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

Anna Tian · August 16, 2013, 06:23

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.

JuPa · August 16, 2013, 06:35

Someone correct me if I'm wrong, but isn't just areaAvg(massflow)@location?

oj.bulmer · August 16, 2013, 07:33

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

Anna Tian · August 17, 2013, 06:08

Quote:

Originally Posted by oj.bulmer

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?

JuPa · August 17, 2013, 06:37

..........

Anna Tian · August 17, 2013, 09:32

Quote:

Originally Posted by RicochetJ

..........

I'm asking about the math averaging algorithm that CFX use to compute. Not the command.

oj.bulmer · August 17, 2013, 09:54

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.

OJ

Anna Tian · August 17, 2013, 09:59

Quote:

Originally Posted by oj.bulmer

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?

oj.bulmer · August 17, 2013, 10:09

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

Anna Tian · September 1, 2013, 10:11

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?

ghorrocks · September 1, 2013, 19:39

Yes, sum up the cell volumes.

Anna Tian · September 3, 2013, 08:05

Quote:

Originally Posted by ghorrocks

Yes, sum up the cell volumes.

How do you know that? Any reference?

ghorrocks · September 3, 2013, 20:08

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.

Anna Tian · September 6, 2013, 14:43

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?

singer1812 · September 6, 2013, 18:06

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.

JuPa · September 6, 2013, 20:07

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

mat_cfd · September 7, 2013, 06:03

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

Anna Tian · September 7, 2013, 06:17

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?

Anna Tian · September 7, 2013, 06:37

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?

JuPa · September 7, 2013, 12:40

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 !

August 16, 2013, 06:23	The formula to compute the mass flow averaged value in a surface	#1
Anna Tian Senior Member Meimei Wang Join Date: Jul 2012 Posts: 494 Rep Power: 16	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

September 3, 2013, 20:08		#13
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	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.

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August 16, 2013, 06:35		#2
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15	Someone correct me if I'm wrong, but isn't just areaAvg(massflow)@location?

August 16, 2013, 07:33		#3
oj.bulmer Senior Member OJ Join Date: Apr 2012 Location: United Kindom Posts: 473 Rep Power: 20	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

August 17, 2013, 06:37		#5
JuPa Senior Member Mr CFD Join Date: Jun 2012 Location: Britain Posts: 361 Rep Power: 15	..........

August 17, 2013, 10:09		#9
oj.bulmer Senior Member OJ Join Date: Apr 2012 Location: United Kindom Posts: 473 Rep Power: 20	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, 19:39		#11
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	Yes, sum up the cell volumes.

September 6, 2013, 18:06		#15
singer1812 Senior Member Edmund Singer P.E. Join Date: Aug 2010 Location: Minneapolis, MN Posts: 511 Rep Power: 21	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 7, 2013, 06:03		#17
mat_cfd New Member Join Date: Oct 2011 Posts: 20 Rep Power: 15	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