CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM

Pressure gradient, channelFoam

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 4, 2012, 03:57
Default Pressure gradient, channelFoam
  #1
New Member
 
Melika
Join Date: Feb 2012
Location: Japan
Posts: 24
Rep Power: 14
libra is on a distinguished road
Hi everyone
I am doing simulations for a pipe with an obstacle inside using LES in openFOAM. I have recently got some results for pressure and velocity. Velocity data looks fine but pressure changes linearly from inlet to the outlet which for sure is not correct.
I am wondering if channelFoam assumes a constant pressure gradient.
I would appreciate any comment about this problem.
Thanks.
libra is offline   Reply With Quote

Old   July 5, 2012, 10:50
Default
  #2
Member
 
Franco Marra
Join Date: Mar 2009
Location: Napoli - Italy
Posts: 68
Rep Power: 17
francescomarra is on a distinguished road
Dear Libra,

channelFoam adds a constant (in space) pressure gradient to the momentum equation and corrects it every time step to ensure the flow rate desired. For a periodic channel without obstacles the pressure have to vary linearly from inlet to outlet. Therefore this pressure contribute tends to be the only one in the average.

Of course, when turbulence develops, the computed pressure (from the elliptic equation) have to equilibrate the flow, so that proper fluctuations have to appear in the computed pressure field.

However I can expect that the intensities of these fluctuations are much smaller than the average pressure variation.

Are you sure you are not missing your pressure variations just because of the way you observe them ?

Best regards,

Franco
francescomarra is offline   Reply With Quote

Old   July 6, 2012, 04:15
Default
  #3
New Member
 
Melika
Join Date: Feb 2012
Location: Japan
Posts: 24
Rep Power: 14
libra is on a distinguished road
Hey Franco
Thanks for your reply. As you said pressure gradient should be corrected for every time step. But it seems that it has not happened in my case.
I checked pressure data for some time steps in paraFoam and for all of them pressure gradient has a constant value.
I have started running my case with paraFoam and I do not have such a problem when using this solver.
I'm still wondering why my pressure data is like this.
Thank you again.
Melika
libra is offline   Reply With Quote

Old   July 6, 2012, 06:40
Default
  #4
Member
 
Franco Marra
Join Date: Mar 2009
Location: Napoli - Italy
Posts: 68
Rep Power: 17
francescomarra is on a distinguished road
Dear Melika,

it is not easy to help with so few info. I will try.

paraFoam is just a postprocessing utility, so I do not understand what exactly you mean when you say that you had no problems computing the pressure with this application.

In the first message you noticed that velocity data looks fine. Now velocity cannot be good if pressure is not correctly solved. Maybe your problem is just to correctly visualize your results.
I would try to use foamToVTK to transform all your time data in the native format of Paraview and then use paraview directly to load and animate all time steps you saved of your computation.

For more help, more details about your case and setup and some images of results are needed.

Best regards,

Franco
francescomarra is offline   Reply With Quote

Old   July 11, 2012, 01:40
Default
  #5
New Member
 
Melika
Join Date: Feb 2012
Location: Japan
Posts: 24
Rep Power: 14
libra is on a distinguished road
Hey Franco

Thanks again for your reply. Actually I think the problem is not about the visualization. There is sth wrong with the data I have got after sampling. As you know after you sample across a line you get this file "line_p_k.xy" that contains the pressure data. Even if I plot the pressure data across the pipe just for one time step, I will get a line!!! Actually this is obvious when I look at the data.
I have attached a view of my case. I have got a long pipe with a half-disk obstacle inside. I also attached this file "line_p_k.xy" for one time step.
In this file the first column is the coordinate on the center line from the inlet to the outlet. As you see if you plot the second column (P) vs. the first column you will get a line.
I will appreciate any comment.
Thanks a lot.

Melika

Mesh.jpg
line_p_k.txt
libra is offline   Reply With Quote

Reply

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
Pressure Outlet Guage pressure Mohsin FLUENT 36 April 29, 2016 18:16
how to get pressure and pressure gradient using UDF Honglin Fluent UDF and Scheme Programming 0 May 24, 2012 07:47
lid-driven cavity in matlab using BiCGStab Don456 Main CFD Forum 1 January 19, 2012 16:00
steam flow in a pipe driven by a pressure gradient between inlet and outlet SalvoCalvo COMSOL 0 March 11, 2010 07:52
Inputting pressure gradient in Fluent Josyula FLUENT 2 December 23, 2009 08:26


All times are GMT -4. The time now is 20:34.