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

root mean square

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

Reply
 
LinkBack Thread Tools Display Modes
Old   May 29, 2002, 13:48
Default root mean square
  #1
John
Guest
 
Posts: n/a
Dear ALL

I am currently trying to model water flowing through a pipe in Fluent 5.5. To validate the model I have found a number of published papers on the subject. Many of the papers use a parameter called the 'root mean square velocity'. What does this mean and how can I get it from Fluent?

Thanks in advance, John
  Reply With Quote

Old   May 29, 2002, 17:03
Default Re: root mean square
  #2
Steve
Guest
 
Posts: n/a
Urms is the root mean square of the turbulent fluctuations in the x direction. It can be recovered from the turbulent kinetic energy if the flow is isotropic (i.e. u'=v'=w') using k=1/2(u'^2+v'^2+w'^2). However the flow is unlikely to be isotropic and so if you are using an eddy viscosity model you will need to know the ratio of u':v':w'(which for example is 4:3:2 for equillibrium flat plate flow).

For eddy viscosity models the flow is assumed to be isotropic so it will be difficult. For second-order closures you can recover Urms from the Reynold stresses.

Steve

  Reply With Quote

Old   May 29, 2002, 17:10
Default Re: root mean square
  #3
Steve
Guest
 
Posts: n/a
Sorry 4:2:3 not 4:3:2
  Reply With Quote

Old   May 30, 2002, 09:55
Default Re: root mean square
  #4
John
Guest
 
Posts: n/a
Hi Steve,

Thank you very much for your response. I am using the k-eps 2 equation model which if I understand correctly assumes the eddy viscosity is the same in all directions. Can I therefore assume that it will be impossible to get the rms of the turbulent fluctuations from Fluent directly (unless I change to the Reynolds stress model) and the only way I can get them indirectly is to know the ratio of u'v'w' for a pipe?

Thanks once again, John
  Reply With Quote

Old   May 30, 2002, 16:14
Default Re: root mean square
  #5
Steve
Guest
 
Posts: n/a
Thats correct, however if the data you are comparing to has all three components of the turbulent fluctuations than you could combine them to give k and compare to k from FLUENT.
  Reply With Quote

Reply

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
OpenFOAM-1.7.0 for CentOS/RHEL/SL 5.x 64bit released linnemann OpenFOAM Installation 66 October 26, 2013 13:04
root mean square velocity Far CFX 11 February 17, 2013 06:50
Installation problem with ParaView 3.8.0 on openSUSE 11.2 aero_ OpenFOAM Paraview & paraFoam 14 August 2, 2010 18:13
Fully developed 3d velocity profile: Square inlet! Taru FLUENT 5 June 23, 2008 06:04
MPI PROBLEMS gtg627e OpenFOAM Running, Solving & CFD 20 October 5, 2007 04:02


All times are GMT -4. The time now is 11:39.