In CFX-post, how to obtain the mass flow rate of a cross section

xyq916 · April 30, 2015, 08:47

In CFX-post, how to obtain the mass flow rate of a cross section?

ghorrocks · April 30, 2015, 18:34

Go to function calculator and choose the massflow function.

mz_uon · January 23, 2022, 17:19

Quote:

Originally Posted by ghorrocks

Go to function calculator and choose the massflow function.

Hi Glenn

I want to calculate the volume flow rate through a zone. I am unable to find a function to do that. Any suggestions?

Thanks

ghorrocks · January 23, 2022, 17:30

I presume by "zone" you mean a boundary face.

For simple flat faces volume flow rate is areaInt(Normal Velocity)@Zone Face, where you replace Normal Velocity with the velocity direction and Zone Face with the appropriate name. You can do curved faces as well, but they will require a bit more thought.

mz_uon · January 24, 2022, 10:04

Quote:

Originally Posted by ghorrocks

I presume by "zone" you mean a boundary face.

For simple flat faces volume flow rate is areaInt(Normal Velocity)@Zone Face, where you replace Normal Velocity with the velocity direction and Zone Face with the appropriate name. You can do curved faces as well, but they will require a bit more thought.

Thanks for your reply. Correct, I have found this across several documents.
However, I am confused when it comes to actual selection of the surface and/or velocity. See the attached snapshot. The 2 methods give different answers, whereas I imagined it should give the same volume flow. What is the difference between them?

Regards

Opaque · January 24, 2022, 10:44

In the first one, you are asking for

Area integral along Z direction of the velocity magnitude

on the second one, you are asking for the

Area integral of Velocity along the Z direction

mz_uon · January 24, 2022, 12:57

Quote:

Originally Posted by Opaque

In the first one, you are asking for

Area integral along Z direction of the velocity magnitude

on the second one, you are asking for the

Area integral of Velocity along the Z direction

So which one is the correct value for volume flow through the window? I observed two formulae in literature (see image), does any of my earlier expression calculate this?

Opaque · January 24, 2022, 13:51

Neither of those is correct (generically). The formalism is

massFlow()@Boundary / Density

or you could also create an expression

VdotN = Velocity u * Normal X + Velocity v * Normal Y + Velocity w * Normal Z

then

areaInt (VdotN)@Boundary

I assume you are working with an incompressible fluid (constant density) since volume flow rate does not mean much (to me) for compressible fluids.

April 30, 2015, 08:47	In CFX-post, how to obtain the mass flow rate of a cross section	#1
xyq916 New Member Bruce Xiao Join Date: Jan 2015 Posts: 26 Rep Power: 11	In CFX-post, how to obtain the mass flow rate of a cross section?

January 23, 2022, 17:30		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	I presume by "zone" you mean a boundary face. For simple flat faces volume flow rate is areaInt(Normal Velocity)@Zone Face, where you replace Normal Velocity with the velocity direction and Zone Face with the appropriate name. You can do curved faces as well, but they will require a bit more thought. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

January 24, 2022, 10:44		#6
Opaque Senior Member Join Date: Jun 2009 Posts: 1,804 Rep Power: 32	In the first one, you are asking for Area integral along Z direction of the velocity magnitude on the second one, you are asking for the Area integral of Velocity along the Z direction __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum.

January 24, 2022, 13:51		#8
Opaque Senior Member Join Date: Jun 2009 Posts: 1,804 Rep Power: 32	Neither of those is correct (generically). The formalism is massFlow()@Boundary / Density or you could also create an expression VdotN = Velocity u * Normal X + Velocity v * Normal Y + Velocity w * Normal Z then areaInt (VdotN)@Boundary I assume you are working with an incompressible fluid (constant density) since volume flow rate does not mean much (to me) for compressible fluids. __________________ Note: I do not answer CFD questions by PM. CFD questions should be posted on the forum. Last edited by Opaque; January 24, 2022 at 14:53.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
CFX turbo - Mass flow rate	Vashishth Patel	CFX	7	April 3, 2014 20:33
Convergence problem with target mass flow rate	ADL	FLUENT	2	May 29, 2012 21:11
Discrete Phase & Mass Flow Rate	MagnusZeus	FLUENT	0	December 2, 2011 17:57
How to change from mass flow to volume flow rate	stanley	FLUENT	1	February 2, 2007 06:44
How to obtain mass flow rate of one phase	Derek Jing	FLUENT	1	May 31, 2002 13:18

April 30, 2015, 18:34		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Go to function calculator and choose the massflow function.