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

Pressure residuals oscillating in natural convection VOF melting case

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   February 18, 2021, 15:19
Question Pressure residuals oscillating in natural convection VOF melting case
  #1
New Member
 
Kai Salscheider
Join Date: Aug 2018
Posts: 13
Rep Power: 8
Skaiwalker is on a distinguished road
Hello forum,

Problem: I want to model the melting process of magnesium ingots in a melting furnaces. Thus I've decided to use OF8 with "buoyantPimpleFoam" and fvOptions "solidificationMeltingSource". After a few timesteps either for Ux, or p_rgh the residuals won't fall under 1e-6 but will oscillate around a constant value in the magnitude of e-5 to e-4 . This happens at first for one timestep, then it converges well again for several time steps. As time progresses the the number of non converging timesteps increases gradually but there are always converging timesteps in between.

The results look not that bad as far as I can tell, nothing explodes or unexpected happens, but from a mathematical standpoint this introduces several sources of errors, if I understand the numerics correctly:
  1. Not well converged results are physically more or less inaccurate
  2. going on from an inaccurate result results in even more inaccurate results
  3. the last pimple step is not computed with an underrelaxation of 1 introducing time continuity errors

Questions: (actually a lot )

Is it possible to converge in every timestep in this case, or am I hitting rock bottom on the residuals? How bad is the error growing through the above mentioned error sources? How can one ensure that pimple doesn't produce time step incontinuities?

Also my underrelaxation seems pretty odd to me (U 0.6, p 0.05) but it works to some extent and I've run a ton of cases to figure this out. Is it adequate to underrelax one parameter that much if it makes trouble? In the end it converges in the "not broken" timesteps.

Additional Info:
I've attached a screenshot on an arbitrary timestep, the x axis is lying. Also the case is attached. Frist run it with Allrun, then runSimulation.sh is enough after the mesh is decomposed. Post processing is done by "gnuplot utilities/plotResiduals_onTheFly.gnu".

Thank you for your help, I appreciate every single bit of info I can gather on OpenFOAM, its so amazing (when it doesn't drive you crazy )

PS: Be careful with ctrl-c on the run-scripts, because the mpi initialized solver processes will not be terminated. I didn't find a workaround up until now.
Attached Images
File Type: jpg residuals.jpg (147.2 KB, 18 views)
Attached Files
File Type: zip pimpleOscillatingResiduals.zip (48.7 KB, 1 views)
Skaiwalker is offline   Reply With Quote

Reply

Tags
buoyantpimplefoam, pimple, residuals, solidificationmeltingsour

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
Natural Convection around two spheres in a box - chtMultiRegioSimpleFoam salvo-K61IC OpenFOAM 4 January 16, 2015 14:27
oscillating pressure residuals buoyant plume mabinty OpenFOAM Running, Solving & CFD 0 August 31, 2009 06:31
Free surface boudary conditions with SOLA-VOF Fan Main CFD Forum 10 September 9, 2006 13:24
Operating Pressure (Natural Convection problem) Andrew Tress Main CFD Forum 1 July 3, 2006 17:00
Hydrostatic pressure in 2-phase flow modeling (long) DS & HB Main CFD Forum 0 January 8, 2000 16:00


All times are GMT -4. The time now is 17:46.