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Note: This discussion is about an older version of the COMSOL Multiphysics® software. The information provided may be out of date.

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Boundary condition T parameter

Posted 2012年11月12日 GMT-5 21:09 Heat Transfer & Phase Change, Parameters, Variables, & Functions, Studies & Solvers Version 4.2a 2 Replies

Ben Harrison
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Firstly: I have created a 'Heat Flux' boundary condition that varies due to an external temperature. My model is a 1D model of heat transfer in solids.

In the field labelled 'External temperature' I placed my expression for T_ext. Is there a notation for the initial value of T I can use here? Something like T_0?

Secondly: I can create a waveform function definition that mimics T_0+deltaT*cos(w*t) in global or local functions, called say T_bc, of a sine wave, with angular frequency = 2*pi/tau (tau is a global parameter), and amplitude = deltaT (another global parameter). I cannot enter this as my T_ext boundary condition in the Heat Flux boundary selection dialogue box. Why?

Ben
2 Replies Last Post 2012年11月13日 GMT-5 23:17
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 1 decade ago 2012年11月13日 GMT-5 15:30
Hi

it's not easy to understand what you want there, as variables can be scalars or fields (dependng on x,y,z,r) or even on t time ...

you can easily define an oscillating expression for any BC entry, only take care with the variable names, "t" is defind by COMSOL for time when you use a time series solver (is not defined els, except if you "pre-define" it as a Parameter. u,v,w are mostly the displacement s (in solid) or the velocity in CFD, etc

In COMSOL most entries can be numbers, scalars or fileds, explicit but mostly implicit (whixh is quite confusing in the beginning ;)

--
Good luck
Ivar
Hi it's not easy to understand what you want there, as variables can be scalars or fields (dependng on x,y,z,r) or even on t time ... you can easily define an oscillating expression for any BC entry, only take care with the variable names, "t" is defind by COMSOL for time when you use a time series solver (is not defined els, except if you "pre-define" it as a Parameter. u,v,w are mostly the displacement s (in solid) or the velocity in CFD, etc In COMSOL most entries can be numbers, scalars or fileds, explicit but mostly implicit (whixh is quite confusing in the beginning ;) -- Good luck Ivar
Ben Harrison
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Posted: 1 decade ago 2012年11月13日 GMT-5 23:17
I have defined a waveform function, you can see the definition in this image:

dl.dropbox.com/u/22417033/LocalDef.png

I tried to use the function's name (wv2) in the definition of my inward heat flux boundary condition:

dl.dropbox.com/u/22417033/HeatFluxDef.png

As you can see it is red. The popup error message says 'Unknown variable'.

I have defined a waveform function, you can see the definition in this image: https://dl.dropbox.com/u/22417033/LocalDef.png I tried to use the function's name (wv2) in the definition of my inward heat flux boundary condition: https://dl.dropbox.com/u/22417033/HeatFluxDef.png As you can see it is red. The popup error message says 'Unknown variable'.

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