CFD Online Logo CFD Online URL
Home > Forums > CFX

convergence problem

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

LinkBack Thread Tools Display Modes
Old   November 4, 2005, 00:39
Default convergence problem
Posts: n/a
Sponsored Links
Hi, i am running inhomogeneous free surface model(steady flow). For some runs, residuals of volume fraction stuck at a high level (~1E-2) but other residuals died off to the target quickly. No matter how I change the time scale for vf and global time scale, it didn't converge. Would you please give your comments/advice? thanks for all. Ken
  Reply With Quote
Sponsored Links

Old   November 5, 2005, 02:05
Default Re: convergence problem
Posts: n/a
In my experience, free surfaces are NEVER steady. There are always wave or ripple effects that are anything but steady-state.

Try running as a transient simulation with 10 coefficent loops max and a smallish time step with a convergence criteria of 1E-5. After the problem starts converging in just a few coefficient loops you can start increasing the time step (better yet use the adaptive algorithm on the volume fraction coefficient loops).

Run the simulation long enough that you get a full volume turn over through the model.

Hope this helps, Jeff
  Reply With Quote

Old   November 7, 2005, 12:38
Default Re: convergence problem
Julian Turnbull
Posts: n/a
I agree with Jeff that free surfacesa re almost never steady. I was also wondering if you have a region in your simulation where there is a change in height or velocity in the free surface. If so, you should choose the timestep to be small enough to `capture' this. For example, if modeling weirm the imestep should be small enough to adequately model the flow where the fluid is fastest.

Don't refine anything until the mass balance is correct, or ou will be wasting time

Use second order transient differencing for a noticable speedup in solution.

  Reply With Quote

Old   November 18, 2005, 07:47
Default Re: convergence problem
Posts: n/a
The vilume fractions is most unstable solution in the two-phase flows. The developers are recommended to set time step for volume fraction equations in order less tha for other equations.
  Reply With Quote


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

Sponsored Links

All times are GMT -4. The time now is 23:29.