted. Also, you mentioned that you weight elements in more tightly meshed regions more than you do elsewhere. I wasn't quite sure how that would work. If that applies over a single contour, then don't you need to keep the integration over the entire contour at the same weight, in order to get the proper "sum" of momentum flux across the boundary?) As far as your question goes, the graph shows force with height. I've accounted for the constant force of gravity by subtracting it from the Femm result in my post-lua script before writing it to my output file. Equilibrium means the force is zero. Stable equilibrium means the force below the equilibrium point is up, the force above the equilibrium point is down (negative). For the diamagnetic materials available, the plate separation needs to be quite small for the equilibrium point to become stable. But the program works beautifully and, having matched the material properties as closely as I could to the experimental setup, the agreement between prediction and observation is excellent. Thanks again for a great tool and great support for it. -----Original Message----- From: Ahmet Cansiz [mailto:acansiz@atauni.edu.tr] Sent: Thursday, February 27, 2003 8:28 AM To: femm@yahoogroups.com Subject: Re: [femm] coenergy vs. stress tensor calculations of magnetic force Dear Jim, I just noticed your work and had some problem running your code due to intense mesh. Any way in your stress tensor simulation how are you choosing stable equilibrium position. The excel file you attached does not give much detail. I am also working on same problemand 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, 20039:02 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.I was 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 in the 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 are just 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 graphsof 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 `Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.` `Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.`

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  - Re: [femm] coenergy vs. stress tensor calculations of magnetic force
    - From: "Ahmet Cansiz" <acansiz@atauni.edu.tr>

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