Please refer to the enclosing sub-package FixedShape
for a description of all elements of this package and to [Ro41]
for derivation and/or coefficients of the equation for permeance
G_m.
For hollow cylindrical flux tubes with a circumferential
magnetic flux, the flux density is a function of the radius. For
that reason, the characteristic mu_r(B) is evaluated
for the flux density at the flux tube's mean radius.
For those flux tube sections of a magnetic device that have a
nonlinear material characteristic mu_r(B) and a large
aspect ratio of outer to inner radius r_o/r_i, the
section can be split up in a series connection of several hollow
cylindrical flux tubes with radial flux. This allows for more
realistic modelling of the dependence of flux density on the radius
compared to modelling with just one flux tube element.