CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

Why zero pressure boundary condition?

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

Like Tree3Likes
  • 2 Post By LuckyTran
  • 1 Post By FMDenaro

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   October 12, 2020, 09:25
Default Why zero pressure boundary condition?
  #1
New Member
 
Join Date: Oct 2020
Posts: 15
Rep Power: 5
SuperSonicCat is on a distinguished road
Hello,


i am currently studying CFD. In many examples for a simulation of internal flows the zero pressure bc at the outlet is set. I am wondering why? In my understanding that means vacuum at the outlet.

When can i set the pressure bc to zero and when i should set the gradient of the pressure to zero?

Lets say i want to simulate a flow through a pipe- segment. I only know the inlet velocity. Which pressure outlet bc should i use if the pressure at the outlet is unknown?


I really want to understand that


Best regards.
SuperSonicCat is offline   Reply With Quote

Old   October 12, 2020, 09:37
Default
  #2
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,664
Rep Power: 65
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
Most CFD software works in gauge pressures. The zero pressure at the outlet is a gauge pressure and not the absolute pressure.


Which outlet BC to use depends on a lot of details which you have aptly omitted about your model: what equation of state you are using, whether your flow is compressible or not, details of the solver, etc. For incompressible flows, the pressure BC doesn't really matter and you just need to apply the BC to make your software happy. For compressible flows, you can't joke around.
cfd.rishabh and SuperSonicCat like this.
LuckyTran is offline   Reply With Quote

Old   October 12, 2020, 09:58
Default
  #3
New Member
 
Join Date: Oct 2020
Posts: 15
Rep Power: 5
SuperSonicCat is on a distinguished road
Hello LuckyTran,


thank you very much for your answer!


So if i want to simulate an incompressible, turbulent flow, zero pressure at the outlet is always a good choice?


Best regrads.
SuperSonicCat is offline   Reply With Quote

Old   October 12, 2020, 16:59
Default
  #4
New Member
 
Join Date: Oct 2020
Posts: 15
Rep Power: 5
SuperSonicCat is on a distinguished road
Hello everyone,


i think i understand it now.


In case of a simulation of a steady incompressible internal flow i can always choose the following boundary conditions:
fixed inlet velocity and zero outlet pressure.
Why:
- in most cases i know the inlet velocity (trivial)
- the pressure in NS- equations is given in form of a gradient, so
it is necessary to specify a fixed pressure value at one point
in order to get one special solution of the equations
- If i set the pressure at outlet to zero, the pressure at the inlet
will be calculated by the solver to satisfy the NS- equations
- I can now calculate the pressure loss and i can analyse the

streamlines
- relative Pressure loss and streamlines are independent of the
chosen pressure outlet value.


Please tell me if something is wrong.


Best regards
SuperSonicCat is offline   Reply With Quote

Old   October 14, 2020, 13:33
Default
  #5
Senior Member
 
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 6,762
Rep Power: 71
FMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura aboutFMDenaro has a spectacular aura about
Quote:
Originally Posted by SuperSonicCat View Post
Hello everyone,


i think i understand it now.


In case of a simulation of a steady incompressible internal flow i can always choose the following boundary conditions:
fixed inlet velocity and zero outlet pressure.
Why:
- in most cases i know the inlet velocity (trivial)
- the pressure in NS- equations is given in form of a gradient, so
it is necessary to specify a fixed pressure value at one point
in order to get one special solution of the equations

- If i set the pressure at outlet to zero, the pressure at the inlet
will be calculated by the solver to satisfy the NS- equations
- I can now calculate the pressure loss and i can analyse the

streamlines
- relative Pressure loss and streamlines are independent of the
chosen pressure outlet value.


Please tell me if something is wrong.


Best regards



No, there is no such a necessary condition to get a solution for the pressure equation. Theoretically, one solution exists provides the compatibility equation is fulfilled. Such solution is determined implicitly by the solver (apart a constant).
SuperSonicCat likes this.
FMDenaro is offline   Reply With Quote

Reply

Tags
pressure boundaries

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Appropriate pressure boundary condition in incompressible flow lonelywing OpenFOAM Running, Solving & CFD 21 June 6, 2022 09:44
Wind turbine simulation Saturn CFX 58 July 3, 2020 01:13
Total Pressure boundary condition in the OpenFOAM dli OpenFOAM Programming & Development 1 December 5, 2017 23:16
Question about adaptive timestepping Guille1811 CFX 25 November 12, 2017 17:38
Question about heat transfer coefficient setting for CFX Anna Tian CFX 1 June 16, 2013 06:28


All times are GMT -4. The time now is 05:11.