Guilin Sun
Ansys Employee

当然前提是通过诊断知道发散是x方向PML引起的,例如可以用视频监视器查看。

如何选?只有StretchedPML才有上面4个选项。一般情况下,所有方向的PML选用同种类型和层数,当然根据需要也可以不同方向选择不同的类型,例如

此时需要不能选Same Settings On All Boundaries,然后点击对应坐标下的Profile那一栏的Standard(缺省的都是Standard)

偶尔需要使用Uniaxial AnisotropicPML:

一般是StretchedPML无论如何调整都得不到需要的效果时才用它测试。我曾经有过这样一个例子。

其它可以参见这几个帖子
FDTD 和 varFDTD 常见的发散原因以及处理
PML应该放在什么地方?
关于varFDTD仿真共振环的问题

此外,实际的PML匹配层经过离散后 仍然有一定的反射 ,因此使用时,既不要对它有苛刻的要求,例如要求反射率不能高于1E-6(也是可以做到的,不过没有必要),也要注意其反射对结果的影响。后者可以通过增加PML的距离等检查。

PML Parameters

Unlike conventional boundary conditions, PML boundaries have a finite thickness. In other words, they occupy a finite volume that surrounds the simulation region. It is within this volume that the absorption of light happens.

•LAYERS: For discretization purposes, PML regions are divided into layers.

•KAPPA, SIGMA, ALPHA : The absorption properties of PML regions are controlled by three parameters. Their definition can be found in the second reference at the top of this page. Kappa is unitless by definition, but sigma and alpha must be entered into the PML settings table as normalized unitless values. Kappa, sigma and alpha are all graded inside the PML regions using polynomial functions. Parameter alpha is sometimes described as a complex frequency shift (CFS) in the literature2. Its main role is to improve numerical stability. Increasing the ratio alpha / sigma will make a a PML boundary more stable, but it will reduce its absorption effectiveness; this is why the stabilized profile requires a larger number of layers. To recover the S.I. unit values of alpha and sigma, it is necessary to multiply by twice the permittivity of free space and divide by the time step employed in the simulation.

•POLYNOMIAL: It specifies the order of the polynomial used to grade kappa and sigma.

•ALPHA POLYNOMIAL: It specifies the order of the polynomial used to grade alpha.

•MIN LAYERS, MAX LAYERS: They enforce a sensible range of values for the number of PML layers.


介绍PML的各个参数时,这句话“Increasing the ratio alpha / sigma will make a a PML boundary more stable, but it will reduce its absorption effectiveness”中表示增大alpha / sigma的比值可以使得PML边界更加稳定。
那么PML边界的稳定性是什么意思?


 
Poplar:

那么PML边界的稳定性是什么意思?

PML穩定的時候應該是吸收光的, 不穩定的時候有機會會造成非物理的增益, 以及彷真發散。