zero-gradient BC at the outlet

lostking18 · April 12, 2021, 17:23

Hi, there

I’m validating a pipe flow simulation with the experiments. At the exit I need to apply the zero-gradient BC at the outlet to indicate the flow has fully developed:
partial-d u/ partial-d x = 0
I saw fluent and openfoam have option to apply zero-gradient BC at the outlet. But in CFX, I didn’t find it. I can only assign the outlet pressure but the outlet pressure value is not shown in the experimental paper. Could you tell me how to apply zero-gradient BC at the outlet?

Thank you in advance!

ghorrocks · April 12, 2021, 18:13

CFX does not have this boundary condition.

If the flow is incompressible then it does not matter - you can use a pressure boundary condition at the outlet and it will set the pressure level (assuming you have not set the pressure anywhere else) and the momentum equation will have zero gradient applied. If the flow is compressible (or material properties are a function of pressure) then things get more complex.

April 12, 2021, 17:23	zero-gradient BC at the outlet	#1
lostking18 Member phd Join Date: Oct 2013 Posts: 76 Rep Power: 12	Hi, there I’m validating a pipe flow simulation with the experiments. At the exit I need to apply the zero-gradient BC at the outlet to indicate the flow has fully developed: partial-d u/ partial-d x = 0 I saw fluent and openfoam have option to apply zero-gradient BC at the outlet. But in CFX, I didn’t find it. I can only assign the outlet pressure but the outlet pressure value is not shown in the experimental paper. Could you tell me how to apply zero-gradient BC at the outlet? Thank you in advance!

April 12, 2021, 18:13		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	CFX does not have this boundary condition. If the flow is incompressible then it does not matter - you can use a pressure boundary condition at the outlet and it will set the pressure level (assuming you have not set the pressure anywhere else) and the momentum equation will have zero gradient applied. If the flow is compressible (or material properties are a function of pressure) then things get more complex. lostking18 and aero_head like this. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

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