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zero-gradient BC at the outlet

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Old   April 12, 2021, 17:23
Default zero-gradient BC at the outlet
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Hi, there

Iím 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 didnít 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!
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Old   April 12, 2021, 18:13
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Glenn Horrocks
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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.
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boundary condition

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