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# Re: mesh density / outside area?

- To: femm@yahoogroups.com
- Subject: Re: mesh density / outside area?
- From: David Meeker <dmeeker@ieee.org>
- Date: Wed, 10 Sep 2003 10:59:11 -0400
- In-reply-to: <bjf1e6+shdh@eGroups.com>
- References: <bjf1e6+shdh@eGroups.com>
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ahmad_m_eid wrote: `Still I have problems in force calculation. you advise me to make several solution using mesh density. ok I made but I don't know which one is good. the stator (linear generator) of my problem is about 38 mm width x120 mm height with an air gap of 1 mm. If I use mesh of 1 mm so there will be one triangular in the air gap al least and David Meeker sugessted that when calculating the force you should be away from the object two triagulars at least. 1- so, in my case if the air gap is 1 mm, what the suitable mesh size density to take?` I often keep increasing the mesh density until the force results converge to whatever accuracy I want. One way to do this is to change the mesh size in all blocks by hitting F3, which halves the mesh size in all blocks, or F4, which doubles the mesh size in all blocks. `2- do you think that the block integral is better than the line integral in calculating the force?` Yes. `3- do you know any reference to calculate this force analytically?` With most designs, you can get a pretty good notion of what the machine should be doing from a somewhat idealized but analytical approach. However, it's difficult to quote you an analytical _expression_ without knowing your machine's topology and geometry. A good reference for you might be Boldea and Nasar's "Linear Electric Actuators and Generators," Cambridge University Press, 1997. This book considers analytical design methods for many different sorts of linear machines. `I notice something, when I change the area surrounding the problem, the values of the parameters changes, especially the force. So, I need help how to choose the air area surrounding the problem. Is it better to make this area small or big?? By how much is it bigger than the objects in the problem?` Check out appendix A3 -- "Open Boundary Problems" in the FEMM 3.3 manual. If you want to approximate an unbounded domain by just putting a lot of air around it, a "rule of thumb" from the literature (Q. Chen and A. Konrad, “A review of finite element open boundary techniques for static and quasistatic electromagnetic field problems,” IEEE Transactions on Magnetics, 33(1):663-676, January 1997) is that the distance from the center of the problem to the outer boundary should be at least five times the distance from the center to the outside of the objects of interest. Other techniques, which allow one to model an "open" boundary in more elegant ways, are also discussed in this appendix. Dave. -- David Meeker email: dmeeker@ieee.org www: http://femm.berlios.de/dmeeker

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    - From: "ahmad_m_eid" <ahmad_m_eid@yahoo.com>

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