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

relative velocity postprocessing

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

Reply
 
LinkBack Thread Tools Display Modes
Old   July 23, 2009, 20:56
Default relative velocity postprocessing
  #1
New Member
 
Salman
Join Date: May 2009
Posts: 11
Rep Power: 8
Salman is on a distinguished road
Hi,
I am simulating an axial fan using Multiple reference frame (MRF). The model consists of 2 stationary zones and 1 rotating (rotor) zone. After having a converged solution, I am unable to view the relative velocity vectors. They are the same as the absolute velocity vectors. I have exported all the related data to ensight and same behaviour is observed there too. Can anybody help me that how can i have the relative velocity data either in Fluent or in ensight for postprocessing like streamlines and relative velocity vectors.
Thanks in advance
Salman is offline   Reply With Quote

Old   July 24, 2009, 10:47
Default Relative Velocity in EnSight
  #2
Member
 
kevincolburn's Avatar
 
Kevin Colburn
Join Date: Mar 2009
Posts: 73
Rep Power: 8
kevincolburn is on a distinguished road
In EnSight, you can use the calculator functions to create the relative velocity vectors, using the absolute velocity (read in), and the motion you prescribed (constant). I suspect that you will have to break out the X and Y components of Cartesian velocity from your Omega velocity. Using the coordinates as well to determine the radius for transferal of your rotational velocity to Cartesian.
-kevin (kevin@ensight.com)
kevincolburn is offline   Reply With Quote

Old   July 24, 2009, 14:46
Default
  #3
New Member
 
Salman
Join Date: May 2009
Posts: 11
Rep Power: 8
Salman is on a distinguished road
Hi,
thanks for the kind advice. Infact I wanted to avoid this vector calculations but i think I must do that.
Anyhow, my axis of rotation is +y. Down here is some sort of attempt to have the relative velocity. Please check, if possible, for errors, if any.

c: abs vel, u: circumferential vel, w: rel vel, N: rpm
c=u +w ------> w = c - u
u= omega * r = [(2*pi*N/60) * sqrt(x^2+z^2)]
u_x = abs(u) * cos(theta)
u_y = abs(u) * sin(theta)

Now : w_x = c_x - u_x
w_z = c_z -u_z

and w = sqrt [(w_x)^2 + (w_y)^2]

Please guide me also if I am doing something wrong or missing anything....
thanks a lot
Salman is offline   Reply With Quote

Old   July 24, 2009, 21:14
Default
  #4
Member
 
kevincolburn's Avatar
 
Kevin Colburn
Join Date: Mar 2009
Posts: 73
Rep Power: 8
kevincolburn is on a distinguished road
I think that you are correct there. On the face of it, your equations look correct. Your method is certainly correct. You can do this either through the calculator, or through python routine (therefore use again/document/adjust).
-kevin
kevincolburn is offline   Reply With Quote

Old   July 25, 2009, 04:44
Default
  #5
New Member
 
Salman
Join Date: May 2009
Posts: 11
Rep Power: 8
Salman is on a distinguished road
Thank you very much for sparing the time...
I have no idea about python routine. I am sorry, I am a sort of beginner in Ensight!!!!
If you think that its a better approach or less time consuming, then I can go through it. I hope I can find it in the user guide documents.
Salman
Salman is offline   Reply With Quote

Old   July 26, 2009, 06:04
Default
  #6
Member
 
kevincolburn's Avatar
 
Kevin Colburn
Join Date: Mar 2009
Posts: 73
Rep Power: 8
kevincolburn is on a distinguished road
Something like this should work for you. I have not tested it out on a real case, but it should work. Save the file off as a ".py" extension. (You can use most editors to edit the text file). Within EnSight, go to File --> command and select the "python" tab. Click on edit python routine (and specify the file), and then select "Run script".
Example python routine attached.
Attached Files
File Type: txt relative.py.txt (1.2 KB, 13 views)
kevincolburn is offline   Reply With Quote

Old   July 27, 2009, 08:59
Default
  #7
New Member
 
Salman
Join Date: May 2009
Posts: 11
Rep Power: 8
Salman is on a distinguished road
thanks for the kind effort. I hope it will work as you guided. I have one more question:
I have two fluid zones, rotating and stationary. In ensight, they are numbered as 1 and 2 respectively (Part ID). Now for the command "ensight.part.select_all()", I am a bit confused. Should it be written as "ensight.part.select_all(1,2)" to select these two parts? Please confirm if I am right otherwise advise. I will be grateful
Salman
Salman is offline   Reply With Quote

Old   July 27, 2009, 09:08
Default
  #8
Member
 
kevincolburn's Avatar
 
Kevin Colburn
Join Date: Mar 2009
Posts: 73
Rep Power: 8
kevincolburn is on a distinguished road
the syntax would be : ensight.part.select_begin(1,2) to grab just parts #1 and #2. You can also reference Chapter 6 in the "Interface Manual" in your EnSight installation for more detail on syntax.
kevincolburn is offline   Reply With Quote

Old   July 27, 2009, 09:35
Default
  #9
New Member
 
Salman
Join Date: May 2009
Posts: 11
Rep Power: 8
Salman is on a distinguished road
Thank you very much.
Salman is offline   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
[UDF] Relative coordinates in logarithmic velocity profile cfdworker Fluent UDF and Scheme Programming 24 February 23, 2015 05:33
relative velocity vectors carno FLUENT 0 May 9, 2008 06:59
Variables Definition in CFX Solver 5.6 R P CFX 2 October 26, 2004 02:13
Terrible Mistake In Fluid Dynamics History Abhi Main CFD Forum 12 July 8, 2002 09:11
Relative velocity errors in cylindrical coords? John FLUENT 0 April 5, 2000 07:46


All times are GMT -4. The time now is 12:56.