Global courant number for large VOF model
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 |
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 |
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