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Complex valued field in Helmholtz PDE interface

Posted Dec 7, 2012, 4:55 p.m. EST Modeling Tools & Definitions, Parameters, Variables, & Functions Version 4.3 2 Replies

Matthew Rogge
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Hello all,

I'm working on a model with the Helmholtz Equation interface in 1D. I'd like to be able to make the variable, u, complex in order to get phase information out of the result. I've looked through the documentation but can't seem to find mention of how to do this. Does anyone know how to accomplish this?

Thanks,

Matt
2 Replies Last Post Feb 12, 2014, 9:48 a.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Dec 8, 2012, 5:08 a.m. EST
Hi

if you read the pdf doc, at least I get the impression that COMSOL assumes by default complex variables for all dependent variables, you have then the option how these are presented (real or complex)

Mostly COMSOL detects any complex BC values and adapts accordingly

See p1045 CMP user manual 4.3a

To be sure, you can in the "solver parameters" node "global settings" - check "Allow complex number" (typically for time series etc)

then if you take sqrt, abs etc functions of negative numbers with complex results check Advanced tab, "Use complex functions with real input"

Now turning everything "complex" means doubling the RAM ;)

--
Good luck
Ivar
Hi if you read the pdf doc, at least I get the impression that COMSOL assumes by default complex variables for all dependent variables, you have then the option how these are presented (real or complex) Mostly COMSOL detects any complex BC values and adapts accordingly See p1045 CMP user manual 4.3a To be sure, you can in the "solver parameters" node "global settings" - check "Allow complex number" (typically for time series etc) then if you take sqrt, abs etc functions of negative numbers with complex results check Advanced tab, "Use complex functions with real input" Now turning everything "complex" means doubling the RAM ;) -- Good luck Ivar
Wenlong Gao
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Posted: 1 decade ago Feb 12, 2014, 9:48 a.m. EST
Hi,

Recently I'm using PDE model to simulate EM wave propagation described by Helmholtz equation.
I exactly followed the steps in a recent webinar givenby COMSOL on PDE, in which they teach how to use helmholtz equation in PDE model.
However, I found the result is not satisfying since you only get the real value solution.
I've also tried to change the input boundary to complex value, and get a complex value solution. However, when I plot abs(E), which should be uniform in the whole domain, it's not so, but rather in a standing wave form.
I thought it's a problem in the boundary condition, and tried to add PML boundary in it, but nothing change.

Do you know how can I set it properly to get a complex value solution of Helmholtz equation?

Thanks!
Hi, Recently I'm using PDE model to simulate EM wave propagation described by Helmholtz equation. I exactly followed the steps in a recent webinar givenby COMSOL on PDE, in which they teach how to use helmholtz equation in PDE model. However, I found the result is not satisfying since you only get the real value solution. I've also tried to change the input boundary to complex value, and get a complex value solution. However, when I plot abs(E), which should be uniform in the whole domain, it's not so, but rather in a standing wave form. I thought it's a problem in the boundary condition, and tried to add PML boundary in it, but nothing change. Do you know how can I set it properly to get a complex value solution of Helmholtz equation? Thanks!

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