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

Unexpected difference between velocity / mass-flow inlet

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

Reply
 
LinkBack Thread Tools Display Modes
Old   December 7, 2010, 23:47
Default Unexpected difference between velocity / mass-flow inlet
  #1
jpo
Member
 
Join Date: Apr 2009
Posts: 93
Rep Power: 8
jpo is on a distinguished road
Hello,

I am modeling a 3D incompressible inviscid airflow in a system. It has 1 velocity inlet and a number of outflow outlets. It converges fine.

Then I check the mass flux via inlet: let's say it is M_dot
and
switch velocity inlet to mass-flow inlet, request a flux equal to M_dot.

Case loses convergence right away and can't converge properly anymore. I don't understand why. Nothing really has changed, except the inlet type.


Where am I going wrong?
Thank you for your thoughts.
jpo is offline   Reply With Quote

Old   December 9, 2010, 02:10
Default
  #2
Super Moderator
 
-mAx-'s Avatar
 
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 2,972
Rep Power: 30
-mAx- will become famous soon enough
try to switch from outflow to pressure outlet.
I always heard that combination velocity-inlet / pressure-outlet was more stable than mass-flow-inlet / pressure-outlet.
Don't ask me why...
__________________
In memory of my friend Hervé: CFD engineer & freerider
-mAx- is offline   Reply With Quote

Old   December 9, 2010, 07:17
Default
  #3
jpo
Member
 
Join Date: Apr 2009
Posts: 93
Rep Power: 8
jpo is on a distinguished road
-mAx-, thank you for your reply and for the time you dedicate in helping others in this forum

I tried this, but there is a problem. How do I set the pressures?


My goal is to switch the whole thing to compressible flow. But velocity inlet/outflows do not work with pressure. I cannot make the transition...
jpo is offline   Reply With Quote

Old   December 9, 2010, 08:44
Default
  #4
Super Moderator
 
-mAx-'s Avatar
 
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 2,972
Rep Power: 30
-mAx- will become famous soon enough
I don't understand what you mean with transition.
Set your outlet with Pressure-Outlet Boundary Condition.
Set your pressure with 0 Pa (outlet free)
__________________
In memory of my friend Hervé: CFD engineer & freerider
-mAx- is offline   Reply With Quote

Old   December 9, 2010, 08:49
Default
  #5
jpo
Member
 
Join Date: Apr 2009
Posts: 93
Rep Power: 8
jpo is on a distinguished road
My system has a number of outlets. Compressible simulation is hard to converge. This is why I do incompressible first and try to find out the pressures on all terminals.

Once this is done, I can retry running compressible calculation with the new pressures.
jpo is offline   Reply With Quote

Old   December 9, 2010, 10:37
Default
  #6
Super Moderator
 
-mAx-'s Avatar
 
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 2,972
Rep Power: 30
-mAx- will become famous soon enough
So you have number of outlets, but you don't know neither their pressure, nor their massfow?
__________________
In memory of my friend Hervé: CFD engineer & freerider
-mAx- is offline   Reply With Quote

Old   December 9, 2010, 11:42
Default
  #7
jpo
Member
 
Join Date: Apr 2009
Posts: 93
Rep Power: 8
jpo is on a distinguished road
that is correct

I only know their approximate massflow percentages (e.g. parts of the mass entering the system)
jpo is offline   Reply With Quote

Old   December 10, 2010, 02:27
Default
  #8
Super Moderator
 
-mAx-'s Avatar
 
Maxime Perelli
Join Date: Mar 2009
Location: Switzerland
Posts: 2,972
Rep Power: 30
-mAx- will become famous soon enough
then try to work with pressure-outlet and enable target massflow option, with giving approximate massflow
__________________
In memory of my friend Hervé: CFD engineer & freerider
-mAx- is offline   Reply With Quote

Old   December 10, 2010, 08:20
Default
  #9
jpo
Member
 
Join Date: Apr 2009
Posts: 93
Rep Power: 8
jpo is on a distinguished road
to be honest, whenever I have tried enabling target mass flow on pressure outlets, it hardly ever works
It didn't work in this case either


But it works with velocity inlet/outflow exits. I am trying to figure out how to use this successful calculation and set up the exits as pressure outlets
jpo is offline   Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Problem with assigned inlet velocity profile as a boundary condition Ozgur_ FLUENT 2 August 3, 2012 05:54
Inlet Velocity in CFX aeroman CFX 12 August 6, 2009 18:42
velocity or mass flow inlet? Silvia FLUENT 1 August 12, 2005 13:55
Variables Definition in CFX Solver 5.6 R P CFX 2 October 26, 2004 02:13
Can reversed flow occur at velocity inlet Bcs. kanok FLUENT 1 September 30, 2003 11:54


All times are GMT -4. The time now is 23:15.