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

time step restriction for implicit icoFoam???

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

Reply
 
LinkBack Thread Tools Display Modes
Old   June 29, 2016, 03:29
Default time step restriction for implicit icoFoam???
  #1
New Member
 
Join Date: Mar 2015
Posts: 14
Rep Power: 4
K.C. is on a distinguished road
Dear Foamers,

during my research I noticed, that there is a time step restriction for the momentum predictor. I tested different meshsizes and timesteps in the cavity case of icoFoam found then following restrictions:

1.) Co = u_wall*dt/dx < 1 (as usuall)
2.) nu*dt/dx < 0.25

The last restriction seems strange, because icoFoam uses an implicit scheme.

Why do we have this restriction, where van-Neumann-stability-analysis predicts unconditional stabilyty for the implicit diffusive term in momentum equation? Do you observe the same restriction? Do you know how to overcome this problem?

Thanks for your help,
K.C.
K.C. is offline   Reply With Quote

Old   July 6, 2016, 03:51
Default
  #2
New Member
 
Join Date: Mar 2015
Posts: 14
Rep Power: 4
K.C. is on a distinguished road
Does anybody have an idea?

By the way an increase in corrector loops seems to soften the last restriction, but nevertheless a reson for this restriction is missing.
K.C. is offline   Reply With Quote

Old   July 12, 2016, 04:06
Default
  #3
New Member
 
Join Date: Mar 2015
Posts: 14
Rep Power: 4
K.C. is on a distinguished road
Please,
could anybody just confirm my observation and try the icoFoam-cavity case with nu*dt/dx > 0.25?

Every literature tells me that implicit treatment of viscous stresses should avoid this time step restriction, but my icoFoam has this restriction.

Thanks to you.
K.C. is offline   Reply With Quote

Old   August 1, 2016, 10:39
Default
  #4
New Member
 
Join Date: Mar 2015
Posts: 14
Rep Power: 4
K.C. is on a distinguished road
This restriction is a huge problem for me and I dont understand, that every literature (e.g. Ferzinger) writes that solving the momentum predictor should be unconditional stable.
K.C. is offline   Reply With Quote

Old   February 23, 2017, 19:19
Default same problem
  #5
New Member
 
longhui zhang
Join Date: Jan 2017
Posts: 1
Rep Power: 0
zzzlllhh is on a distinguished road
Dear K.C.

Did you find the answer to this problem? I tested with icoFoam and found the same restriction on time step. Could you please tell me what produces this restriction if you already got the answer? Thank you very much!

Zhang
zzzlllhh 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
Star cd es-ice solver error ernarasimman STAR-CD 2 September 12, 2014 00:01
Help for the small implementation in turbulence model shipman OpenFOAM Programming & Development 25 March 19, 2014 11:08
AMI interDyMFoam for mixer nu problem danny123 OpenFOAM Programming & Development 8 September 6, 2013 02:34
AMI speed performance danny123 OpenFOAM 19 October 24, 2012 07:44
plot over time fferroni OpenFOAM Post-Processing 7 June 8, 2012 07:56


All times are GMT -4. The time now is 09:45.