Interface and Boundary Condition On same Surface?

steady-state · May 12, 2012, 18:29

Hi Folks,

Just a question about interfaces and boundary conditions. I am working on a CHT in CFX with workbench, and have an exterior air volume around a finned enclosure with some heat generation inside of it. I have a fluid-solid interface to translate heat from the solid to exterior air and all is good, but now I wish to add boundary conditions to the exterior enclosure that simulate solar radiation. Since the radiation falls on only some surfaces and with different irradiation values I need to add individual BCs to each surface (I think anyways), But, since I am already using those 200 some surfaces for the fluid solid interface I cant add more BCs...

Any ideas on how I can get a heat flux applied to some of the surfaces in the interface, but not the whole interface, without creating multiple interfaces for each group of surfaces with equal irradiation?

If I'm not being clear, please let me know and I'll rephrase.

ghorrocks · May 13, 2012, 06:49

You can use a heat source term, and make the heat source a function of position either by breaking the surface into different source term areas or using a conditional expression to nominate the heating sections.

Alternately you could use radiation modelling and put an irradiation BC where the radiation enters, then the heat loads will be calculated as part of the simulation.

steady-state · May 13, 2012, 16:54

Thanks for your reply Glenn,

Can you give an example of how I would break the surface into different source term areas? Or how I would create the conditional expression to nominate the heating sections?
I'm confused as to how I would practically implement that...

Thanks in advance!

ghorrocks · May 13, 2012, 18:32

Have a look into source terms. You can use them to specify a heat source or sink anywhere you like. If the heat source is an expression then you can add stuff to the expression so it only applies to certain regions, such as step(x)*(heat source) - the step term evaluates to 1 where x>0, or 0 if x<0 so the source term only applies to positive x. You can make more complex regions using combinations of step and if statements. Alternately you can use a 1D or 3D interpolation function which returns 1 where the heat source is applied and 0 where it is not. Then you can use an interpolation function to control where the heat source is applied.

May 12, 2012, 18:29	Interface and Boundary Condition On same Surface?	#1
steady-state New Member Scott Join Date: Mar 2012 Posts: 9 Rep Power: 14	Hi Folks, Just a question about interfaces and boundary conditions. I am working on a CHT in CFX with workbench, and have an exterior air volume around a finned enclosure with some heat generation inside of it. I have a fluid-solid interface to translate heat from the solid to exterior air and all is good, but now I wish to add boundary conditions to the exterior enclosure that simulate solar radiation. Since the radiation falls on only some surfaces and with different irradiation values I need to add individual BCs to each surface (I think anyways), But, since I am already using those 200 some surfaces for the fluid solid interface I cant add more BCs... Any ideas on how I can get a heat flux applied to some of the surfaces in the interface, but not the whole interface, without creating multiple interfaces for each group of surfaces with equal irradiation? If I'm not being clear, please let me know and I'll rephrase.

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May 13, 2012, 06:49		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	You can use a heat source term, and make the heat source a function of position either by breaking the surface into different source term areas or using a conditional expression to nominate the heating sections. Alternately you could use radiation modelling and put an irradiation BC where the radiation enters, then the heat loads will be calculated as part of the simulation.

May 13, 2012, 16:54		#3
steady-state New Member Scott Join Date: Mar 2012 Posts: 9 Rep Power: 14	Thanks for your reply Glenn, Can you give an example of how I would break the surface into different source term areas? Or how I would create the conditional expression to nominate the heating sections? I'm confused as to how I would practically implement that... Thanks in advance!

May 13, 2012, 18:32		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,700 Rep Power: 143	Have a look into source terms. You can use them to specify a heat source or sink anywhere you like. If the heat source is an expression then you can add stuff to the expression so it only applies to certain regions, such as step(x)*(heat source) - the step term evaluates to 1 where x>0, or 0 if x<0 so the source term only applies to positive x. You can make more complex regions using combinations of step and if statements. Alternately you can use a 1D or 3D interpolation function which returns 1 where the heat source is applied and 0 where it is not. Then you can use an interpolation function to control where the heat source is applied.