THREDE: A Free-Field EMP Coupling and Scattering Code

THREDE is a time-domain, linear, finite-difference, three-dimensional EMP coupling and scattering code. In its present form, it can accomodate a problem space consisting of a 30×30×30 mesh. Differencing is linear. Problem-space boundaries are provided with a radiating condition which does not generate ficticious mathematical echos at late times. The scatterer must be a perfect conductor, although a nonideal ground plane (runway) may be close by. The present article describes the mathematical basis of THREDE, and shows the results of applying it to predict the response of an F-111 shell in the horizontally-polarized dipole (HPD) EMP simulator. Amplitude agreement between experiment and prediction for this example is typically on the order of 20%; resonant frequencies are predicted more closely than this. THREDE costs about 1 second of CDC 7600 computer time per program cycle; most practical analyses require 500 to 1000 program cycles.