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easy question, I think...

Posted Nov 3, 2011, 12:19 p.m. EDT Heat Transfer & Phase Change Version 4.2 5 Replies

Juan-Luis Valera
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Dear friends,


I have a problem with COMSOL 4.2. It should be a silly thing, but I cannot solve it. I am absolutely new at comsol, besides.

I am starting using Comsol 4.2 and I have prepared a simple model: 1D, heat transfer in solids, a hot long bar with instantaneous cooling at the top. I prepare the mesh, choose Time-Dependent study, compute the model and T behaves correctly. I plot the heat flux (ht.fluxx) wrt time and it is correct. But I now tried to compute the time derivative of the ht.fluxx d(ht.fluxx,t) and I only get zeros. I use the Derived Values node to get numerical values and still get zeros. I'm storing time derivatives, also.

Some people told m that probably my results are beyond the mathematical resolution, but I don't think so. The heat fluxx variation is of the order of 0.1 and the time-step is of the order of 3e7. Moreover, I have change the time-step, I have used finer meshes. But still I get zeros.

Could somebody give some clue?

Thank you
5 Replies Last Post Nov 7, 2011, 3:10 p.m. EST
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Nov 4, 2011, 3:14 a.m. EDT
Hi

COMSOL extracts (mostly ?) derivatives from the shape functions you use. If your shape function is linear you might not manage to get out any derivative, so first issue check the shape function setting, try to use a higher order 2nd or 3rd it should easy work for your 1D model

Also check if COMSOl does not already have a derivative defined internally, but I'm not sure "Tt" or "Ttt" exist, look up the doc

--
Good luck
Ivar
Hi COMSOL extracts (mostly ?) derivatives from the shape functions you use. If your shape function is linear you might not manage to get out any derivative, so first issue check the shape function setting, try to use a higher order 2nd or 3rd it should easy work for your 1D model Also check if COMSOl does not already have a derivative defined internally, but I'm not sure "Tt" or "Ttt" exist, look up the doc -- Good luck Ivar
Juan-Luis Valera
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Posted: 1 decade ago Nov 7, 2011, 9:27 a.m. EST
Thank you, Ivar.

I have found the shape functions at the Equation View. They are lagrange type. But I cannot change them there, at the "equation view" of the "Heat Transfer in solids" node. Is there any trick? Or maybe I cannot choose another one because it is 1D?
I do not know...
Thank you, Ivar. I have found the shape functions at the Equation View. They are lagrange type. But I cannot change them there, at the "equation view" of the "Heat Transfer in solids" node. Is there any trick? Or maybe I cannot choose another one because it is 1D? I do not know...
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Nov 7, 2011, 10:39 a.m. EST
Hi

you probably need to turn on the "Discretisation" tabs, see the preferences or the view icon

--
Good luck
Ivar
Hi you probably need to turn on the "Discretisation" tabs, see the preferences or the view icon -- Good luck Ivar
Juan-Luis Valera
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Posted: 1 decade ago Nov 7, 2011, 11:35 a.m. EST
Thank you for your time, Ivar. Thank you....

I turned on everything about the discretisation I have found (at the preferences, at the show icon...) but still I cannot edit the type of shape function...
Thank you for your time, Ivar. Thank you.... I turned on everything about the discretisation I have found (at the preferences, at the show icon...) but still I cannot edit the type of shape function...
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago Nov 7, 2011, 3:10 p.m. EST
Hi

could it be this one ? (I'm in v4.2a)

--
Good luck
Ivar
Hi could it be this one ? (I'm in v4.2a) -- Good luck Ivar

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