CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

OutletBoundary Condition Causes Mass Imbalance

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   March 11, 2004, 15:05
Default OutletBoundary Condition Causes Mass Imbalance
  #1
Andy
Guest
 
Posts: n/a
Hello there,

I have been trying to validate a CFD solver using the flow through an ordinary pipe problem (at very low Reynolds numbers).

The simulation is as expected until the flow reaches the outlet where a large velocity accumulates causing mass imbalance.

I am using the outlet boundary condition, specifying pressure equal to zero at the outlet.

I have found the following information whilst surfing the internet:

INTERNET INFORMATION

If significant gradients are calculated near a boundary, a mass imbalance can occur. This is due to the implied condition of fully developed flow for a constant pressure boundary. If the flow has not fully developed, FLOTRAN is forced to adjust it across the last row of elements to satisfy the boundary condition. Occasionally, this adjustment may cause a mass imbalance.

To prevent this from occurring, you can add a development length to the exit. This addition does not actually need to match the physical flow geometry. You can subtract the pressure drop in this "chimney" to obtain the desired pressure drop. The pressure drop in the "chimney" will vary over its full cross section. You will have to choose a representative value of the pressure drop to match the desired outlet conditions. You will not know this pressure drop, of course, until after you examine the results from the chimney. You can opt to apply an exit pressure profile, based on these results, to the original geometry.

You can calculate the required length of the chimney (L) using the characteristic diameter (D) from the development length formula:

Laminar flow: L/D ~ 0.06 Re Turbulent flow: L/D ~ 4.4 Re1/6

END OF INTERNET INFORMATION

I have tried using a 'chimney' as the above information suggests however, this does not remove the mass imbalance instead, a large velocity occurs at the new outlet position.

Has anyone come across this problem before?

Does anyone have any ideas on how to overcome this problem?

How can I reduce gradients close to the outlet boundary?

Could it be something to do with my mesh, I am using under 1000 nodes and no clustering?

Kind Regards

Andy
  Reply With Quote

Reply


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
Transient Mass flow inlet boundary condition nikhil FLUENT 1 April 16, 2014 10:58
CFL condition: can a mass balance problem arise if violated? bzz77 Main CFD Forum 3 August 24, 2011 16:12
mass transfer coefficient for boundary condition niek Main CFD Forum 0 March 29, 2011 06:33
mass imbalance reactions ?? Abel FLUENT 0 December 21, 2008 10:32
Relative Error and Mass Imbalance Giordano Bruno FLUENT 0 March 3, 2005 12:01


All times are GMT -4. The time now is 09:35.