CST時域算法的探討

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使用CST以來，關(guān)于其所使用的時域算法聽到過兩種說法：一種是說CST使用的是FIT（Finite Integration Technique—時域有限積分法），另一種說法是CST使用的是FVTD（Finite-Volume Time-Domain—時域有限體積法）。這兩種說法哪種更準確？還是說這兩個就是一回事？

以幫助文件為準吧。

IEEE Standard 1597系列的兩個協(xié)議里有簡單的介紹：
finite integration technique (FIT): Unlike the FDTD method, which uses the differential form of Maxwell’s equations, the FIT discretizes Maxwell’s equations written in their original (integral) form, on a 3D domain. The unknowns are thus electric voltages and magnetic fluxes, rather than field components along the three space directions. Like all full 3D methods (FEM, FDTD, TLM, etc.), the entire 3D domain needs to be meshed. For Cartesian grids, however, a special technique called Perfect Boundary Approximation (PBA) eliminates the staircase approximation of curved boundaries, for both PEC/dielectric and dielectric/dielectric interfaces. It allows even strongly non-uniform meshes, thus maintaining a manageable computational size. The FIT can be applied in both time domain (as the FDTD) and frequency domain (like FEM), on Cartesian, non-orthogonal-hexahedral, or tetrahedral grids. In the time domain, the explicit formulation leads to small memory requirements, and allows solving very large problems. From the time-domain results, broad-band, high-resolution frequency-domain quantities are obtained by DFT, virtually at no extra cost. If the FIT is used directly in the frequency domain, the resulting matrices are sparse. The FIT is applicable to a variety of electromagnetic problems: in bounded or unbounded domains, for electrically small or very large structures, in inhomogeneous, lossy, dispersive, or anisotropic materials. It performs well from dc up to the terahertz region.
finite-volume time domain (FVTD): This technique, an extension of the FDTD approach, permits each element in the grid to have an arbitrary shape. Frequency-domain results are obtained by applying a discrete Fourier transform to the time-domain results. This requires additional computation, but a wideband frequency-domain analysis can be obtained by transforming the system’s impulse response. The FVTD (and FDTD) methods are widely used for RCS analysis although they have been applied to a wide range of EM modeling problems. Flexibility is their primary advantage. Arbitrary signal waveforms can be modeled as they propagate through complex configurations of conductors, dielectrics, and lossy nonlinear, nonisotropic materials. Another advantage is that they are readily implemented on massively parallel computers, particularly vector processors and single-instruction-multiple-data machines. The only significant disadvantage is that the problem size can easily become unwieldy for some configurations. Grid resolution is generally determined by the dimensions of the smallest features to be modeled. The volume of the grid must be large enough to encompass the entire object and most of the near field. Large objects with regions containing small, complex geometries may require large, dense grids. When this is the case, other numerical techniques may be much more efficient than the FVTD (or FDTD) methods.

根據(jù)上面EDATOP的資料以及我查的一下資料總結(jié)一下FIT和FVTD的異同：
相同點：
1、FIT和FVTD都是從Maxwell積分方程出發(fā)導(dǎo)出；
2、FIT和FVTD都是既可以使用六面體網(wǎng)格又可以使用四面體網(wǎng)格。FVTD可以更容易的構(gòu)造共形網(wǎng)格。
不同點：
1、FIT計算的未知量是電磁流，F(xiàn)VTD計算的未知量是電磁場值。
2、FIT可以使用PBA技術(shù)。
3、FIT既可以用于時域（時域有限積分法）又可以用于頻域（頻域有限積分法）。
3、FVTD可以看做是FDTD（時域有限差分法）的一種擴展形式，可以更容易與FDTD結(jié)合得到混合方法。