Discussion Forum

Paul,

you can convert the quadrilateral elements to triangles, check the available functions in the meshing.

The other question is whether you really need to resolve the eddy currents. If the skin depth is much smaller than the 5 mm plate thickness you might consider to use impedance BCs at the plate surfaces and exclude the plate volumes from the model.

Cheers
Edgar

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Edgar J. Kaiser
emPhys Physical Technology
www.emphys.com

Hello Edgar,
thanks for your reply!
Unfortunately this model is supposed to become more and more adapted to a situation with high frequency interfering fields (100Khz-5Ghz) but low frequency magnetic field pulses aswell. Also the wall thickness might get smaller later on. Therefore I have to resolve the eddy currents because one important question will be how quick they will decay.

Some comments: Accurately modeling small imperfections in RF shielded chambers via FE is not very straightforward, in part for the reasons that you have already noted. Leakage at relatively high frequencies is dominated by gaps, cracks, oxidized/corroded (quite possibly hidden) joints, holes, etc. If you get the geometries and properties of those features wrong, (which is usually the case, since those holes/gaps are generally unplanned/accidental) then your model results will be wrong too! If the chamber is not loaded with lossy items/materials, the leaked fields will ring about inside it (if they are above cavity mode cutoffs) and there will be strong dependencies upon small features. The complexity is even worse if your chamber isn't empty. (And an empty chamber is mostly a useless chamber!) So frankly, you are unlikely to model it correctly. Also, if the shielding is fairly good (which is the usual design goal), then leaked fields will be quite weak (which is as they should be). However, this introduces yet another problem: numerical noise. Comsol Multiphysics is much better at computing accurately the stronger fields in just about any problem than the much weaker ones. If what you are looking for is, for example, 60 dB (or worse, say 90 dB) down from the numbers you are working with on the strong-field side, you may run into all sorts of nonsensical values in your outputs, due to numerical noise. This is especially problematic in time domain models. Leakage at lower frequencies, such as directly through the sealed/solid/unbroken material walls is easier to model, but at the same time, seldom of much interest, unless your chamber's objective is blocking signals such as 60 Hz background fields or similar low-frequency noise. So... what to do? Stick to modeling the things you can test and verify and for which the geometries and materials are well defined. Things like specific sizes of gaps and holes in specific materials. You can also do some models of individual cases of larger problems, paying close attention to the kinds of issues noted above, so that you don't produce a bunch of computed curves that have virtually nothing to do with reality. In other words, be very careful and don't get too ambitious or careless with these kinds of models. Good luck.