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Global courant number for large VOF model

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Old   January 13, 2014, 15:15
Default Global courant number for large VOF model
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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.


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
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Old   January 20, 2014, 00:08
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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.

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