Learning Forum

[johnny02]

Hello guys,

I am running a simulation of a mixing tank in unsteady-state( transient) with turbulence model SST K-omega and multiphase model VOF. And I gave a number of time steps as 10000 and time step size as 0.0003 and max iteration as 50 and the solution is converging at each time step. So my question is how do I know my simulation is completed ? or I need to wait till 10000-time steps to complete?

Thanks in advance

Regards

Johnny

[DrAmine]

You need to run for the desired /expected physical time. Imagine you have a fogging / defogging cycle which lasts 60 minutes: I need to run my model for 60 minutes. If your intention is just to bring the run to quasi steady state then you can interrupt once key monitors are not changing any more and not because your residuals are reducing every time step.

[johnny02]

Thank you Dr.Amine. Furthermore, I have a question regarding the courant number calculation. I know that the courant number is an important parameter to choose the proper time step size and the courant number is calculated using this formula CFL = Flow velocity * time step size/cell size and my question is how do I find the flow velocity. Since I am doing a mixing simulation on different rotational speed so I need to take those rotational speeds as a flow velocity or is there is any other way to find flow velocity?

[DrAmine]

Based on tangential velocity (Angular speed * maximum radius) you can have a rough estimate for the velocity. However local values are important for that reason I won't first both much about that especially as we are dealing with implicit solvers. CFL numbers can be as high as 20 if you want to have a rapid run. Accuracy will require smaller CFL numbers (and depending on which VOF formulation you are using). For Multiphase there is an adaptive time stepping method which might of great benefit for you.