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Which BC needed?

Posted Nov 24, 2014, 2:28 a.m. EST Fluid & Heat Version 4.4 3 Replies

Jonas Lölsberg
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Hey there,

I am quite new to comsol so maybe there is an easy way to do it. I have a pipe with an internal helix structure which is repeated after a 180° twist. I want to use this symmetry to save computational power. Therefore I need to find boundary conditions, where the flow field at the outlet is equal to the flow field at the inlet.

I already tried periodic flow conditions but it resolves in a flow speed of zero since the inlet/outlet pressure difference is equal.

The model is attached. Do you have any suggestions for me?

Best regards
Jonas

Attachments:

3 Replies Last Post Nov 25, 2014, 2:11 a.m. EST
Mikael Noerregaard Nielsen
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Posted: 10 years ago Nov 24, 2014, 3:44 a.m. EST
Hey Jonas

A symmetry boundary condition exists in COMSOL which can be selected like any other surface boundary condition.

Just to clarify: What are you trying to model, meaning which results are you looking for?

Best regards

Mikael

Hey Jonas A symmetry boundary condition exists in COMSOL which can be selected like any other surface boundary condition. Just to clarify: What are you trying to model, meaning which results are you looking for? Best regards Mikael
Lasse Murtomäki
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Posted: 10 years ago Nov 24, 2014, 3:54 a.m. EST
Hi Jonas

What you are asking is not possible: Your input flow profile is flat, 1 m/s over the entire inlet face. Along the tube the flow profile develops, so outlet cannot be same as inlet. My suggestion is that you add another similar geometry object, and use the first one only as a flow development step, and do the rest of your simulation in the second one. In the case of a plain tube, a fully developed Poisseulle flow can be used as a BC, and then there is no more change in the flow profile within the tube.

I wish I understood your problem set-up properly.

br
Lasse
Hi Jonas What you are asking is not possible: Your input flow profile is flat, 1 m/s over the entire inlet face. Along the tube the flow profile develops, so outlet cannot be same as inlet. My suggestion is that you add another similar geometry object, and use the first one only as a flow development step, and do the rest of your simulation in the second one. In the case of a plain tube, a fully developed Poisseulle flow can be used as a BC, and then there is no more change in the flow profile within the tube. I wish I understood your problem set-up properly. br Lasse
Ivar KJELBERG COMSOL Multiphysics(r) fan, retired, former "Senior Expert" at CSEM SA (CH)
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Posted: 10 years ago Nov 25, 2014, 2:11 a.m. EST
Hi

I believe still your remark above, about the pressure drop, is correct, that is: you must ensure the pressure is not "reset" by the periodic conditions.
But I believe this is possible (I do not have access to COMSOL just now, all licences fully booked and used , it's popular here :)

In the "Equation" view you should be able to check which variables are concerned by the periodic condition, and if needed correct the equation at will. Be aware in that case you "block" e equations and COMSOL will no longer adapt the equations if you change your model, that becomes then your responsibility, a small "lock" reminds you about this in the model tree in front of the node.

On the other side you must ensure fully developed flow, as your periodic condition means you are somewhere in the "middle" of your tubing

--
Good luck
Ivar
Hi I believe still your remark above, about the pressure drop, is correct, that is: you must ensure the pressure is not "reset" by the periodic conditions. But I believe this is possible (I do not have access to COMSOL just now, all licences fully booked and used , it's popular here :) In the "Equation" view you should be able to check which variables are concerned by the periodic condition, and if needed correct the equation at will. Be aware in that case you "block" e equations and COMSOL will no longer adapt the equations if you change your model, that becomes then your responsibility, a small "lock" reminds you about this in the model tree in front of the node. On the other side you must ensure fully developed flow, as your periodic condition means you are somewhere in the "middle" of your tubing -- Good luck Ivar

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