CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Mass flow rate: calculation v/s computation

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree4Likes
  • 4 Post By oj.bulmer

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   April 21, 2010, 08:33
Default Mass flow rate: calculation v/s computation
  #1
New Member
 
Annon
Join Date: Apr 2010
Posts: 18
Rep Power: 16
beguxa is on a distinguished road
Hi all,
In my simulations density (Rho) is constant. I have computed mass flow rate (M) through an interior face using standard mass flow rate calculation option from fluent. Then I calculated mass averaged velocity (Vm), area weighted averaged velocity (Va) and facet averaged velocity (Vf). To recheck, I calculated the mass flow rate as M' = Rho*A*V. By using all the three above mentioned velocities in the above formula I am not able to get both mass flow rates the same (i.e. M' and M are not equal). Which is the apropriate value of velocity that one should consider to re-calculate accurately the mass flow rate?
I guess, the mass flow rate is computed in fluent as summation of (Rho*A*V) values at all the facets. Could any body throw some light on this issue?

Thanks.
-aditya
beguxa is offline   Reply With Quote

Old   March 23, 2013, 11:31
Default
  #2
New Member
 
Andre Joubert
Join Date: Mar 2013
Posts: 4
Rep Power: 13
AJoubert is on a distinguished road
Hi beguxa, I believe the velocity you want to use is actually the axial velocity. When running my current simulation of flow rate through a nozzle I used surface integrals in the report menu to get an accurate reading but when calculating the flow through use of a custom field function the velocity which returned the same value for flow rate was the axial velocity.
AJoubert is offline   Reply With Quote

Old   March 24, 2013, 04:44
Default
  #3
Member
 
Thiagu
Join Date: Oct 2012
Location: India
Posts: 60
Rep Power: 14
jthiakz is on a distinguished road
you can use standard massflow option in fluent. but to check this use velocity normal to the surface in your calculation. I think both should agree.
jthiakz is offline   Reply With Quote

Old   March 28, 2013, 12:04
Default
  #4
Senior Member
 
OJ
Join Date: Apr 2012
Location: United Kindom
Posts: 473
Rep Power: 20
oj.bulmer will become famous soon enough
beguxa, There are subtle differences in all four values.

For correct mass flow rate, we want addition of all the mass flow rates going into the every surface element of your surface.


Let's compare all four approaches with this bold statement.

Area averaged velocity is calculated as \frac{1}{A} \int vdA. This is a good representation of average velocity over the surface. But depending on velocity distribution and mesh, smaller mesh elements may have higher velocities and vice versa, so this doesn't equate to bold sentence at the beginning.

Facet average is calculated as \frac{\sum v} {n}, where n is no. of facets in the surface. If your mesh is not uniform and if there are more mesh elements in regions of high velocity, your facet average velocity will be unnecessarily higher than the average velocity. You can't multiply this with area and still get the definition of bold statement at the beginning.

Mass weighted average is \frac{\int v \rho | \overline{v}.\overline{dA}|}{\int \rho | \overline{v}.\overline{dA}|}. We are slowly getting closer now, since there is a slight reference of actual mass going into every element - \int \rho | \overline{v}.\overline{dA}| - in the denominator. But again since it is "weighted" it bears same limitations as above.

Mass flow rate is calculated as: \int \rho \overline{v}.\overline{dA}. This definition is closest to the bold statement at the beginning. And hence is recommended.

Although, if mesh is of high quality, structured and if solution is fully converged, the differences between these should become smaller. But in real world, with often-used unstructured meshes, this is not always possible.

OJ
oj.bulmer is offline   Reply With Quote

Old   June 8, 2013, 17:17
Default
  #5
New Member
 
behnam
Join Date: Sep 2011
Location: Iran
Posts: 20
Rep Power: 15
Behnam Ghadimi is on a distinguished road
Dear beguxa
If you define a variable in the Define> Custom Field Functions... as Density*Velocity, and then integrated this variable in the report>surface integral using the "integral" in the "Report type" you can observe that this value is equal to the mass flow rate in your desired surface, which is reported by FLUENT post processing from Report> FLuxes
Behnam Ghadimi is offline   Reply With Quote

Old   December 2, 2018, 22:02
Default
  #6
New Member
 
che.shugang
Join Date: Oct 2018
Posts: 2
Rep Power: 0
Che.Shuguang is on a distinguished road
beguxa,What is your final conclusion about your question? I have encountered a problem similar to yours. Thank you!
Che.Shuguang is offline   Reply With Quote

Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
UDF to measure Mass Flow Rate a.lynchy Fluent UDF and Scheme Programming 31 October 4, 2018 15:10
The reporting of mass flow rate summer FLUENT 1 July 12, 2007 10:35
mass flow rate error Masood FLUENT 0 May 22, 2005 01:32
Target mass flow rate Saturn FLUENT 0 December 10, 2004 05:18
Mass flow rate Neser CFX 4 February 14, 2004 01:27


All times are GMT -4. The time now is 21:50.