How exactly the "pressure outlet" bdry condition compute properties on the boundary?

yating9901 · January 12, 2010, 14:53

Hello, Fluent users,

I wrote a Fortran code which can simulate 3-D unsteady flow. In order to validate it, I use both my Fortran code and Fluent to simulate a very simple case, the development of the buoyant flow against a vertical hot wall in an open domain. I know I have to set my computational domain large enough to mimic an open domain. It turns out the "pressure outlet" boundary condition in Fluent is more "efficient" to do that. i.e. 20 cm domain size in the vertical direction is enough in Fluent while my own Fortran code requires 50 cm domain size.

In my code, the boundary conditions at the exit of the domain (X=Xmax) are all set to be gradient free (i.e. dUi/dx=0, dT/dx=0). After studying the manual and doing some research online, I realized my BC's correspond to the "outflow" boundary condition in Fluent.

So I am very curious how Fluent treats properties on the boundary when "pressure outlet" is chosen. I know what happens when backflow occurs, the temperature is assigned and the backflow direction is specified. But I can not find anything about how Fluent computes everything when backflow does not occur.

Here is an earlier thread which relates to my question.
http://www.cfd-online.com/Forums/flu...re-outlet.html
It helped me a little bit but did not completely solve my problem.

Sorry, my English is not very good and I may not explain my question very clearly. Here is my question:

I know for "outflow" boundary condition, diffusion free assumption (d/dx=0) is imposed on the boundary. But how does Fluent treat properties on the boundary when "pressure outlet" is chosen. What is the mathematical form of the boundary condition for each governing equation? I know pressure is specified to a certain value, how about the rest, u, v, w, and T?!

This question has bothered me for quite a while. Any advices will be greatly appreciated!

yating9901 · January 13, 2010, 14:39

Here is something I found after posting my question.

"At pressure outlets, FLUENT uses the boundary condition pressure you input as the static pressure of the fluid at the outlet plane, ps, and extrapolates all other conditions from the interior of the domain."

It answered my question. Hope it may help people who have the same question.

yvonne · February 9, 2010, 01:32

thanx... it was helpful

engahmed · June 28, 2010, 13:26

Hi, can you please explain more what does "exporlates the other conditions from the interior of the domain" mean? so how does it treat v, w, and T???

January 12, 2010, 14:53	How exactly does the "pressure outlet" bdry condition treat properties on the bdry?	#1
yating9901 New Member Join Date: Jan 2010 Posts: 2 Rep Power: 0	Hello, Fluent users, I wrote a Fortran code which can simulate 3-D unsteady flow. In order to validate it, I use both my Fortran code and Fluent to simulate a very simple case, the development of the buoyant flow against a vertical hot wall in an open domain. I know I have to set my computational domain large enough to mimic an open domain. It turns out the "pressure outlet" boundary condition in Fluent is more "efficient" to do that. i.e. 20 cm domain size in the vertical direction is enough in Fluent while my own Fortran code requires 50 cm domain size. In my code, the boundary conditions at the exit of the domain (X=Xmax) are all set to be gradient free (i.e. dUi/dx=0, dT/dx=0). After studying the manual and doing some research online, I realized my BC's correspond to the "outflow" boundary condition in Fluent. So I am very curious how Fluent treats properties on the boundary when "pressure outlet" is chosen. I know what happens when backflow occurs, the temperature is assigned and the backflow direction is specified. But I can not find anything about how Fluent computes everything when backflow does not occur. Here is an earlier thread which relates to my question. http://www.cfd-online.com/Forums/flu...re-outlet.html It helped me a little bit but did not completely solve my problem. Sorry, my English is not very good and I may not explain my question very clearly. Here is my question: I know for "outflow" boundary condition, diffusion free assumption (d/dx=0) is imposed on the boundary. But how does Fluent treat properties on the boundary when "pressure outlet" is chosen. What is the mathematical form of the boundary condition for each governing equation? I know pressure is specified to a certain value, how about the rest, u, v, w, and T?! This question has bothered me for quite a while. Any advices will be greatly appreciated! Last edited by yating9901; January 12, 2010 at 16:34.

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January 13, 2010, 14:39		#2
yating9901 New Member Join Date: Jan 2010 Posts: 2 Rep Power: 0	Here is something I found after posting my question. "At pressure outlets, FLUENT uses the boundary condition pressure you input as the static pressure of the fluid at the outlet plane, ps, and extrapolates all other conditions from the interior of the domain." It answered my question. Hope it may help people who have the same question.

February 9, 2010, 01:32		#3
yvonne New Member @p N Join Date: Jan 2010 Location: United States Posts: 27 Rep Power: 16	thanx... it was helpful

June 28, 2010, 13:26		#4
engahmed Member engahmed Join Date: Mar 2010 Location: Canada,Ontario Posts: 31 Rep Power: 16	Hi, can you please explain more what does "exporlates the other conditions from the interior of the domain" mean? so how does it treat v, w, and T???