XFDTD中外部边界条件的设置.docx

原文地址：XFDTD中外部边界条件的设置作者：浜嗕簡闈炰簡XFDTD中默认的外部边界条件是二阶稳定Liao辐射边界条件。它可以将辐射或散射的电磁波几乎完全吸收，只有很少的反射。XFDTD中还有完全匹配层（PML）外部边界条件、完全电导体（PEC）和完全磁导体（PMC）边界条件。在同一个计算中，不能将Liao和PML边界混合使用。而且，PML不能和PMC混用，Liao不能和PEC混用。不恰当的边界条件可能导致不稳定的计算结果。Liao BoundaryIt is important to understand the differences between the Liao and PML options. Liao is an estimation method. By looking into the FDTD space and back in time, it estimates the electric fields just outside the limits of the FDTD mesh. These estimated values are then used in the FDTD equations inside the space. The Liao estimation assumes that waves are allowed to travel outward from the space but not reflect back in. The Liao method works well provided that there is enough space between the radiating geometry and the outer boundary. Typical limits are at least 15 cells spacing.A homogeneous dielectric may be located against the Liao boundary. For example, in a lossy earth or strip line calculation, the earth or dielectric layer may touch the outer boundary. Liao will usually function well in this situation provided that there are no air gaps within 5 cells of the Liao boundary. Liao assumes homogeneous material within 5 cells, and if this is not the case then the FDTD calculation will usually be unstable with rapidly rising field amplitudes.Since Liao is an estimation method, the size of the FDTD mesh is not increased by using it. Some storage is needed for saving electric values at previous time steps, but this is usually negligible in a typical calculation.PML BoundaryPML is an artificial absorbing material. It absorbs the incident energy as it propagates through the PML layers. Better absorption, that is, smaller reflection, is obtained by adding more layers at the expense of increasing the size of the FDTD mesh.PML 是人工吸波材料。当波传播到PML 层时，入射能量被吸收。通过添加更多的PML可以得到更好的吸收，即意味着较小的反射，代价是增加FDTD的网格大小。For example, consider an FDTD calculation on a mesh using the Liao absorber that is 50 x 60 x 70 cells or a total of 210,000 cells. There is a 15 cell free