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

Convergence problem

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

Reply
 
LinkBack Thread Tools Display Modes
Old   August 28, 2013, 09:36
Default Convergence problem
  #1
Member
 
Harry
Join Date: May 2013
Posts: 68
Rep Power: 5
star is on a distinguished road
Hi friends. I am simulating flow over 2d airfoil at Reynold number of 1000. Flow is laminar and steady state. I run with two different pressure schemes. First I used PRESTO Pressure scheme and 2nd order upwind and the residual became stable (very small oscillations) at 10e-6 and were not decreasing any more. I was monitoring CL and was constantly decreasing with a very minor change.
Then I restarted the whole process with 2nd order Pressure scheme and the solution converged but the result is over predicted from experimental. I know this is steady state case and not transient and my mesh quality, skewness, and Angle is also fine.
Anyone know where is the problem? Thanks in advance
star is offline   Reply With Quote

Old   August 28, 2013, 10:34
Default
  #2
Senior Member
 
flotus1's Avatar
 
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,241
Rep Power: 22
flotus1 will become famous soon enoughflotus1 will become famous soon enough
Why do you thing that this is a problem with convergence? You say that both simulations converge

How high is the error between experiment and simulation?
flotus1 is offline   Reply With Quote

Old   August 28, 2013, 10:51
Default
  #3
Member
 
Harry
Join Date: May 2013
Posts: 68
Rep Power: 5
star is on a distinguished road
Thanks Alex for your response. The correct CL is about 0.1634 (at 3 degree angle of attack) but i got 0.167 with 2nd order pressure and momentum schemes. My value was lower when i used coarser mesh but with refining it become over predected. I think it should be equal or less than correct value. When I changed pressure scheme to PRESTO, i got about 0.1620 but not totally stable, it still decreasing (very minor change 0.1% decrease in 1000 iterations) but residuals not decreasing. I run through about 32000 iterations. What do you say about this?
star is offline   Reply With Quote

Old   August 28, 2013, 13:41
Default
  #4
Senior Member
 
flotus1's Avatar
 
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,241
Rep Power: 22
flotus1 will become famous soon enoughflotus1 will become famous soon enough
I say that a mismatch of 2.2% between experiment and simulation is very good.
Unless you know for certain that the error of the measurement is lower than those 2.2%, there is no reason in pushing the simulation towards the "exact" measurement value.
flotus1 is offline   Reply With Quote

Old   August 28, 2013, 23:20
Default
  #5
Member
 
Harry
Join Date: May 2013
Posts: 68
Rep Power: 5
star is on a distinguished road
Actually I am doing grid validation at angle of attack 3 degree. one of my friend did the same case and he got almost the same result to the actual (He is not here). I wonder why with two different pressure schemes result is different. In my opinion, using PRESTO pressure scheme the result looks ok but the residuals become stable as i told before so i don't think this is correct.
star is offline   Reply With Quote

Old   August 29, 2013, 04:06
Default
  #6
Member
 
Harry
Join Date: May 2013
Posts: 68
Rep Power: 5
star is on a distinguished road
Quote:
Originally Posted by star View Post
Thanks Alex for your response. The correct CL is about 0.1634 (at 3 degree angle of attack) but i got 0.167 with 2nd order pressure and momentum schemes. My value was lower when i used coarser mesh but with refining it become over predected. I think it should be equal or less than correct value. When I changed pressure scheme to PRESTO, i got about 0.1620 but not totally stable, it still decreasing (very minor change 0.1% decrease in 1000 iterations) but residuals not decreasing. I run through about 32000 iterations. What do you say about this?
Oh i am sorry i came to know that i had made a small mistake with using 2nd order pressure scheme that's why i got a little over predected CL, I corrected it and now it's in process.. .. but the problem with using PRESTO scheme is still there.
star is offline   Reply With Quote

Old   September 4, 2013, 05:34
Wink
  #7
Member
 
Centurion2011's Avatar
 
Join Date: Jul 2011
Posts: 73
Blog Entries: 1
Rep Power: 11
Centurion2011 will become famous soon enough
try here
__________________
I'M NOT A GYNECOLOGIST BUT I'LL TAKE A LOOK.
Centurion2011 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
convergence problem when use pisoFoam, LES for wind tunnel case Forrest_Lei OpenFOAM 3 July 19, 2011 06:00
convergence problem commonyue Main CFD Forum 1 December 1, 2009 04:54
Convergence of CFX field in FSI analysis nasdak CFX 2 June 29, 2009 01:17
3D Fluid Flow Convergence problem Emily FLUENT 2 March 21, 2007 23:18
Non Convergence of 3D Heat transfer cfd problem Balraj Main CFD Forum 3 December 9, 2004 01:24


All times are GMT -4. The time now is 02:38.