Ulf Andersson <ulfa [at] nada.kth.se> and others at the Parallel and Scientific Computing Institute (PSCI) in Sweden.
Computational Electromagnetics (CEM)
GemsFDTD solves the Maxwell equations in 3D in the time domain using the finite-difference time-domain (FDTD) method. The radar cross section (RCS) of a perfectly conducting (PEC) object is computed. GemsFDTD is a subset of the code GemsTD developed in the General ElectroMagnetic Solvers (GEMS) project.
The code consists of three steps, initialization, timestepping and post-processing. More than 99% of the time is spent in the timestepping.
The core of the FDTD method are second-order accurate central-difference approximations of the Faraday's and Ampere's laws. These central-differences are employed on a staggered Cartesian grid resulting in an explicit finite-difference method. These updates are performed in the module update_mod. The FDTD method is also referred to as the Yee scheme. It is the standard time-domain method within CEM.
An incident plane wave is generated using so-called Huygens' surfaces. This means that the computational domain is split into a total-field part and a scattered field part, where the scattered field part surrounds the total-field part. This part only takes a few percent of the total execution time. It uses the excite_module to compute the shape of the incident fields.
The computational domain is truncated by an absorbing layer in order to minimize the artificial reflections at the boundary. The Uni-axial perfectly matched layer (UPML) by Gedney is used here.
A time-domain near-to-far-field transformation computes the RCS according to the Martin and Pettersson. This is handled by the module NFT_mod. Fast Fourier transforms (FFT) are employed in the post-processing of the NFT_mod.
The execution time is concentrated to five subroutines, two update routines, two UPML routines, and the routine NFT_store.
The train case uses a slightly smaller computational domain and a thinner absorbing-boundary-condition layer. Furthermore, the near-to-far-field transformation is computed closer to the PEC sphere and for different far-field locations (angles).
A main input file called yee.dat is needed. A number of PRIMARY keyword can be given. They are always written in capital letters. There are a total of nine primary keywords available. With the exception of PROGRESS, they must all be present in yee.dat. A PRIMARY keyword may have one or several Secondary keyword.
The PRIMARY keywords are used to define problem size, number of time steps to be taken, the cell size, and the CFL value. Furthermore, they are used for definitions of the excitation, an incident plane wave, the absorbing layer at the outer boundary, and the near-to-far-field transform. Finally the primary keyword PEC and its secondary keyword Filename are used to specify a file that contains a description of the PEC object.
The order in which PRIMARY keywords appear in yee.dat is arbitrary. The same is true for the Secondary keywords.
In total, there are two input files, the main input file yee.dat and the PEC description file.
Output is an ASCII file containing the requested RCS data. The name of the output file is <Filenamebase>.nft where <Filenamebase> is given in the input file under the PRIMARY keyword NFTRANS_TD and the Secondary keyword Filenamebase.
Two Matlab scripts, farfieldgemsTD.m and rcsmain.m, are included in 459.GemsFDTD/docs/matlab, which can be used to plot the RCS of the PEC object. For the PEC sphere, an analytical reference solution is supplied.
Allen Taflove, Computational Electrodynamics: The Finite-Difference Time-Domain Method, Artech House, 2000
T. Martin and L. Pettersson, IEEE Trans. Ant. Prop. Vol. 48, No. 4, pp. 494-501, Apr. 2000.
S. Gedney, IEEE Trans. Ant. Prop., vol. 44, no. 12, pp 1630-1639, Dec. 1996.
A report on a subset of GemsFDTD may be found at http://www.pdc.kth.se/publications/pdc-technical-report-series/trita-repository/PDC_TRITA_2002_1.pdf
Last updated: $Date: 2011-08-16 18:23:17 -0400 (Tue, 16 Aug 2011) $