Hybrid computation of low-frequency field penetration through open ended metallic cylinders

Computation of low-frequency field penetration through conducting and magnetic materials is important for accessing interference within devices and facilities. Another use is the prediction of naval vessel vulnerability to detection caused by the field signatures of their internal electrical machinery. A hybrid procedure is used for predicting the penetration of low-frequency fields through a cylindrical structure. The structure is assumed to be rotationally symmetric but otherwise can be inhomogeneous, including layers or rings of differing material. The internal source is an axially oriented multi-turn coil. Field computation is formulated using a finite element solution within the interior region coupled to a spheroidal field expansion in the unbounded exterior. Comparisons are made to field measurements for a coil inside a steel pipe at frequencies of 1Hz and 50Hz.