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

units in OpenFOAM

Register Blogs Community New Posts Updated Threads Search

Like Tree4Likes
  • 2 Post By megacrout
  • 1 Post By akidess
  • 1 Post By fabian_roesler

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 15, 2011, 06:44
Default units in OpenFOAM
  #1
Member
 
Tibo
Join Date: Jun 2011
Posts: 68
Rep Power: 15
megacrout is on a distinguished road
Hi,

I am getting confused about the units used in OpenFOAM for some parameters, especially p and phi.

1. p has to be given with units m2/s2 in simpleFoam (which actually are the units of p/rho) but with the usual units kg/m/s2 in reactingFoam for example.
I´m pretty sure this has something to do with incompressibility but I am not quite sure and I don´t get why it is programmed this way.

2. phi is rho.U (e.g. User Guide 1.7.1 p.115 or 2.0.0 p.118), right? It thus should have units kg/m2/s but has units m3/s in the time step folders. why is that so?
This question might result from a bad understanding of what phi is. Complementary information about it too would be appreciated.

Thanks for your help.

Tibo
rafa13 and hulli like this.
megacrout is offline   Reply With Quote

Old   July 15, 2011, 08:14
Default
  #2
Senior Member
 
akidess's Avatar
 
Anton Kidess
Join Date: May 2009
Location: Germany
Posts: 1,377
Rep Power: 30
akidess will become famous soon enough
1. With constant density, you can divide the entire equation with rho and you will end up with a modified pressure, and thus the "uncommon" units.

2. phi is U * Sf (velocity times surface area) in all the solvers I worked with.
rafa13 likes this.
akidess is offline   Reply With Quote

Old   July 18, 2011, 06:37
Default
  #3
Member
 
Tibo
Join Date: Jun 2011
Posts: 68
Rep Power: 15
megacrout is on a distinguished road
Hi Anton,

Thanks for your answer.
I´m a bit confused though. As I mentioned, phi is defined as rho * U in the User Guide (p.115 or p.118). Does that mean the definition varies depending on the situation studied or the solver used?

Best regards.

Tibo
megacrout is offline   Reply With Quote

Old   July 19, 2011, 06:44
Smile Phi units
  #4
Senior Member
 
Fabian Roesler
Join Date: Mar 2009
Location: Germany
Posts: 213
Rep Power: 18
fabian_roesler is on a distinguished road
Hi,

in simpleFoam you assume constant density.

phi = rho*U*Sf [kg/s] in the convective term

as rho is constant you can divide the whole equation by rho as akidess posted before. And so you get

phi = U*Sf [m³/s]

Regards

Fabian
rafa13 likes this.
fabian_roesler is offline   Reply With Quote

Old   July 19, 2011, 09:04
Default
  #5
Member
 
Tibo
Join Date: Jun 2011
Posts: 68
Rep Power: 15
megacrout is on a distinguished road
All right!
Crystal clear.
thx
megacrout is offline   Reply With Quote

Reply

Tags
phi, pressure, units


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
Cross-compiling OpenFOAM 1.7.0 on Linux for Windows 32 and 64bits with Mingw-w64 wyldckat OpenFOAM Announcements from Other Sources 3 September 8, 2010 07:25
Modified OpenFOAM Forum Structure and New Mailing-List pete Site News & Announcements 0 June 29, 2009 06:56
64bitrhel5 OF installation instructions mirko OpenFOAM Installation 2 August 12, 2008 19:07
Adventure of fisrst openfoam installation on Ubuntu 710 jussi OpenFOAM Installation 0 April 24, 2008 15:25
OpenFOAM Debian packaging current status problems and TODOs oseen OpenFOAM Installation 9 August 26, 2007 14:50


All times are GMT -4. The time now is 14:27.