CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Running, Solving & CFD

Convergence problem residuals stay at a certain level

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

Reply
 
LinkBack Thread Tools Display Modes
Old   April 13, 2006, 11:30
Default Hi! I still try to calculat
  #1
Member
 
Andreas Hauffe
Join Date: Mar 2009
Location: Dresden, Germany
Posts: 36
Rep Power: 7
andimb is on a distinguished road
Hi!

I still try to calculate the flow arround an airfoil and have a convergence problem. If I use another scheme then upwind all residuals decrease quite well but suddenly they increase to a certain level and stay there. The values of the solution are great (cl,cd,pressure distribution), but the residuals won't decrease.

I think the aspect ratio of the elements behind the airfoil is really bad, so it acts nearly like a wall. This could be a problem. Is there an other solution than use much more elements behind the profil which is uninteressting for me?
Or could it be another problem?
The values of the solution are really great, but I need the convergence for an automatic calculation.

Thanks
Andreas
andimb is offline   Reply With Quote

Old   April 13, 2006, 11:35
Default Hi! Sorry, I forgot to say,
  #2
Member
 
Andreas Hauffe
Join Date: Mar 2009
Location: Dresden, Germany
Posts: 36
Rep Power: 7
andimb is on a distinguished road
Hi!

Sorry, I forgot to say, I'm using a c-grid. That's the reason for the thin elements.

Andreas
andimb is offline   Reply With Quote

Old   April 27, 2006, 03:56
Default Hi! Sorry, for starting thi
  #3
Member
 
Andreas Hauffe
Join Date: Mar 2009
Location: Dresden, Germany
Posts: 36
Rep Power: 7
andimb is on a distinguished road
Hi!

Sorry, for starting this thread new, but I'm still having the same problems. I reduced the mesh resolution so that there are no bad elements, but the convergence isn't better. The same behavior of the residuals. I tried other turbulence models to check if this is a problem. The only scheme for div(phi,U), which converge, is upwind.

Does anyone have an idea?

Thanks
Andreas
andimb is offline   Reply With Quote

Old   April 28, 2006, 17:13
Default I've pasted some possible expl
  #4
Senior Member
 
Srinath Madhavan (a.k.a pUl|)
Join Date: Mar 2009
Location: Edmonton, AB, Canada
Posts: 697
Rep Power: 10
msrinath80 is on a distinguished road
I've pasted some possible explanations for the residual behavior you've observed. These are from the Fluent manual. Assuming your problem setup is correct, here are some possible reasons for the obseved behavior:

[1] If you have provided a very good initial guess, the residuals may not drop three orders of magnitude.

[2] If the governing equation contains non-linear source terms which are zero at the beginning of the calculation and build up slowly during computation, the residuals may not drop three orders of magnitude.

[3] If the variable of interest is nearly zero everywhere, the residuals may not drop three orders of magnitude. In fully-developed flow in a pipe, for example, the cross-sectional velocities are zero. If these velocities have been initialized to zero, initial (and final) residuals are both close to zero, and a three-order drop cannot be expected.

In such cases, it is wise to monitor integrated quantities, such as drag or overall heat transfer coefficient, before concluding that the solution has converged. It may also be useful to examine the un-normalized unscaled residual, and determine if the residual is small compared to some appropriate scale.

Hope this helps!
msrinath80 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
Residuals for convergence of segregated solvers almanstoetter OpenFOAM Running, Solving & CFD 11 April 16, 2009 14:59
convergence and residuals ebm CFX 2 February 2, 2009 17:55
How to set convergence not using residuals Stephen Gillen FLUENT 0 November 28, 2008 07:03
Convergence and residuals christian OpenFOAM Running, Solving & CFD 1 April 15, 2008 13:28
Convergence-Residuals Madhukar FLUENT 7 May 17, 2007 03:13


All times are GMT -4. The time now is 14:23.