
[Sponsors] 
September 18, 2008, 15:02 
CFXPost Variables

#1 
Guest
Posts: n/a

I have discovered what appears to be an inconsistency between the Velocity variables in CFXPost. 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 udirection 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. 

September 18, 2008, 19:12 
Re: CFXPost Variables

#2 
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 

September 18, 2008, 19:29 
Re: CFXPost Variables

#3 
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. 

September 18, 2008, 23:54 
Re: CFXPost Variables

#4 
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. 

September 19, 2008, 04:28 
Re: CFXPost Variables

#5 
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 )


September 15, 2015, 10:12 

#6  
Senior Member
Maxim
Join Date: Aug 2015
Location: Germany
Posts: 380
Rep Power: 5 
Quote:
Thank you for this post to clarify it. We can rely on this forum 

September 15, 2015, 12:28 

#7 
Member
Attique Javaid
Join Date: Apr 2015
Location: Pakistan
Posts: 64
Rep Power: 4 
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 cfxpost.
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. 

September 15, 2015, 12:51 

#8  
Senior Member
Join Date: Feb 2011
Posts: 460
Rep Power: 11 
Quote:


September 15, 2015, 14:33 

#9 
Member
Attique Javaid
Join Date: Apr 2015
Location: Pakistan
Posts: 64
Rep Power: 4 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
New Variable in CFX Post  Ben  CFX  9  June 20, 2012 18:10 
chart for transiente simulation  CFX Post  rogbrito  CFX  0  June 22, 2009 12:12 
Assign new variable in CFX Post  Ben  CFX  1  August 25, 2008 14:59 
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 