CST时域算法的探讨

使用CST以来，关于其所使用的时域算法听到过两种说法：一种是说CST使用的是FIT（Finite Integration Technique—时域有限积分法），另一种说法是CST使用的是FVTD（Finite-Volume Time-Domain—时域有限体积法）。这两种说法哪种更准确?还是说这两个就是一回事?

以帮助文件为准吧。

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IEEE Standard 1597系列的两个协议里有简单的介绍：

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.

根据上面hefang的资料以及我查的一下资料总结一下FIT和FVTD的异同：
相同点：
1、FIT和FVTD都是从Maxwell积分方程出发导出；
2、FIT和FVTD都是既可以使用六面体网格又可以使用四面体网格。FVTD可以更容易的构造共形网格。
不同点：
1、FIT计算的未知量是电磁流，FVTD计算的未知量是电磁场值。
2、FIT可以使用PBA技术。
3、FIT既可以用于时域（时域有限积分法）又可以用于频域（频域有限积分法）。
3、FVTD可以看做是FDTD（时域有限差分法）的一种扩展形式，可以更容易与FDTD结合得到混合方法。