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

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 August 16, 2013, 05:23 The formula to compute the mass flow averaged value in a surface #1 Senior Member     Meimei Wang Join Date: Jul 2012 Posts: 494 Rep Power: 8 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. __________________ Best regards, Meimei

 August 16, 2013, 05:35 #2 Senior Member     Mr CFD Join Date: Jun 2012 Location: Britain Posts: 312 Rep Power: 7 Someone correct me if I'm wrong, but isn't just areaAvg(massflow)@location?

 August 16, 2013, 06:33 #3 Senior Member   OJ Join Date: Apr 2012 Location: United Kindom Posts: 475 Rep Power: 12 Mass weighted average of quantity on surface: = massFlowAve()@ Area weighted average of quantity on surface: =areaAve()@ OJ

August 17, 2013, 05:08
#4
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Meimei Wang
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Quote:
 Originally Posted by oj.bulmer Mass weighted average of quantity on surface: = massFlowAve()@ Area weighted average of quantity on surface: =areaAve()@ 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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 August 17, 2013, 05:37 #5 Senior Member     Mr CFD Join Date: Jun 2012 Location: Britain Posts: 312 Rep Power: 7 ..........

August 17, 2013, 08:32
#6
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Meimei Wang
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Quote:
 Originally Posted by RicochetJ ..........
I'm asking about the math averaging algorithm that CFX use to compute. Not the command.
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August 17, 2013, 08:54
#7
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OJ
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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 in the formula as v. Thus, more formally, the formula is

=

where are the local velocity vector, local density and the area vector at facet of the local cell respectively.

OJ

August 17, 2013, 08:59
#8
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Meimei Wang
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Quote:
 Originally Posted by oj.bulmer Touche! That is what I would call lost in translation (in LaTex) Please read the second in the formula as v. Thus, more formally, the formula is = where 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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 August 17, 2013, 09:09 #9 Senior Member   OJ Join Date: Apr 2012 Location: United Kindom Posts: 475 Rep Power: 12 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

September 1, 2013, 09:11
#10
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Meimei Wang
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Quote:
 Originally Posted by oj.bulmer 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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 September 1, 2013, 18:39 #11 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,718 Rep Power: 99 Yes, sum up the cell volumes.

September 3, 2013, 07:05
#12
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Meimei Wang
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Quote:
 Originally Posted by ghorrocks Yes, sum up the cell volumes.
How do you know that? Any reference?
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 September 3, 2013, 19:08 #13 Super Moderator   Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 12,718 Rep Power: 99 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. hmasenger likes this.

September 6, 2013, 13:43
#14
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Meimei Wang
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Quote:
 Originally Posted by ghorrocks 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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 September 6, 2013, 17:06 #15 Senior Member   Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 512 Rep Power: 12 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.

September 6, 2013, 19:07
#16
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Mr CFD
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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

 September 7, 2013, 05:03 #17 New Member   Join Date: Oct 2011 Posts: 20 Rep Power: 7 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

September 7, 2013, 05:17
#18
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Meimei Wang
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Quote:
 Originally Posted by mat_cfd 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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September 7, 2013, 05:37
#19
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Meimei Wang
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Quote:
 Originally Posted by oj.bulmer 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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September 7, 2013, 11:40
#20
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Mr CFD
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Quote:
 Originally Posted by Anna Tian These links do not work now. Where could I find the documentation?
In the Ansys theory or solver modelling guide !

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