Fully Developed B.C. in CFX

safikhani_hamed · July 5, 2012, 01:30

Dear friends,
I am going to simulate fluid flow in a circular tube using CFX, at the outlet of the tubes "Fully developed" boundary condition should be used, in CFX what boundary condition is related to fully developed flow ? in fact in fluent there is an " outflow" B.C. but in CFX I do not know.

Sincerely

ghorrocks · July 5, 2012, 05:59

Use an outlet.

safikhani_hamed · July 5, 2012, 07:03

thank you but are you sure that "outlet" means fully developed condition ?

ghorrocks · July 5, 2012, 08:01

Read the documentation for what an outlet means mathematically, but unfortunately I think you will find the description incomplete. By fully developed most people seem to think zero normal gradient. CFX takes a bit of a different approach and makes the boundary condition part of the solution variables, that is the boundary value for the other variables are convected/diffused from the domain.

safikhani_hamed · July 5, 2012, 12:56

thank you gorrocks , I read the documentation but I have another question, in the documentation speak just about velocity fully developed boundary condition, but do you know can impose the thermally fully developed boundary condition ?

ghorrocks · July 5, 2012, 18:37

As discussed in the documentation, CFX uses the value convected from the domain as the thermal condition on an outlet boundary. This is different to a zero normal gradient boundary which some other codes use.

But in fully developed flow both approaches are equivalent.

So to call the zero normal gradient approach a "fully developed flow boundary" and the CFX approach as something else is missing the point.

safikhani_hamed · July 5, 2012, 21:25

Dear ghorrocks, thank you from your help and sorry for consuming your time, in fact I test what you have told but unfortunately there is not "zero gradient" item in the outlet boundary condition of a laminar flow, where is that?

Sincerely

ghorrocks · July 6, 2012, 02:03

CFX does not have an option with zero gradient at the outlet.

July 5, 2012, 01:30	Fully Developed B.C. in CFX	#1
safikhani_hamed Member Hamed Join Date: Oct 2009 Posts: 64 Rep Power: 16	Dear friends, I am going to simulate fluid flow in a circular tube using CFX, at the outlet of the tubes "Fully developed" boundary condition should be used, in CFX what boundary condition is related to fully developed flow ? in fact in fluent there is an " outflow" B.C. but in CFX I do not know. Sincerely

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July 5, 2012, 05:59		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,711 Rep Power: 143	Use an outlet.

July 5, 2012, 07:03		#3
safikhani_hamed Member Hamed Join Date: Oct 2009 Posts: 64 Rep Power: 16	thank you but are you sure that "outlet" means fully developed condition ?

July 5, 2012, 08:01		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,711 Rep Power: 143	Read the documentation for what an outlet means mathematically, but unfortunately I think you will find the description incomplete. By fully developed most people seem to think zero normal gradient. CFX takes a bit of a different approach and makes the boundary condition part of the solution variables, that is the boundary value for the other variables are convected/diffused from the domain.

July 5, 2012, 12:56		#5
safikhani_hamed Member Hamed Join Date: Oct 2009 Posts: 64 Rep Power: 16	thank you gorrocks , I read the documentation but I have another question, in the documentation speak just about velocity fully developed boundary condition, but do you know can impose the thermally fully developed boundary condition ?

July 5, 2012, 18:37		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,711 Rep Power: 143	As discussed in the documentation, CFX uses the value convected from the domain as the thermal condition on an outlet boundary. This is different to a zero normal gradient boundary which some other codes use. But in fully developed flow both approaches are equivalent. So to call the zero normal gradient approach a "fully developed flow boundary" and the CFX approach as something else is missing the point.

July 5, 2012, 21:25		#7
safikhani_hamed Member Hamed Join Date: Oct 2009 Posts: 64 Rep Power: 16	Dear ghorrocks, thank you from your help and sorry for consuming your time, in fact I test what you have told but unfortunately there is not "zero gradient" item in the outlet boundary condition of a laminar flow, where is that? Sincerely

July 6, 2012, 02:03		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,711 Rep Power: 143	CFX does not have an option with zero gradient at the outlet.