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

VELOCITY vs VELOCITY IN STN FRAME vs RELATIVE VELOCITY

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   July 13, 2015, 07:07
Unhappy VELOCITY vs VELOCITY IN STN FRAME vs RELATIVE VELOCITY
  #1
New Member
 
everest
Join Date: Mar 2014
Posts: 3
Rep Power: 7
everest20 is on a distinguished road
Hi guys,

In FLUENT we have option to display VELOCITY MAGNITUDE and RELATIVE VELOCITY MAGNITUDE, while in CFD-POST there is additionally VELOCITY IN STN FRAME.

Here's short dscription of my test-case model.
  • I have one moving reference frame in 2D with translational motion along y-direction and I am using MRF appraoch.
  • For one case I assume zero velocity and for the second one some non-zero velocity of the reference frame.
  • There are no interior wall boundaries insisde the moving domain (e.g. rotor walls), so there is just a rectangular domain with prescribed velocity in MRF panel.
  • I would expect that, since there are no walls inside, the resultant velocity (absolute one) should be the same in both cases. This is true in CFD Post when I plot contours for VELOCITY IN STN FRAME and comapre it for the two cases being analysed. I always thought that VELOCITY in FLUENT also relates to absolute velocity viewed from stationary frame, however when the two cases are compared, the resulting velocity contours are different. Can you please explain what is the exact difference between VELOCITY MAGNITUDE (inside fluent) and VELOCITY IN STN RAME (inside cfd-post) ?

In Fluent documentation regarding MFR the ABSOLUTE VELOCITY is defined as \vec{v} = \vec{v_r}  +  \vec{v_t}.
Where \vec{v_r} is the relative velocity and \vec{v_t} is the translational velocity, note that there is no rotational motion in my case. I accept this definition however I cannot understand how Fluent understands VELOCITY MAGNITUDE in case of of MRF.

Thanks in advance for any help
everest20 is offline   Reply With Quote

Old   July 13, 2015, 09:35
Default
  #2
New Member
 
everest
Join Date: Mar 2014
Posts: 3
Rep Power: 7
everest20 is on a distinguished road
OK! So I was actually wrong about the difference in VELOCITY IN STN FRMAE (cfd-post) and VELOCITY MAGNITUDE (fluent) - both look alike and are different between the two analysed cases.

However there is still a question: why there is a difference between velocities in both cases. See the attached figure which presents y-velocity, As we can see the behavior of the flow is different?

Once again thanks in advance for any help, and sorry for the previous confusion between VELOCITY (fluent) and VELOCITY IN STN FRAME (cfd-post).
Attached Images
File Type: jpg velocities.jpg (28.2 KB, 106 views)
everest20 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
How to display relative flow velocity vectors? sarah4 FLUENT 0 July 10, 2012 12:53
Calculating Relative Velocity of a Bubble SMM STAR-CD 3 August 29, 2011 12:38
N-S equations in rotating frame Li Yang Main CFD Forum 11 September 28, 2002 10:25
Terrible Mistake In Fluid Dynamics History Abhi Main CFD Forum 12 July 8, 2002 10:11
Fluent-V5,Turbomachinery (stagnation pressure live its own life) Alberto Tamm Main CFD Forum 19 May 27, 1999 15:42


All times are GMT -4. The time now is 21:44.