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Old   July 5, 2015, 15:14
Default Average Velocity in CFD Post
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Hi there,

Can anyone help me to find out CFD-Post 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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Old   July 5, 2015, 18:32
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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.
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Old   July 5, 2015, 19:27
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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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Old   July 6, 2015, 00:51
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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.
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Old   July 6, 2015, 01:43
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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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Old   July 6, 2015, 06:20
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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.
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Old   July 6, 2015, 12:58
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Do you have any idea where do I can find any explanation about averaging method?
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Old   July 6, 2015, 18:40
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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
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Old   July 7, 2015, 12:10
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OK, Thanks friend
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Old   July 7, 2015, 14:47
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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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Old   July 7, 2015, 18:57
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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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