# Neumann Pressure Boundary Condition with UDF

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 October 17, 2006, 14:30 Neumann Pressure Boundary Condition with UDF #1 Balder Guest   Posts: n/a Dear all, I wonder if someone is able to give me an UDF coding example to force a Neumann b.c. to the pressure field on a pressure outlet, I mean: dp/dn = 0 where n is the normal of a "pressure outlet" b.c. Thanks for your attention. Balder

 June 1, 2009, 12:57 #2 New Member   souviktor Join Date: Mar 2009 Posts: 11 Rep Power: 17 Hello, I am stuck with the same problem except the fact that I want to implement du/dx=0 for a laminar pipe flow at the outlet(fully developed condition). Have you got any work around for your problem so far?

 November 3, 2009, 11:09 #3 New Member   Santhosh Join Date: Oct 2009 Posts: 14 Rep Power: 16 Even I've the same problem, haven't got much help, how about you guys. The average static pressure=0 is the nearest matching BC. But I am not sure whether it serves any purpose.

 February 4, 2010, 04:39 #4 New Member   Zahra Join Date: Feb 2010 Posts: 3 Rep Power: 16 Why do'nt you use an outflow bc (if u dont have backflow)? this bc sets both dp/dn=0 and dv/dn=0 automatically. if you have backflow through your boundary, then these Boundary Conditions make the problem ill-posed. happy likes this.

 February 4, 2010, 09:44 #5 New Member   Santhosh Join Date: Oct 2009 Posts: 14 Rep Power: 16 Maintaining static pressure=0 will get you the neumann boundary condition. I confirmed it with the Ansys CFX tech people. thank you all..!

 August 24, 2010, 14:20 #6 Member   Behrang Asgharian Join Date: Oct 2009 Posts: 38 Rep Power: 16 I have the same problem with Temperature at the inlet for compressible flow. adiabatic: dT/dx=0

March 19, 2012, 01:37
questions
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Quote:
 Originally Posted by san.pirate Maintaining static pressure=0 will get you the neumann boundary condition. I confirmed it with the Ansys CFX tech people. thank you all..!
So what about K, zitha and temp.?does P=0 covers all variables?
IDo you meant CFX or FLUENT?

 June 25, 2012, 05:43 #8 Member   Satish Gupta Join Date: Jun 2012 Posts: 30 Rep Power: 13 I have written the UDF for convective B.C using DEFINE_PROFILE.....Can any1 tell me how to apply it at the outlet?

July 6, 2012, 11:21
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Quote:
 Originally Posted by Rahul123 I have written the UDF for convective B.C using DEFINE_PROFILE.....Can any1 tell me how to apply it at the outlet?
Do you mean a convective boundary condition for a scalar value? By scalar value I mean temperature, species mass fraction or user defined scalar. If so, You can define both the value itself and the derivative with respect to the normal direction to the surface. Firstly, of course you need to compile or interpret it. Then you will see the name of your udf at the relevant boundary condition panel.