General > General Technical Chat
EM solver, FDTD vs FEM
niconiconi:
--- Quote from: switchabl on February 26, 2024, 12:46:36 am ---It is also not particularly well suited for resonant structures (think filters, antennas). If the quality factor is high, the simulation time required for the source energy to decay can become very long.
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Great point. I completely forgot to mention it. Yeah, waiting for the energy to decay takes almost forever in an FDTD simulation when you have a resonator... |O |O |O
selcuk:
--- Quote from: metebalci on February 25, 2024, 01:30:00 pm ---
I know about openems, another is I think (not tried yet) meep, https://meep.readthedocs.io/en/latest/
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Thank you for the recommendation for another open source tool.
There is a detailed book chapter about the topic. It compares different solver tools tested on planar antennas.
A Practical Guide to 3D Electromagnetic Software Tools (Guy A. E. Vandenbosch and Alexander Vasylchenko)
metebalci:
--- Quote from: switchabl on February 26, 2024, 12:46:36 am ---The problem with large(-ish) FDTD simulations is that you run out of memory really fast. This is true for all volume methods to some degree but having to refine the cartesian grid around small/curved structures makes it worse.
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It seems the optimization Ansys (in Lumerical FDTD) has is to change the mesh in different materials based on the refractive index and also using a much finer mesh when there is a high contrast interface.
--- Quote from: switchabl on February 26, 2024, 12:46:36 am ---It is also not particularly well suited for resonant structures (think filters, antennas). If the quality factor is high, the simulation time required for the source energy to decay can become very long.
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Yes, I am aware of this, in general I guess if the energy does not leave the grid quickly, then there is this issue, when to stop the simulation. I dont know if it is correct to say this, but on the other hand, this is probably the result one is looking for in the transient response, so maybe it is expected or natural.
--- Quote from: switchabl on February 26, 2024, 12:46:36 am ---MoM can be very efficient if you can decompose your problem into multiple regions of homogeneous material because you only need to discretize the surfaces separating them and not the volume itself. You also get radiation boundary conditions basically for free. There is no need to simulate the sorrounding space and no need for PMLs. That makes it very attractive for things like antenna design and EMC simulations.
You do end up with a dense matrix in principle but it is still possible to solve it in O(n) using the fast multipole method (FMM). It is not exactly trivial to implement this. Building a MoM code with FMM from scratch is much, much harder than an FDTD code.
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I didnt know MoM discretizes only the boundary, I should read more about it. I thought it is similar to FEM but other than being in frequency domain it sounds they are pretty different.
--- Quote from: switchabl on February 26, 2024, 12:46:36 am ---FDTD is much more flexible though. It is reasonably easy to include inhomogeneous, anisotropic and even non-linear materials. That is a big reason why it is so popular in the photonics space.
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I know nothing about photonics but what you said sounds very relevant. I saw Ansys also mentions this, nonlinear materials and anisotropy in Lumerical product pages. Is this a need for electronics ? I dont remember I saw something about these for signal integrity in PCBs but I dont know much about RF and Microwave.
metebalci:
--- Quote from: niconiconi on February 25, 2024, 11:14:33 pm ---Also, do you remember those "current density on the reference plane at different frequencies" visualization images from EMC books? The near-DC case needs a simulation under 1 MHz. Unmodified textbook FDTD can't do those.
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This was actually the first thing I wondered, if they are created using FDTD, or if not if they can be created using FDTD.
Thanks for sharing the link to the Agilent white paper, it is a very short but a good summary, what I was looking for with specific applications. It also answers my question why at least the large companies (Ansys, Agilent etc.) do not provide all solver types for electronics. I thought Agilent only has FEM (and maybe MoM), I read they also have an FDTD solver.
In the interview with Taflove, I like the part: "Not many people know that Kane Yee was simply learning how to program in Fortran, as he told me 20 years later. He chose Maxwell’s time-dependent curl equations as the basis of his self-study because he wanted an initial-value problem that had both time and space derivatives.". I will give the example if someone asks me how to start learning a new programming language.
Also I read somewhere (Edit: quoting a paper [1], using 1/100 lambda grid, it has a solution within +- 1.5dB of the exact solution) that the numerical dispersion in FDTD can be an issue, in the sense that its error increases with time and it can reach an unacceptable level comparing to FEM or MoM. I dont know to what extent this is true but this made me wonder if the issue with simulating resonant structures is also because of this. Maybe the time it takes is half of the problem, and the other half is the increasing error with increasing simulation time.
[1] https://opg.optica.org/oe/fulltext.cfm?uri=oe-12-7-1214&id=79442
Nominal Animal:
--- Quote from: metebalci on February 25, 2024, 01:27:53 pm ---Is there an application or area specific or common reason to mainly select one of these methods or does it all depend on the problem (and the type of results one looking for) ? For example, in antenna research or radar etc. is one of these methods mainly used ?
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Properties of the system being investigated dictates mostly which methods are most efficient and produce most useful results. Some do parallelize/distribute better than others, though.
I don't know these simulations well enough to say exactly what properties are key in selecting the method.
--- Quote from: metebalci on February 25, 2024, 01:27:53 pm ---The reason I wonder this is, I understand if a company or research group most often develops, uses and promotes only one method, but as far as I understand Ansys [HFSS] has all types of solvers but still either offers or recommends (I only know Ansys a bit from the free version, I dont know if it is possible to use any method for electronics in a commercially licensed version) FDTD for photonics and FEM/MoM for high-freq electronics.
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Perhaps that is because FTDT requires explicit modeling of the EM wave propagation in materials. (For optics, this is the dispersion stuff, and well known for typical materials. I imagine the full EM spectrum frequency-dependent behaviour is not fully known for materials used in high-freq electronics. So, it may not be an issue with the method per se, but simply a consequence of not having the necessary real-world data that the method needs.)
Just a guess, though.
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