COMSOL Forums: Problem with Mie Model

Re: Problem with Mie Model

Fri, 16 Apr 2010 09:13:48 +0000

Hi everyone!

I have the same problem with the model; for some wavelengths the scattering cross section becomes negative which makes, of course, nonsense. First I thought it was probably a mistake I made while defining the global expressions or the integration ones, so I repeated the process several times but I obtained the same result over and over. Later I thought that maybe one of the steps in the documentation file was wrong, so I ended downloading the file .mph directly from the model gallery instead of trying to reproduce it myself. Everything was already defined in the file so I just had to mesh the model and solve it. To do so I followed the instructions (mesh size, number of layers in the exterior subdomain, solver parameters,...) exactly as they were written in the documentation. It was a big surprise to find out that I had exactly the same problem as before. I wanted to attach the scattering cross-sections obtained with the downloaded file (Mie_scattering_plasmonic_reset_35a1.mph) to illustrate the problem but the attach button is not responding. Anyway, any idea about what could be wrong?

Thanks a lot,

Ramon

Re: Problem with Mie Model

Wed, 31 Mar 2010 20:31:21 +0000

Hi to all!
I've got some problems too!
There is serious discrepancy between my results and in results given in the presentation (different field enhancment and cross-sections that are negative at some wavelengths). I used my own data for the refraction index though it also was teken from johnson&christy (1972) paper - I've built interpolated functions with dependence on the wavelength.

Any ideas?

Besides that I can't download the xml library for some reasons (!) and I can't compare dispersion data. Could anybody send it to me. Maybe that will solve the problem...
Thks!

trisha.petrov@gmail.com

Re: Problem with Mie Model

Mon, 15 Mar 2010 07:54:29 +0000

Hi

I believe there is a quick reply for your units issue the "r/1[m]" this gives [m] if "r" is without units, or [m^2] as I expect "r" is in [m] and is different from "r/(1[m])" which is without units or equivalent to understanding it as (r[m])/(1[m])

Will have to look closer for the global expression, but I'm a t a Comsol course just now ;) and Ihave to liten to the speaker too ;)

Have a nice day
Ivar

Re: Problem with Mie Model

Mon, 15 Mar 2010 05:26:50 +0000

Hi, I am happy to find your example about radar cross section. But I met a problem when I fllowed your example. It happend in Global Expression(dsigma_near, dsigma_far). When I want to plot 4*pi*sigma_back, I met the problem as fllowed: disgma_far is not a valid global experssion. I do not know why.

Re: Problem with Mie Model

Mon, 18 Jan 2010 19:43:37 +0000

Play with meshing more.
Define it manually with care, sometimes automatic meshing give you better results (avoiding inverted elements) but could be not enough for nanoscale.

Problem with Mie Model

Mon, 18 Jan 2010 15:52:22 +0000

Hi!
I only got started with COMSOL and was very happy to find the Model 'Mie Scattering off Plasmonic Nanoparticle and Radar Cross-Section Computations'. I followed all the instructions, but when I tried to do the mesh, it wouldn't let me do the swept mesh at first, saying there are involuted elements (even though I followed the instructions to the letter)...
I tried the swept mesh some more and suddenly, it was working fine - until, after 2 hours of calculation time, I saw the results: lots of warnings about an 'inverted mesh element near coordinate (9.24802e-008,9.24802e-008,2.6999e-007)' (these coordinates change slightly when I solve again) and a global variables plot which looks completely unlike the one in the documentation (see attached). What is going on here? I'm at my wit's end...
Thanks in advance for any help!
Kind regards,
Martina