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July 5, 2015, 15:14 
Average Velocity in CFD Post

#1 
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Amin
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Hi there,
Can anyone help me to find out CFDPost method for calculating average velocity in a line or plane for multiphase flows? I use function calculator for this purpose. Thanks in advance
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July 5, 2015, 18:32 

#2 
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Glenn Horrocks
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Please post the CEL expression you are trying to evaluate. There are many different ways of doing this, and there are many different velocities you could be talking about.


July 5, 2015, 19:27 

#3 
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Amin
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I have a multiphase flow in a tube, water as primary phase and air as secondary phase. I need to calculate average velocity of each phase in a section,
I use "ave" option in function calculator and then "velocity" option. The results are logical, but I guess it has a advanced method for calculating velocity. Because I think it should include volume fraction for each phase in the equations.
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July 6, 2015, 00:51 

#4 
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Glenn Horrocks
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If you are running inhomogeneous multiphase then each phase has its own velocity field. Volume fraction does not come into it.
So, make sure you understand what you are doing on two main fronts: Integration: You can do arithemetic averages, line averages, area averages, volume averages, mass averages and other weightings as well. Make sure you understand which type of integration is suitable for the average you want. Velocity: Each phase has its own velocity, but there is drift velocity, slip velocity, superficial velocity, and just plain old velocity. Make sure you are using teh correct velocity for what you want. If you do not know what these things are look them up in the CFX documentation. They are well explained. 

July 6, 2015, 01:43 

#5 
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Amin
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Thanks dear Glenn,
I've studied "CFX Reference guide" but I didn't find good explanation about it, I have average velocity of each phase and slip velocity, so I want to obtain these values from CFD model in order to verify simulation results, I've used axisymmetric model for the tube, so I think the phases just have axial velocity, so line average and plane average must be equal! is this right? and in your idea which model is suitable for my model? which model does include volume fraction in average velocity?
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July 6, 2015, 06:20 

#6 
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Glenn Horrocks
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If the paper does not define what velocity average it used then it is probably a mass flow average.
I have no idea what you are modelling and what you expect to see so cannot say anything about what I would expect to see in your simulation. 

July 6, 2015, 12:58 

#7 
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Amin
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Do you have any idea where do I can find any explanation about averaging method?
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July 6, 2015, 18:40 

#8 
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Glenn Horrocks
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The documentation has extensive discussions on this.
Arithmetic  sum of all instances divided by count line  length weighted integral based average area  area weighted integral based average volume  volume weighted integral based average mass  mass weighted (ie density*volume) integral based average 

July 7, 2015, 12:10 

#9 
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Amin
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OK, Thanks friend
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July 7, 2015, 14:47 

#10 
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Amin
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and a little question!
when I use "massFlowAve" there is an option and I can select: 1.All Fluid 2.Air 3.Water But when there are water velocity and air velocity separately, what's meaning of this option? why we should select something like it? Which of them I should use?
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July 7, 2015, 18:57 

#11 
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Glenn Horrocks
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Simply the mass flow ave is based on the mass flux of either the air or water phases, or both.
You choose choose the option most relevant to your application  is the thing you are averaging associated with air, water or both? Then choose the option to suit. 

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