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Problem with temperature dependent viscosity

Posted 8 avr. 2011, 04:18 UTC−4 Heat Transfer & Phase Change, Computational Fluid Dynamics (CFD), Parameters, Variables, & Functions, Results & Visualization, Studies & Solvers Version 3.5a, Version 4.2 7 Replies

OD Tuna
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I am trying to model a simple glass furnace using Incompressible navier-stokes and heat transfer modules of COMSOL 3.5a.

As I proceeded further on my model, to make it more physically realistic, I added a temperature dependent viscosity term and enabled convective heat transfer. My viscosity values are based on the VFT relation, and for the type of glass that I am using, I entered a value as follows:

10^(-2.49474+(4099.768/(T[1/K]-544.0237)))[Pa*s]

This causes "negative inside sqrt" error either during calculation of various expressions or the residual term, depending on the type of solver that I use, whenever I enable convective heating.

When I disable convective heat transfer, the solver seems to work fine but gets stuck at certain times and I observe convergence value getting bigger and bigger at those times. When I give a constant value for viscosity (5-10-20-30), the bigger viscosity I give, the less problematic the convergence becomes during solver operation.

Has anyone faced with a similar problem? Any help would be greatly appreciated.

PS: One of my colleagues suggested that I should change my time unit from seconds to minutes or even hours, as glass processes are slow in real life. I do not know how to do that in COMSOL as well.
7 Replies Last Post 14 sept. 2011, 07:06 UTC−4
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 8 avr. 2011, 04:49 UTC−4
Hi

For the time constant, in 3.5 there is normally a time multiplier, setting it to 60 means default min, 3600 default hours, but I find it easier to use units (at least in V4 you have min, hour, day, year) and just make "t" largeer

--
Good luck
Ivar
Hi For the time constant, in 3.5 there is normally a time multiplier, setting it to 60 means default min, 3600 default hours, but I find it easier to use units (at least in V4 you have min, hour, day, year) and just make "t" largeer -- Good luck Ivar
OD Tuna
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Posted: 1 decade ago 8 avr. 2011, 04:55 UTC−4
hi, thanks for your fast reply. where can i find this time multiplier?
hi, thanks for your fast reply. where can i find this time multiplier?
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 8 avr. 2011, 05:54 UTC−4
Hi

Sorry I cannot find it back, 3.5 is getting old ;) I could not find it either in the doc,. maybe it was/is a application mode specific expression/parameter.

Anyhow what works OK by using t=2[day] or t=36[hour] in both 3.5 and 4.

--
Good luck
Ivar
Hi Sorry I cannot find it back, 3.5 is getting old ;) I could not find it either in the doc,. maybe it was/is a application mode specific expression/parameter. Anyhow what works OK by using t=2[day] or t=36[hour] in both 3.5 and 4. -- Good luck Ivar
OD Tuna
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Posted: 1 decade ago 11 avr. 2011, 07:48 UTC−4
thank you, my model works OK now.
thank you, my model works OK now.
MURACA Gaelle
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Posted: 1 decade ago 24 mai 2011, 15:41 UTC−4
Please can you explain how do you change the "t" unit ? I have the same error and I don't find how change the time unit. Thank you

Gaelle
Please can you explain how do you change the "t" unit ? I have the same error and I don't find how change the time unit. Thank you Gaelle
Joke Lee
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Posted: 1 decade ago 14 sept. 2011, 06:58 UTC−4

Hi

Sorry I cannot find it back, 3.5 is getting old ;) I could not find it either in the doc,. maybe it was/is a application mode specific expression/parameter.

Anyhow what works OK by using t=2[day] or t=36[hour] in both 3.5 and 4.

--
Good luck
Ivar


I am searching this thing and now found it. Thank you so much Ivar.

Best regards.

[QUOTE] Hi Sorry I cannot find it back, 3.5 is getting old ;) I could not find it either in the doc,. maybe it was/is a application mode specific expression/parameter. Anyhow what works OK by using t=2[day] or t=36[hour] in both 3.5 and 4. -- Good luck Ivar [/QUOTE] I am searching this thing and now found it. Thank you so much Ivar. Best regards.
Joke Lee
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Posted: 1 decade ago 14 sept. 2011, 07:06 UTC−4

Please can you explain how do you change the "t" unit ? I have the same error and I don't find how change the time unit. Thank you

Gaelle


I changed time unit by this way, normally (default) time unit is second, now when I want to set up time to solve is minute, hour, day, year... I setup time like this: 3[min] or 5[d] or 8[h]... etc

For example: when the time unit is default, I type time like this (in the quotation mark): Times: "range(0,0.1,120)" second
Now: I can type: Times: "range(0,0.1,2[min])" second.

Hope you can find out what you need.
[QUOTE] Please can you explain how do you change the "t" unit ? I have the same error and I don't find how change the time unit. Thank you Gaelle [/QUOTE] I changed time unit by this way, normally (default) time unit is second, now when I want to set up time to solve is minute, hour, day, year... I setup time like this: 3[min] or 5[d] or 8[h]... etc For example: when the time unit is default, I type time like this (in the quotation mark): Times: "range(0,0.1,120)" second Now: I can type: Times: "range(0,0.1,2[min])" second. Hope you can find out what you need.

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