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Re: [femm] plot x-y

salloker wrote:

I am analysing a axisymmetric system and want to know the field
strength along the axis.

I define a contour along the axis and get strange results depending
on the direction of my contour. For one direction I get reasonable
results, while for the oposite direction (i.e. changing only the
beginning and the end of my contour) the field is zero.

Is there any explanation for this?


Michael

This is actually a feature, not a bug. What the program does when it evaluates the field "along a line" is that it actually evaluates the field at a distance of like 10^(-6) units to the normal side of the contour (a 90 degree counter-clockwise rotation away from the contour, see p. 33 in manual for a picture). The idea here is that if the contour is drawn along a boundary between different materials, you can get field results on one side of the interface by drawing the contour in one direction, and results in the other material by drawing the contour in the reverse direction. If you tried to draw the contour exactly on the boundary, you'd probably get worthless results--a mix of results from both sides, possibly varying which side of the boundary that is used on a point-by-point basis. The way that I've done it is an attempt to remove the ambiguity.

What's going on in your case is when you draw the contour along the axis in the direction of decreasing z, you get the results that you'd expect, becuase the field is being evaluated at points which lie just inside the solution domain. When you draw the contour in the direction of increasing z, the program is attempting to evaluate the field for points (just slightly) out of the solution domain, so the results are all zero (the default result).

Dave.