# Outflow condition CfX-Fluent

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 March 6, 2011, 06:16 Outflow condition CfX-Fluent #1 Member   anonymous Join Date: Jan 2011 Posts: 42 Rep Power: 7 Hi All ; till now no response about the equivalent of outflow condition to setup a BC in CFX Pre witch means ( a zero first derivative of all variables flow ). this conditition is used in fluent. this is the post of a CFD user . I'm moving from Fluent to CFX. There is this boundary condition in Fluent named outfow where the solver sets the normal derivative of flow variables to zero for that boundary edge. How is it possible to use this boundary condition in CFX. As far as I've understood there is only INLET, OUTLET, OPENING, WALL and SYMMETRY boundary conditions in CFX. Any help is really appreciated. many thanks. rgd likes this.

 March 6, 2011, 18:14 #2 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,638 Rep Power: 98 Incompressible or compressible flow? If compressible, sub-sonic or supersonic?

 March 7, 2011, 05:57 #3 Member   anonymous Join Date: Jan 2011 Posts: 42 Rep Power: 7 incompressible and sub-sonic flow.

 March 7, 2011, 19:57 #4 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,638 Rep Power: 98 The outlet in CFX is similar. It can be set up to use zero gradient on all parameters except pressure which needs to be defined.

 March 8, 2011, 07:02 #5 Member   anonymous Join Date: Jan 2011 Posts: 42 Rep Power: 7 how can I set up CFX outlet to use a zero gradient on all parameters ? more details please.

 March 8, 2011, 18:01 #6 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,638 Rep Power: 98 That is a symmetry plane. You have to define at least one parameter at an outlet.

June 10, 2011, 16:05
#7
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Joseph Tipton
Join Date: Jun 2010
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Dr. Horrocks,

I'm a little confused by your statement here regarding a symmetry plane BC in CFX. I don't see how it could be used as an outlet BC since it requires zero normal velocity at the symmetry plane in addition to zero normal gradients of all variables.

It is my understanding that, in Fluent, you can specify a "outflow" boundary condition which specifies a zero diffusion flux for all flow variables along with an overall mass balance correction. From the Fluent User's Guide, Section 7.3.2:
Quote:
 Outflow boundary conditions are used to model flow exits where the details of the flow velocity and pressure are not known prior to solution of the flow problem. They are appropriate where the exit flow is close to a fully developed condition, as the outflow boundary condition assumes a zero streamwise gradient for all flow variables except pressure. They are not appropriate for compressible flow calculations.
The CFX documentation I have read does not seem to address how this could be implimented. Do you have any insight that you could share?

 June 10, 2011, 16:21 #8 New Member   Joseph Tipton Join Date: Jun 2010 Posts: 14 Rep Power: 8 I'll add that this question also relates to a previous topic: Outflow B.C. and CFX This issue is that, when modeling pipe flow, I often don't know the pressure. How is it possible to specify a velocity or mass flow rate at the inlet along with zero gradients in all variables at the outlet?

June 11, 2011, 07:40
#9
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Glenn Horrocks
Join Date: Mar 2009
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Quote:
 I'm a little confused by your statement here regarding a symmetry plane BC in CFX. I don't see how it could be used as an outlet BC since it requires zero normal velocity at the symmetry plane in addition to zero normal gradients of all variables.
The comment was "zero gradient on all parameters". That is a symmetry plane, but I guess the symmetry plane has the additional constraint of no flow normal to it. I guess a bit cheeky of me to change the subject like that but the question was not very specific.

Quote:
 This issue is that, when modeling pipe flow, I often don't know the pressure. How is it possible to specify a velocity or mass flow rate at the inlet along with zero gradients in all variables at the outlet?
No, you need to specify pressure at the outlet in your case. But for incompressible flows the pressure is relative anyway, so the outlet just becomes a reference pressure. You obviously need to be more careful in compressible flows.

 December 22, 2014, 10:54 #10 New Member   saleh Join Date: Nov 2014 Posts: 16 Rep Power: 3 Dear All What is the best outlet boundary condition for transonic(subsonic inlet and outlet but transonic passage) compressor and in general transonic turbomachines? why? I would like to have specified inlet mass flow rate. I use total pressure(because of more stable and better convergence behavior than inlet mass flow rate) at inlet but by applying static pressure at outlet, desired mass flow rate is not be obtained. Last edited by sfallah; December 23, 2014 at 01:11.

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