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

CFX-Post Variables

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 18, 2008, 16:02
Default CFX-Post Variables
  #1
LKF
Guest
 
Posts: n/a
I have discovered what appears to be an inconsistency between the Velocity variables in CFX-Post. I assume that: Velocity = sqrt[(Velocity u)^2 + (Velocity v)^2 + (Velocity w)^2]

Using either areaAve or massFlowAve I get the following data:

areaAve( Velocity u ) 8606.64 [ ft min^-1 ]

areaAve( Velocity v ) 80.62 [ ft min^-1 ]

areaAve( Velocity w ) -213.75 [ ft min^-1 ]

areaAve( Velocity ) 9738.15 [ ft min^-1 ]

However, calculation of the magnitude using the formula above gives a value of: 8609.67 [ft min^-1].

Strangely the areaAve Velocity value of 9738.15 [ft min^-1] is closer to the expected velocity of 9620 [ft min^-1] based on the flow area and the u-direction mass flow rate boundary condition I've applied.

Does anyone have an explanation for this?

I haven't been able to find a mathematical definition for Velocity in the CFX documentation.
  Reply With Quote

Old   September 18, 2008, 20:12
Default Re: CFX-Post Variables
  #2
Glenn Horrocks
Guest
 
Posts: n/a
Hi,

Velocity is defined as sqrt(u^2+v^2+w^2). I would look at how the area averages are being calculated. What surface is this area average over? An inlet or outlet? Is the surface planar?

Glenn Horrocks
  Reply With Quote

Old   September 18, 2008, 20:29
Default Re: CFX-Post Variables
  #3
LKF
Guest
 
Posts: n/a
Thanks for your response. I have tried the previous calculations on 3 different surfaces - an inlet, a midplane, and an outlet. All are planar, this is just a simple duct flow problem. Checking my discretised area using the area()@... function shows negligable difference from the true planar area (<.5%).

The specific data I posted was for an outlet surface but all 3 surfaces were showing the same behaviour. Since I posted I have also tried another test where I explicitly defined the outlet u velocity to be 49 m s^-1. Following that run, the areaAve(Velocity u)@.. came out at 43 m s^-1 there were small v and w components and the areaAve(Velocity)@ returned a value near the 49 m s^-1 input value.

Your insight is appreciated.
  Reply With Quote

Old   September 19, 2008, 00:54
Default Re: CFX-Post Variables
  #4
johnny
Guest
 
Posts: n/a
The averages should not be the same.

Sum(Ui^2+Vi^2+Wi^2)/n

does not equal

(Sum(Ui)/n)^2 + (Sum(Vi)/n)^2 + (Sum(Wi)/n)^2

where i = 1,2,...,n

This is essentially what you are trying to compare by area averging the components of velocity and area averaging the resultant vector.
  Reply With Quote

Old   September 19, 2008, 05:28
Default Re: CFX-Post Variables
  #5
Rui
Guest
 
Posts: n/a
areaAve( Velocity ) = areaAve ( sqrt[(Velocity u)^2 + (Velocity v)^2 + (Velocity w)^2] ) , but this is different of sqrt ( (areaAve( Velocity u ))^2 + (areaAve( Velocity v ))^2 + (areaAve( Velocity w ))^2 )

  Reply With Quote

Old   September 15, 2015, 11:12
Default
  #6
Senior Member
 
Maxim
Join Date: Aug 2015
Location: Germany
Posts: 415
Rep Power: 12
-Maxim- is on a distinguished road
Quote:
Originally Posted by LKF View Post
I haven't been able to find a mathematical definition for Velocity in the CFX documentation.
I have also tried to find an exact definition for velocity in CFX. I think it's just not there. They assume that everybody knows - which we do - but to confirm that CFX actually uses that defintion as well, it would have been nice if they put it in there.
Thank you for this post to clarify it. We can rely on this forum
-Maxim- is offline   Reply With Quote

Old   September 15, 2015, 13:28
Default
  #7
Member
 
Attique Javaid
Join Date: Apr 2015
Location: Pakistan
Posts: 66
Rep Power: 10
attiquejavaid08 is on a distinguished road
in first attempt to check yplus, after getting results, when i go to contour plot and check yplus in variable drop down list. there was no yplus option in cfx-post.
then i checked variable tab where Yplus was there, clicking on 'calculate' i got maximum and minimum values of yplus. but how can i plot it to see where is maximum yplus value?
please.
attiquejavaid08 is offline   Reply With Quote

Old   September 15, 2015, 13:51
Default
  #8
Senior Member
 
Join Date: Feb 2011
Posts: 495
Rep Power: 18
Antanas is on a distinguished road
Quote:
Originally Posted by attiquejavaid08 View Post
in first attempt to check yplus, after getting results, when i go to contour plot and check yplus in variable drop down list. there was no yplus option in cfx-post.
then i checked variable tab where Yplus was there, clicking on 'calculate' i got maximum and minimum values of yplus. but how can i plot it to see where is maximum yplus value?
please.
There is ... button near drop down list.
Antanas is offline   Reply With Quote

Old   September 15, 2015, 15:33
Default
  #9
Member
 
Attique Javaid
Join Date: Apr 2015
Location: Pakistan
Posts: 66
Rep Power: 10
attiquejavaid08 is on a distinguished road
Quote:
Originally Posted by Antanas View Post
There is ... button near drop down list.
i got it. the whole variable list appeared and selecting yplus from that list gave what actually i was looking for.
thank you.
attiquejavaid08 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
New Variable in CFX Post Ben CFX 9 June 20, 2012 19:10
chart for transiente simulation - CFX Post rogbrito CFX 0 June 22, 2009 13:12
Assign new variable in CFX Post Ben CFX 1 August 25, 2008 15:59
CFX definitions of Rotating Variables: Confusing! Prakash CFX 0 November 8, 2005 02:24
CFX POST mpeg Revisited henry CFX 2 November 22, 2002 12:42


All times are GMT -4. The time now is 13:36.