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

Velocity Magnitude for Steady state RANS

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

Like Tree4Likes
  • 1 Post By Manu4CFD
  • 3 Post By LuckyTran

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   June 12, 2019, 00:34
Default Velocity Magnitude for Steady state RANS
  #1
New Member
 
Join Date: Feb 2018
Posts: 29
Rep Power: 8
Manu4CFD is on a distinguished road
While performing a steady state RANS CFD simulation with RNG k-ε turbulence model in Ansys Fluent, if I plot the velocity magnitude (V = SQRT(Ux^2 + Uy^2 + Uz^2) ) at a random point within the computational domain, does this velocity magnitude represents the mean and fluctuating velocity components or is it just representing the mean velocity at that particular point?
Manu4CFD is offline   Reply With Quote

Old   June 13, 2019, 11:30
Default
  #2
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,746
Rep Power: 66
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
only the mean
LuckyTran is offline   Reply With Quote

Old   June 14, 2019, 06:31
Default
  #3
New Member
 
Join Date: Feb 2018
Posts: 29
Rep Power: 8
Manu4CFD is on a distinguished road
Quote:
Originally Posted by LuckyTran View Post
only the mean
So, it means that as the instantaneous velocity "u" is split into mean velocity "U" and fluctuating velocity u' i.e., (u = U + u'), while time averaging over a sufficiently long time, say "T", the fluctuating component u' tends to zero and the time averaged mean velocity U is treated as a constant. So in short, while performing a steady state RANS simulation, the velocity magnitude plotted for a point or even on a contour tends to provide us only the mean velocity "U".
Is my interpretation of your statement correct?
amandahan likes this.
Manu4CFD is offline   Reply With Quote

Old   June 14, 2019, 13:08
Default
  #4
Senior Member
 
Lucky
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 5,746
Rep Power: 66
LuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura aboutLuckyTran has a spectacular aura about
Quote:
Originally Posted by Manu4CFD View Post
So in short, while performing a steady state RANS simulation, the velocity magnitude plotted for a point or even on a contour tends to provide us only the mean velocity "U".
Is my interpretation of your statement correct?

Yes. The "velocity" that you get when you do RANS is only the mean velocity.


RANS means Reynolds-averaged Navier Stokes. When you solve RANS, you are not solving the time-accurate Navier Stokes equations but the Reynolds-averaged ones, written in terms of the reynolds-averaged variables (which you are calling the time average velocity).


If you do URANS (unsteady RANS) you still get unsteady reynolds-averaged, which is averaged over a finite T instead of infinite T. Here the velocity you get is still the reynolds-averaged velocity (containing no fluctuating part), although it does vary in time slowly.
Ionut G, amandahan and Manu4CFD like this.
LuckyTran is offline   Reply With Quote

Old   June 15, 2019, 01:11
Default
  #5
New Member
 
Join Date: Feb 2018
Posts: 29
Rep Power: 8
Manu4CFD is on a distinguished road
Quote:
Originally Posted by LuckyTran View Post
Yes. The "velocity" that you get when you do RANS is only the mean velocity.


RANS means Reynolds-averaged Navier Stokes. When you solve RANS, you are not solving the time-accurate Navier Stokes equations but the Reynolds-averaged ones, written in terms of the reynolds-averaged variables (which you are calling the time average velocity).


If you do URANS (unsteady RANS) you still get unsteady reynolds-averaged, which is averaged over a finite T instead of infinite T. Here the velocity you get is still the reynolds-averaged velocity (containing no fluctuating part), although it does vary in time slowly.
Thanks a lot for your valuable feedback.
Manu4CFD 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
Multiphase flow - incorrect velocity on inlet Mike_Tom CFX 6 September 29, 2016 02:27
Simple piston movement in cylinder- fluid models arun1994 CFX 4 July 8, 2016 03:54
the mean of injection velocity magnitude and why wall film can't be used in steady? sope111 CFX 0 August 21, 2012 06:04
Two-Phase Buoyant Flow Issue Miguel Baritto CFX 4 August 31, 2006 13:02
Terrible Mistake In Fluid Dynamics History Abhi Main CFD Forum 12 July 8, 2002 10:11


All times are GMT -4. The time now is 19:01.