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

Global courant number for large VOF model

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

Reply
 
LinkBack Thread Tools Display Modes
Old   January 13, 2014, 14:15
Default Global courant number for large VOF model
  #1
New Member
 
Join Date: Jan 2014
Posts: 2
Rep Power: 0
skljar is on a distinguished road
Hi all,
I am creating a transient model for filling and emptying a sedimentation basin. My current setup consists of 1.7 million cell mesh, 1 second time step, and max velocity at inlet of about 1 m/s. Now, right at the start of the simulation, I get a global courant number of over 250, which results in an error - this is with a a flow velocity of .01 something. So question 1 - to help with the courant number situation, would making the mesh coarser help with this issue (instead of dropping the time step)? Also, from everything I gathered (I am very new to CFD) a 1 second time step is too large - however we would like to load the sedimentation basin with a hydrograph which is over 1 hour long... Is this a reasonable model to run on 62 average speed cores distributed over 2 nodes ? (This question is further aggravated because we plan on using DPM injections after the hydraulic model is verified). Question 2 - in general, what kind of settings would one recommend for a VOF model of a sedimentation basin (My journal log is below). Please let me know if I left out any information.

Thanks


define operating-conditions gravity yes 0 0 -9.81
define models unsteady-1st order yes
define models multiphase model vof
define models multiphase number-of-phases 2
define models multiphase volume-fraction-parameters explicit .25 yes ,
define models multiphase body-force-formulation yes
define models viscous ke-realizable yes
define materials copy fluid water-liquid
define phases phase-domain phase-2 phase-2 yes water-liquid
define phases interaction-domain 0 yes yes no no yes constant .072 no
define operating-conditions operating-density yes 1.225
define boundary-conditions zone-type inlet mass-flow-inlet
define boundary-conditions zone-type outlet pressure-outlet
define boundary-conditions mass-flow-inlet inlet mixture yes no 0 no yes no no yes 10 10
define boundary-conditions pressure-outlet outlet mixture no 0 no yes no no yes 10 10 yes
define boundary-conditions mass-flow-inlet inlet phase-1 yes no 0
solve set discretization-scheme mom 0
solve set discretization-scheme mp 16
solve set p-v-coupling 22
skljar is offline   Reply With Quote

Old   January 19, 2014, 23:08
Default
  #2
Member
 
Marcus
Join Date: Dec 2012
Posts: 61
Rep Power: 3
beer is on a distinguished road
Hi

It seems that you use the explicit VOF-calculation. Explicit time-stepping doesn't go with CFL-numbers larger than 1 (Fluent says two, but that can already lead to convergence problems in some cases). The explicit time scheme is for sharp surfaces including reconstruction. I think it is not that important to have a sharp surface for sedimentation, so maybe try an implicit scheme. The compressive scheme is not to bad and still provide a more or less sharp surface due to an interface compression (some kind of bounded negative diffusion).
With this you will get rid of the need for small time steps.

Cheers
beer 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
AMI interDyMFoam for mixer danny123 OpenFOAM Running, Solving & CFD 4 June 19, 2013 04:49
same geometry,structured and unstructured mesh,different behaviour. sharonyue OpenFOAM Running, Solving & CFD 13 January 2, 2013 22:40
pisoFoam with k-epsilon turb blows up - Some questions Heroic OpenFOAM Running, Solving & CFD 26 December 17, 2012 03:34
Multiple floating objects CKH OpenFOAM 10 September 21, 2011 23:13
Global courant number what mean erica FLUENT 0 November 24, 2007 05:41


All times are GMT -4. The time now is 21:59.