CFX-Post Variables
 

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.

Re: CFX-Post Variables
 

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

Re: CFX-Post Variables
 

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.

Re: CFX-Post Variables
 

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.

Re: CFX-Post Variables
 

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 )

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 ;)

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.

i got it. the whole variable list appeared and selecting yplus from that list gave what actually i was looking for.
thank you.