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Mirror surrounding by PMLs overrides Boundary Conditions

Posted Apr 24, 2020, 6:06 p.m. EDT RF & Microwave Engineering Version 5.5 2 Replies

Georgia
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Hi,

I am new to COMSOL. I am designing a mirror inside air and then surrounded by PMLs. Before I added the PMLs , I added the PMC/PECS and Impedance boundary conditions at the mirror and I was thinking of using a SBC at the outer PML boundary for radiation to enter. However it seems now that all the boundaries but the ones that refer to outer PMLS are disabled.

What am I missing here?

Thanks!

2 Replies Last Post Apr 30, 2020, 5:46 p.m. EDT
Robert Koslover Certified Consultant
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Posted: 4 years ago Apr 27, 2020, 12:15 p.m. EDT
Updated: 4 years ago Apr 27, 2020, 12:20 p.m. EDT
  1. It might be helpful for you to post your model to the forum so that others can look at it and identify specific issues.
  2. Just a guess, but you may be overspecifying (overconstraining) your BCs. For example, a typical "ideal mirror" in a real-world RF problem would be a PEC condition. It would not simultaneously be a PMC condition, nor would it be an impedance boundary condition. A wall of a somewhat lossy real-world conductor (e.g., aluminum) might be modeled as an impedance boundary condition (at least if you want to compute or account for the losses in it) but it would not at the same time be assigned to be a PEC, and (as already noted) you wouldn't assign it to be a PMC either.
  3. In regard to choosing the most appropriate setting of the outermost BC of a PML, I encourage you to look at how this is handled in the library examples provided by Comsol Multiphysics. When uncertain what to do, copy the experts. ( Well, most of the time!) :)
  4. Finally, if all your "internal" BCs are being disabled, it is because they are internal. They are called boundary conditions for a reason! If you already know there is zero field inside a region of your model, that region should be outside your model (not meshed). You can use the geometry tools and Boolean operations to remove arbitrary areas or volumes from your models as needed.
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Scientific Applications & Research Associates (SARA) Inc.
www.comsol.com/partners-consultants/certified-consultants/sara
1. It might be helpful for you to post your model to the forum so that others can look at it and identify specific issues. 2. Just a guess, but you may be overspecifying (overconstraining) your BCs. For example, a typical "ideal mirror" in a real-world RF problem would be a PEC condition. It would not simultaneously be a PMC condition, nor would it be an impedance boundary condition. A wall of a somewhat lossy real-world conductor (e.g., aluminum) might be modeled as an impedance boundary condition (at least if you want to compute or account for the losses in it) but it would not *at the same time* be assigned to be a PEC, and (as already noted) you wouldn't assign it to be a PMC either. 3. In regard to choosing the most appropriate setting of the outermost BC of a PML, I encourage you to look at how this is handled in the library examples provided by Comsol Multiphysics. When uncertain what to do, copy the experts. ( Well, *most* of the time!) :) 4. Finally, if all your "internal" BCs are being disabled, it is because they are *internal*. They are called *boundary* conditions for a reason! If you already know there is zero field inside a region of your model, that region should be *outside* your model (not meshed). You can use the geometry tools and Boolean operations to remove arbitrary areas or volumes from your models as needed.
Georgia
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Posted: 4 years ago Apr 30, 2020, 5:46 p.m. EDT

Hi Robert , thanks for your response!

I managed to have control of the BECS by going under Geometry and activate them as boundaries. You were right about overconstraining them.

Hi Robert , thanks for your response! I managed to have control of the BECS by going under Geometry and activate them as boundaries. You were right about overconstraining them.

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