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The formula to compute the mass flow averaged value in a surface

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Old   August 16, 2013, 05:23
Question The formula to compute the mass flow averaged value in a surface
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Hi

May I ask where could I find the formula that CFX-post use to compute the mass flow averaged value in a surface? I checked the CFX tutorial but didn't find it. If tutorial does include this formula, please let me which tutorial and which page. Thanks.
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Old   August 16, 2013, 05:35
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Someone correct me if I'm wrong, but isn't just areaAvg(massflow)@location?
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Old   August 16, 2013, 06:33
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Mass weighted average of quantity \phi on surface: \frac{\int \phi \rho \phi dA}{\int \rho \phi dA} = massFlowAve(\phi)@<Surface name>

Area weighted average of quantity \phi on surface: \frac{1}{A} \int \phi dA=areaAve(\phi)@<Surface name>

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Old   August 17, 2013, 05:08
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Quote:
Originally Posted by oj.bulmer View Post
Mass weighted average of quantity \phi on surface: \frac{\int \phi \rho \phi dA}{\int \rho \phi dA} = massFlowAve(\phi)@<Surface name>

Area weighted average of quantity \phi on surface: \frac{1}{A} \int \phi dA=areaAve(\phi)@<Surface name>

OJ
Are you sure about the mass flow averaging formula? There is no velocity term in it?

Where did you find it? Any CFX tutorial?
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Old   August 17, 2013, 05:37
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Old   August 17, 2013, 08:32
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Quote:
Originally Posted by RicochetJ View Post
..........
I'm asking about the math averaging algorithm that CFX use to compute. Not the command.
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Old   August 17, 2013, 08:54
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Quote:
Are you sure about the mass flow averaging formula? There is no velocity term in it?
Touche! That is what I would call lost in translation (in LaTex)

Please read the second \phi in the formula as v. Thus, more formally, the formula is


\frac{\int \phi \rho | \overline{v}.\overline{dA}|}{\int \rho | \overline{v}.\overline{dA}|}=\frac{\sum \phi m }{\sum m}

where \overline{v}, \; \rho, \;  \overline{dA} are the local velocity vector, local density and the area vector at facet of the local cell respectively.

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Old   August 17, 2013, 08:59
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Quote:
Originally Posted by oj.bulmer View Post
Touche! That is what I would call lost in translation (in LaTex)

Please read the second \phi in the formula as v. Thus, more formally, the formula is


\frac{\int \phi \rho | \overline{v}.\overline{dA}|}{\int \rho | \overline{v}.\overline{dA}|}=\frac{\sum \phi m }{\sum m}

where \overline{v}, \; \rho, \;  \overline{dA} are the local velocity vector, local density and the area vector at facet of the local cell respectively.

OJ

Where did you find this formula? How do you know CFX use this formula?
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Old   August 17, 2013, 09:09
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Have a look at documentation, stitch the information together and apply judgement...

CFX:
http://www.sharcnet.ca/Software/Flue.../i1308570.html

FLUENT:
http://www.sharcnet.ca/Software/Flue...th/node416.htm

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Old   September 1, 2013, 09:11
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Quote:
Originally Posted by oj.bulmer View Post
Have a look at documentation, stitch the information together and apply judgement...

CFX:
http://www.sharcnet.ca/Software/Flue.../i1308570.html

FLUENT:
http://www.sharcnet.ca/Software/Flue...th/node416.htm

OJ
By the way, how does CFX calculate the volume of the fluid domain? Just sum up all the cells size in the fluid domain?
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Old   September 1, 2013, 18:39
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Yes, sum up the cell volumes.
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Old   September 3, 2013, 07:05
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Yes, sum up the cell volumes.
How do you know that? Any reference?
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Old   September 3, 2013, 19:08
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Isn't it obvious? If you break up a volume into zillions of little volumes then the sum of those little volumes is the total volume. It is hardly rocket science.
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Old   September 6, 2013, 13:43
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Isn't it obvious? If you break up a volume into zillions of little volumes then the sum of those little volumes is the total volume. It is hardly rocket science.

Yes. It is obvious that we can compute it in this way. But it is not obvious that CFX choose this way to compute the volume. Any CFX reference about it?
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Old   September 6, 2013, 17:06
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If you are worried about how CFX calculates the total volume and need a reference for it, then I think you have your work cut out for you.

You are heading towards questioning every single equation, which is quite a task.

Are you getting a huge discrepency in the CFX calculated volume as opposed to the CAD? If so, I would question your mesh resolution first prior to blaming the software.
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Old   September 6, 2013, 19:07
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Quote:
Originally Posted by singer181250281
If you are worried about how CFX calculates the total volume and need a reference for it, then I think you have your work cut out for you.

You are heading towards questioning every single equation, which is quite a task.

Are you getting a huge discrepency in the CFX calculated volume as opposed to the CAD? If so, I would question your mesh resolution first prior to blaming the software.
Absolutely this - 100%. Most cfd issues I have witnessed have been either:
1. Mesh issues
2. A lack of understanding
3. A combination of 1 and 2
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Old   September 7, 2013, 05:03
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Remember only mesh goes into solver not real geometry. It must be cell volume or calculation through cell edges but if ur mesh is close to geometry solver volume will match the real geometry.
Anyhow domain volume in not used in discretized eqn so it wont effect CFX solution. But If you have compromised on geometry to improve mesh quality then you should use volume from preprocessing for calculation or expresion in CFX
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Old   September 7, 2013, 05:17
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Quote:
Originally Posted by mat_cfd View Post
Remember only mesh goes into solver not real geometry. It must be cell volume or calculation through cell edges but if ur mesh is close to geometry solver volume will match the real geometry.
Anyhow domain volume in not used in discretized eqn so it wont effect CFX solution. But If you have compromised on geometry to improve mesh quality then you should use volume from preprocessing for calculation or expresion in CFX
I ask for the reference because I use CFX-post to compute volume and I need to refer it and put the volume calculating formula on my thesis. Where could I find it?

And what is the formula to compute the surface averaged heat flux?
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Old   September 7, 2013, 05:37
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Quote:
Originally Posted by oj.bulmer View Post
Have a look at documentation, stitch the information together and apply judgement...

CFX:
http://www.sharcnet.ca/Software/Flue.../i1308570.html

FLUENT:
http://www.sharcnet.ca/Software/Flue...th/node416.htm

OJ
These links do not work now. Where could I find the documentation?
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Old   September 7, 2013, 11:40
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Originally Posted by Anna Tian View Post
These links do not work now. Where could I find the documentation?
In the Ansys theory or solver modelling guide !
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