Ansys Insight: About override mesh in FDTD: its use and settings
Many users ask if they can just make a curved surface smoother using override mesh, eg:
It is easy in DEVICE, but it is not possible in FDTD Solutions (and MODE Solutions). This is because, DEVICE uses finite element grids, whereas FDTD and MODE use rectangular mesh. That said, if an override mesh is added in the simulation region, any coordinates position that falls inside the defined override mesh will use the defined mesh:
- Priority: For geometric objects with the same mesh order, if they have overlap in space, the lower one in the object tree will take the priority to override others. The override mesh does not follow this rule: the finest mesh takes the priority, eg, FDTD will use the finest mesh if the override meshes have overlaps.
2: How to know the mesh size to be used
In some cases users know what the mesh sizes to be used, eg, if you want resolve a small feature. However in many cases users do not know exactly the sizes to be used. I recommend to use the default size with rounding to meaningful digits. eg, if the default size is 0.012597um , simply use 0.013 or 0.012um.
3: How to know what the equivalent index is used.
In most cases, FDTD uses the number of points per wavelength for meshing. The larger the refractive index of a geometry, the smaller (finer) the mesh size. So the equivalent index will depend on how small the mesh size you want to set in the override region.
The best thing using this method is, with the background mesh accuracy changes, the override mesh size also changes, which is good for converging test.
4: about the override mesh region
Only applies the override mesh to the desired region. Although with higher mesh accuracy the result can be more accurate provided other settings are proper, it requires more memory.
5: directly defined vs. based on a structure
You can directly define the override region (in its region all the mesh is governed by its settings, if it is finer than other override mesh). If you want to make a specific geometry to have finer mesh, you can use “base don structure”, simply add the name of the structure. In case more than one geometry has the same name, it applies to all.
You can also add a buffer region around the geometry to have the same defined mesh.
6: can you make the override coarser than the background mesh?
Yes for FDTD, VarFDTD, but right now not for FDE and EME. This is useful in some cases.
Override mesh has higher priority than the background mesh (from “mesh type” )
7: Make sure the override mesh size is what you really expect
Sometimes users want to use finer mesh by specifying the mesh size but actually this makes the mesh coarser ! If you are not sure, please zoom in the mesh, and use the “ruler” on the left toolbar to measure it.
In addition, be careful when you use override mesh (larger mesh size) to reduce the memory requirement, except that you know the reason. Please refer this example: https://apps.lumerical.com/cmos_psf_example_3d.html
8: mesh size vs simulation time
In general, the smaller the mesh size, the larger the memory requirement, and the slower the simulation since the time step size is proportional to the smallest mesh size, that is, the dt becomes smaller. When finer mesh is used, the memory may be increased too high to handle for the computer, and the total CPU simulation time will be longer to reach the same autoshutoff level or the set simulation time (FDTD/General).