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

Convergence problems!!!

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

Reply
 
LinkBack Thread Tools Display Modes
Old   June 8, 2011, 15:03
Default Convergence problems!!!
  #1
New Member
 
Join Date: Dec 2009
Posts: 26
Rep Power: 7
king lui is on a distinguished road
Hello everybody,

I investigate a time-dependent two-phase flow with the vof-model and I have some problems with the convergence. I donīt know why but my simulation runs good for the first few time steps (between 2 and 50), but suddenly the continuity residual raises and results in a floating point error.At the beginning I had also the problem with a too high global courant number but this is gone since I am using the variable time step method. Anyway, now the floating point error is the problem, for example the last simulation I did runs very good, at the end the time steps converged quicker and quicker but as I said did the continuity increase suddenly.

I hope someone can help me with this problem as I tried already many things like different solver settings, changing the under-relaxation factors, minimise the time step size, etc.
king lui is offline   Reply With Quote

Old   June 8, 2011, 15:14
Default
  #2
Senior Member
 
Join Date: Nov 2010
Posts: 418
Rep Power: 8
me3840 is on a distinguished road
The problem may lie with your grid. Check the velocity vectors for discontinuities or weird flow fields, and refine or change the grid in that location.
me3840 is offline   Reply With Quote

Old   June 8, 2011, 17:42
Default
  #3
Member
 
Neil
Join Date: Jun 2009
Location: The Moon
Posts: 42
Rep Power: 8
Trev is on a distinguished road
Depending on the type of flow you could also try initially running the solution as single phase (deactivate the VOF equation) then later patch the other phase into the domain when the solution is stable.

But as me3840 says check the quality of the mesh first. You could try looking for regions where large gradients exist which may not physically occur. But the state of your solution will probably not provide much useful information considering you have only done a handful of time steps and the solver will have barely processed the boundary conditions.
Trev is offline   Reply With Quote

Old   June 9, 2011, 02:44
Default
  #4
New Member
 
Join Date: Dec 2009
Posts: 26
Rep Power: 7
king lui is on a distinguished road
Hello, at first thank you for your suggestions to solve this problem, I will try now to alter the mesh. About the answer of Trev, how do I kow when the solution is already stable? Yesterday my last try looks quite good for the first 260 time steps, but than the global courant number increases from 2 to 2e8. Another question is, if there is a difference between the "normal" courant number and the global courant number? To display my problem more clearly for you, I attach a picture from the residuals of my last try.
Thank you in advance for your help. Residuals.jpg
king lui is offline   Reply With Quote

Old   June 9, 2011, 09:38
Default
  #5
Member
 
Neil
Join Date: Jun 2009
Location: The Moon
Posts: 42
Rep Power: 8
Trev is on a distinguished road
The solution is generally stable when the residuals are flat (as low as the residual error will go) or at least oscillating by a small amount and your mass flux error is at an acceptable value (I normally use <10-6). From the look of yours I would say that for 1st order discretisation the averaged residual errors are ok but not for anything higher. In which case you should be using 1st order upon the initial run of the solution. You have large oscillations around the magnitude of an order which straight away says that your time step is too large as the solver can't properly resolve the flow. So you will have to drop the CFL number to account for this as a rough estimate i would say set it to something like 0.1 initially just to get the solution going. As well as use a fixed time step but experiment with it too see how the residual oscillations are affected. You can increase both at a later stage as all you want to do initially is get the solution running without the worry of divergence.

I don't know how you have set the solver up but have used these guidelines in the past to do VOF work once you have accepetable convergence values at each stage.

1) Run singlephase as steady-state with 1st order discretisation.
2) Switch to transient solver.
3) Increase to 2nd order.
4) Patch appropriate region with secondary phase.

I'm not too sure on the difference between the standard CFL and global CFL number. But would hazard a guess that the solver uses the standard CFL number as a baseline value and deviates from it accordingly at parts of the mesh where it is having trouble resolving the flow. Then applies a global CFL when things are going wrong to try and prevent divergence. But like I said I'm not sure so hopefully someone else could help you out on that one?
Trev is offline   Reply With Quote

Old   June 11, 2011, 08:24
Default
  #6
New Member
 
Join Date: Dec 2009
Posts: 26
Rep Power: 7
king lui is on a distinguished road
Thank you guys for your help, now my residuals are stable if I use your suggestions but unfortunately with a very small time step so that my simulation needs several months and if I alter the time step size the same problem occurs as I already explained.

So if you have maybe some more ideas please let me know.
But anyway thank you for your help.
king lui is offline   Reply With Quote

Old   June 12, 2011, 10:49
Default
  #7
Member
 
Neil
Join Date: Jun 2009
Location: The Moon
Posts: 42
Rep Power: 8
Trev is on a distinguished road
Although your time step is small at the moment to keep the solution stable this should only be temporary and you will probably find that you can start to increase it as the solution progresses. So I suggest experimenting with seeing how much you can increase it by every now and again. A good indicator of what you can get away with using is to see how much oscillation it causes in the residuals. If it speeds up the solution you can allow oscillations in the residuals as long as they are not too large or cause divergence.
Trev is offline   Reply With Quote

Old   June 14, 2011, 03:21
Default
  #8
New Member
 
Join Date: Dec 2009
Posts: 26
Rep Power: 7
king lui is on a distinguished road
I try this now, unfortunately I can increase the time steps just with a very small amount. But anyway, thanks a lot for your help.
king lui is offline   Reply With Quote

Old   June 15, 2011, 05:00
Default
  #9
Senior Member
 
kbaker's Avatar
 
Khalid Baker
Join Date: Mar 2009
Location: IRAQ
Posts: 118
Rep Power: 8
kbaker is on a distinguished road
What type of flow regime you working with?
kbaker is offline   Reply With Quote

Old   June 15, 2011, 05:05
Default
  #10
New Member
 
Join Date: Dec 2009
Posts: 26
Rep Power: 7
king lui is on a distinguished road
Hello,
its a gas-liquid flow regime.
king lui is offline   Reply With Quote

Old   June 15, 2011, 08:38
Default
  #11
Senior Member
 
kbaker's Avatar
 
Khalid Baker
Join Date: Mar 2009
Location: IRAQ
Posts: 118
Rep Power: 8
kbaker is on a distinguished road
dude i mean with what type of flow regime is it stratified-annular-slug flows...........etc?
kbaker is offline   Reply With Quote

Old   June 15, 2011, 10:30
Default
  #12
New Member
 
Join Date: Dec 2009
Posts: 26
Rep Power: 7
king lui is on a distinguished road
its a stratified annular flow
king lui 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 problems... need help urgent!! HSeldon FLUENT 1 March 15, 2007 02:04
NACA0012 Convergence Problems StudentAndrew CFX 6 November 21, 2005 07:49
Convergence problems Simone CD-adapco 5 June 29, 2005 10:48
convergence problems with transonic missile Emma FLUENT 0 July 12, 2004 10:33
Convergence problems Chetan FLUENT 3 April 15, 2004 19:13


All times are GMT -4. The time now is 17:15.