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Old   September 18, 2008, 15:02
Default CFX-Post Variables
  #1
LKF
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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.
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Old   September 18, 2008, 19:12
Default Re: CFX-Post Variables
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Glenn Horrocks
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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
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Old   September 18, 2008, 19:29
Default Re: CFX-Post Variables
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LKF
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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.
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Old   September 18, 2008, 23:54
Default Re: CFX-Post Variables
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johnny
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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.
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Old   September 19, 2008, 04:28
Default Re: CFX-Post Variables
  #5
Rui
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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 )

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