Steve Ryder wrote: > Hello, > I've been trying to model an alnico V slug (.187" dia. by .750" long) > for a guitar pickup and keep getting an unlikely field pattern. The > missing component could be boundary conditions but I'm at a loss as to > how to define them. Any hints would be helpful. Eckart's suggestion ("add a closed rectangular frame all around the geometry, with sufficient distance for leakage fields, and set A=0 on this") ought to work fine. However, I have a sneaky suspicion that boundary conditions are not the issue with your problem. You have to define the orientation of the magnet as part of the material properties. If you just pull the entry for Alinco 5 out of the materials library, the default is that the magnetization direction is 0 degrees. You need to change the direction to 90 degrees (or -90 degrees) to get the right results for your problem (so that the magnetization is pointing along the magnet's axis). Since it's only about 600 bytes, I've attached a zipped sample model of your magnet. As far as the boundary conditions, I have made the edge of the domain a half-circle, and applied a "mixed" boundary condition to this edge. The parameters for this boundary condition are specified in eqs. (42) and (43) on p. 49 of the manual. This boundary condition is meant to mimic the influence of the air outside the solution domain so that the solution that you get looks like the field of a magnet in unbounded free space (an asymptotic boundary condition). Along the axis, A=0 is automatically applied by the program (so you don't have to define a boundary condition explicitly here). Dave.
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