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June 12, 2002, 07:34 
Velocity Magnitude Definition

#1 
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Dear Nices,
Is the velocity magnitude in FLUENT defined as following (for nonswirl cases): Velocity Magnittude=ABS(Ux)+ABS(Uy)+ABS(Uz), where ABS() means the absolute value and Ux, Uy and Uz are the velocitiey components at xyz Cartesian coordinate. Best regards Koorosh 

June 12, 2002, 12:57 
Re: Velocity Magnitude Definition

#2 
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Dear Koorosh,
If it is ok to patch velocity magnitude in initialization for calculating domain, what do you think about to use "User Field Function" in 'Define' menu? If velocity magnitude needs to be assigned as boundary condition, I think UDF is needed. Thanks, J.W.Ryu 

June 12, 2002, 15:43 
Re: Velocity Magnitude Definition

#3 
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The correct definition of the velocity magnitude is SQRT(Ux^2 + Uy^2 + Uz^2).


June 13, 2002, 03:17 
Re: Velocity Magnitude Definition

#4 
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Dear Janos,
Basically the definition you wrote is the magnitude of velocity vector U=Uxi+Uyj+Uzk in 3D cartesian space. So it seems that the velocity magnitude has to be equal to the [U]=(Ux^2+Uy^2+Uz^2)^0.5. But as I checked by FLUENT software for several simulation (those I'm doing or the solved problems) the "velocity magnitude" is different from that you wrote. To check it, just please open a solved problem in FLUENT and write the x,y,z velocities and also velocity magnitude. Next to it you will find that SQR(Ux^2+Uy^2+Uz^2) is so different from "velocity magnitude" reported by FLUENT. Best regards Koorosh 

June 13, 2002, 03:27 
Re: Velocity Magnitude Definition

#5 
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Yep, I just checked it directly in a Fluent simulation and my definition is correct.
Remember to uncheck the "nodevalues" option (if you report nodevalues interpolation errors can cause differences). 

June 13, 2002, 04:26 
Re: Velocity Magnitude Definition

#6 
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Dear Janson,
You are exactlly right. I did switch on the node values and the problem was so. Now as you told me I did switch off those node values in order to prevent the interpolations, and now the velocity magnitude is OK. For an example, with nodes value the velocity magnitude reported by FLUENT and that one obtained by SQR(Ux^2+Uy^2+Uz^2) has an average error nera 0.81%. even though it seems OK, but this error for the case of NO NODE VALUES is 0.00008%. So thanks a lot. But I have another question. I will writte it in another massage. Best regards Koorosh 

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