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

under relaxation factors

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   November 22, 2009, 15:36
Default under relaxation factors
  #1
New Member
 
sateesh
Join Date: Oct 2009
Posts: 9
Rep Power: 16
sateesh is on a distinguished road
Dear All,

I am getting convergence for once of my problem only if i reduce the under relaxation factors to ten times in middle of the simulation. can i reduce the under relaxation factor to any fraction?
sateesh is offline   Reply With Quote

Old   November 23, 2009, 07:26
Lightbulb
  #2
Senior Member
 
teguhtf's Avatar
 
teguh hady
Join Date: Aug 2009
Location: Saga, Japan
Posts: 222
Rep Power: 17
teguhtf is on a distinguished road
Hi
I've read in books that decrease alpha will increase convergence. the limitation of value to decrease is must be >= 0.2. maybe it can be used for your limitations to set alpha.
teguh
__________________
Now Or Never!!!
teguhtf is offline   Reply With Quote

Old   November 23, 2009, 17:42
Default
  #3
Senior Member
 
Chris
Join Date: Jul 2009
Location: Ohio, USA
Posts: 169
Rep Power: 16
Chris D is on a distinguished road
The under-relaxation factors are used to make the solution more stable. They should not affect your final solution, only how many iterations it takes to reach it. Decreasing the URF's will reduce the residuals, but the reduction is artificial. By what measure are you saying that your simulation is converged?
Chris D is offline   Reply With Quote

Old   November 23, 2009, 19:13
Default URF doubts
  #4
Member
 
R. Roy
Join Date: Mar 2009
Location: India
Posts: 52
Rep Power: 17
rr123 is on a distinguished road
yes, i also do not understand these URF parameters. for the moment, i feel like i am just trying to achieve conevrgence through trial and error using these URFs. also what would be the connection between these parameters among each other ?
can you refer me any book/articles where I can learn the concepts behind these parameters ?
rr123 is offline   Reply With Quote

Old   November 23, 2009, 21:11
Default
  #5
Senior Member
 
Chris
Join Date: Jul 2009
Location: Ohio, USA
Posts: 169
Rep Power: 16
Chris D is on a distinguished road
In the FLUENT 6.3 User's Guide, see Section 25.4.4. The under-relaxation factors are used to prevent large changes in solution values from one iteration to the next. Large fluctuations of the solution variables are possible in the initial stages of the simulation, so the under-relaxation factors keep it from blowing up during this time. After the flow has settled down and is nicely headed towards steady-state, they shouldn't be necessary.

If you're finding that your simulation will only "converge" if you decrease the under-relaxation factors to a very small amount, then there is probably something wrong. By setting low values of the URF's, you're only allowing the variables to change by a very small amount, so the only reason that it looks like the residuals/surface monitors have stopped changing is because you have forced them to stop changing.
Chris D is offline   Reply With Quote

Old   November 24, 2009, 13:02
Default
  #6
New Member
 
sateesh
Join Date: Oct 2009
Posts: 9
Rep Power: 16
sateesh is on a distinguished road
Hi,
Thanks for your valuable information on URF. I am trying to solving flow, turbulence, energy, species transport equations simultaneously in a irregular 3-D geometry. after long time, there is no change in the residuals. the plot becomes straight line. in this case if i reduce the URF by 10 times, i am getting the required convergence. this is my problem. can you suggest me what should i do? or where is the problem? Thanks in Advance- Sateesh
sateesh is offline   Reply With Quote

Old   November 24, 2009, 14:37
Default
  #7
Senior Member
 
Chris
Join Date: Jul 2009
Location: Ohio, USA
Posts: 169
Rep Power: 16
Chris D is on a distinguished road
Quote:
Originally Posted by sateesh View Post
Hi,
Thanks for your valuable information on URF. I am trying to solving flow, turbulence, energy, species transport equations simultaneously in a irregular 3-D geometry. after long time, there is no change in the residuals. the plot becomes straight line. in this case if i reduce the URF by 10 times, i am getting the required convergence. this is my problem. can you suggest me what should i do? or where is the problem? Thanks in Advance- Sateesh
Do you mean that, after the residuals decrease to some value (say 10^-2), you reduce the URF and your residuals decrease to some lower value (say 10^-6)?

If that's true, then your simulation is not necessarily converged. The residuals are only decreasing because you decreased the URF. To monitor convergence, you need to consider other factors besides the residuals. Just because your residuals dropped below some value does not mean that your simulation is converged.

You should monitor some other solution values to help you judge convergence. If you are performing a steady-state calculation on an open system, for example, you should monitor the net mass flow rate, which should converge to zero.

I would suggest monitoring some parameters during the simulation that are important to you. You can setup the monitors under the solve-->monitors menu. If these monitors converge, then that is the best you can do with that particular grid, and with those particular boundary conditions and solver settings. Changing any of those things may change your solution, and you should investigate what happens when you change them, but your URF should not influence your converged solution.
Chris D is offline   Reply With Quote

Old   November 25, 2009, 08:30
Default Shaddow Wall / Help Sought
  #8
New Member
 
sateesh
Join Date: Oct 2009
Posts: 9
Rep Power: 16
sateesh is on a distinguished road
Hi,

I have two adjacent zones one is solid (coal) and another one is fluid (Occupaid by Oxygen).
A reaction is taking place at the solid surface/interface i.e. coal is getting combusted with oxygen (which is coming from the fluid zone).
It is a exothermic reaction. because of this reaction, heat transfer 'll takes place in both the zines.
I need to obtain the temperature profile in side the two zones.
I need to create a geometry in a way that only heat should get transfer through the interface between the two zones.
How can i create the shadow wall for on side interface? or can any body suggest me how can i solve this problem in FLUENT?

Thanks in Advance
sateesh 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
Relaxation Factors for Transient solvers philippose OpenFOAM Running, Solving & CFD 19 March 20, 2014 05:39
relaxation factors and time accuracy Mike Main CFD Forum 7 May 21, 2005 13:41
Relaxation Factors Tim Phoenics 3 June 30, 2004 03:03
relaxation factors zhujianguo Phoenics 1 August 5, 2003 07:17
relaxation factors adjust zhujianguo Phoenics 1 July 15, 2003 12:11


All times are GMT -4. The time now is 14:09.