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Laser Transmission Welding / Overshooting? -> Lumped Mass Matrix

Posted Jul 26, 2010, 1:59 p.m. EDT Heat Transfer & Phase Change, Materials, Studies & Solvers Version 4.0a 4 Replies

Anita Samba
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Hello everybody,

currently I'm working on a simulation of the laser transmission welding process. In this process a laser beam passes through a transmitting part and heats up an absorbing part which then starts to melt. Due to heat conduction this in turn heats up the transmitting part and a welding seam occurs.

I only simulate temperature profiles via the general heat transfer module (transient) , no mechanical behaviour is regarded.

My problem is the following:
I'm constantly getting negative temperatures near the welding zone. When I highly increase the mesh density in this area, this effect can be weakened but the computional time increases - of course - a lot.

I think this effect can be due to the steep increase of the absorption coefficient at the interface of the two joining partners. Somebody told me that this leads to overshooting/undershooting in the welding zone and this effect could be weakened by using a lumped mass matrix.

So my question is:
Can I tell COMSOL to use a lumped mass matrix in its calculations? If yes, how?

Thank you very much,
Anita

P.S.: Any other solutions for this problem are VERY welcome, too :-)
4 Replies Last Post Jan 10, 2011, 9:31 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 Jul 28, 2010, 2:53 a.m. EDT
Hi

well I cannot comment on the lumped mass, but my experience with "negative temperatures" mostly came from badly defined BC or wroing initial conditions (the default mostly "0" is not always the best starting point for temperatures).
There is one particularity with temperature that it can be used as an absolut value (in Kelvin) or a relative step in K or degC (or other units). Even knowing this well I often get it wrong when I'm typing too quick (see the discussion in the V4a doc).

You are right that you might get some numerical induced "oscillations" around large, steep steps or at edges, there are also several pages about "stabilisation technics" in the doc, and how to adapt to this

--
Good luck
Ivar
Hi well I cannot comment on the lumped mass, but my experience with "negative temperatures" mostly came from badly defined BC or wroing initial conditions (the default mostly "0" is not always the best starting point for temperatures). There is one particularity with temperature that it can be used as an absolut value (in Kelvin) or a relative step in K or degC (or other units). Even knowing this well I often get it wrong when I'm typing too quick (see the discussion in the V4a doc). You are right that you might get some numerical induced "oscillations" around large, steep steps or at edges, there are also several pages about "stabilisation technics" in the doc, and how to adapt to this -- Good luck Ivar
Anita Samba
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Posted: 1 decade ago Jul 28, 2010, 2:38 p.m. EDT
Hello Ivar,

first of all, thank you for your help! :-)

I'm not sure, if I understand you right. What do you exactly mean with "starting point for temperatures"? I'm doing a transient simulation, so I need to give the starting temperature for the wohle subdomain. In the laser welding process, all joining parts have ambient temperature (23°C) at the beginning. So I assign a temperature of 23[degC] to the whole subdomain. Or do you mean something else?

I use SI base units. Should I assign the starting temperature in [K] then? I thought there would be an automatic conversion in 3.5a.

Where exactly can I find the topic about stabilization in the comsol user guide?

Thanks again,
Anita



Hello Ivar, first of all, thank you for your help! :-) I'm not sure, if I understand you right. What do you exactly mean with "starting point for temperatures"? I'm doing a transient simulation, so I need to give the starting temperature for the wohle subdomain. In the laser welding process, all joining parts have ambient temperature (23°C) at the beginning. So I assign a temperature of 23[degC] to the whole subdomain. Or do you mean something else? I use SI base units. Should I assign the starting temperature in [K] then? I thought there would be an automatic conversion in 3.5a. Where exactly can I find the topic about stabilization in the comsol user guide? Thanks again, Anita
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Jul 28, 2010, 4:30 p.m. EDT
Hi

You are right: you should read "starting temperature" = initial conditions, and you are confirming that you set them to some reasonnable value.

and if you state degC then Comsol look aftrer the translation to Kelvin so you are doing again right :)

The "stabilization techniques" is in the 3.5a doc Modelingref.pdf chapter 17, but do an indexed search on the doc, there are other related pages in the specific physics docs, for V4 chap 7 in "mph_ref.pdf"

--
Good luck
Ivar
Hi You are right: you should read "starting temperature" = initial conditions, and you are confirming that you set them to some reasonnable value. and if you state degC then Comsol look aftrer the translation to Kelvin so you are doing again right :) The "stabilization techniques" is in the 3.5a doc Modelingref.pdf chapter 17, but do an indexed search on the doc, there are other related pages in the specific physics docs, for V4 chap 7 in "mph_ref.pdf" -- Good luck Ivar
Sankaranarayanan Raman
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Posted: 1 decade ago Jan 10, 2011, 9:31 a.m. EST
Hi Anita,
I'm working on modeling TIG welding using COMSOL 4.0a. I have difficulties in modeling the arc shape and size, and also in giving velocity for the arc motion. . it would be really helpful if you could share ur knowledge about this software with me. my mail id is sankar1990@gmail.com.
thank you in advance. . :)
Hi Anita, I'm working on modeling TIG welding using COMSOL 4.0a. I have difficulties in modeling the arc shape and size, and also in giving velocity for the arc motion. . it would be really helpful if you could share ur knowledge about this software with me. my mail id is sankar1990@gmail.com. thank you in advance. . :)

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