CFD Online Logo CFD Online URL
Home > Forums > Software User Forums > ANSYS > FLUENT

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

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Display Modes
Old   January 12, 2010, 14:53
Default How exactly does the "pressure outlet" bdry condition treat properties on the bdry?
New Member
Join Date: Jan 2010
Posts: 2
Rep Power: 0
yating9901 is on a distinguished road
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.
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.
yating9901 is offline   Reply With Quote

Old   January 13, 2010, 14:39
New Member
Join Date: Jan 2010
Posts: 2
Rep Power: 0
yating9901 is on a distinguished road
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.
yating9901 is offline   Reply With Quote

Old   February 9, 2010, 01:32
New Member
@p N
Join Date: Jan 2010
Location: United States
Posts: 27
Rep Power: 10
yvonne is on a distinguished road
thanx... it was helpful
yvonne is offline   Reply With Quote

Old   June 28, 2010, 12:26
Join Date: Mar 2010
Location: Canada,Ontario
Posts: 31
Rep Power: 9
engahmed is on a distinguished road
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???
engahmed is offline   Reply With Quote


Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
Boundary Conditions Thomas P. Abraham Main CFD Forum 20 July 7, 2013 05:05
inlet velocity boundary condition murali CFX 5 August 3, 2012 08:56
Transient outlet boundary condition problem jwillie2000 CFX 1 December 7, 2009 18:07
spanwiperiodic boundary condition for LES Sungho Yoon CFX 5 July 8, 2008 06:21
How to set boundary condition in Fluent for the fo Peiyong FLUENT 1 November 10, 2006 12:44

All times are GMT -4. The time now is 07:18.