Learning Forum

[SR786]

Hello,

I am currently modelling a semi-batch precipitation process in an agitated vessel, where I have multiphase VOF (free surface tracking and volume tracking), SST k-w CC turbulence model, species transport (mass balance) and user defined scalars (population balance) with UDFs for source terms and I want to introduce a change by closing feed inlet boundary condition. I have a velocity-inlet (0.00877 m/s) for feed pipe face and pressure-outlet for top.

As I understand there are two ways to do this:

1. Change velocity-inlet to wall
2. Set velocity-inlet to 0 m/s

I tried the first option but it causes the solver to be very unstable and I cannot get reproducible results even with controlled time step size. Is the second option viable and what is the most robust way of going about this?

[SR786]

 

Method 2 doesn’t work either. I have tried method 1 but on a case which does not involve UDS, source terms or UDS (for precipitation). Instead it is relatively simpler semi-batch feeding simulation involving VOF and species transport equation. After changing boundary condition, the solver is able to handle the transition much better without exploding suggesting that the UDS is the primary culprit. Is there anyway to help handle this or is it just a limitation of the solver itself due to the complexity of the simulation.

Edit I have one last option I am thinking of. For the feed-inlet pipe, instead of automatically switching from velocity-inlet to wall I should keep velocity-inlet but change mass fractions of species to 0 (pure solvent). This will make feed jet inert. Thenn after readjustment period, when all species concentrations on feed face reduce to negligble values then switch to wall bc.

 

[prae]

I think you have answered your own question. Changing the Bcn to wall and back to velocity boundary is not a good idea for multi fluid VOF cases. I will recommend writing a transient expressions for inlet made of nested if and else statements to adjust the velocity and the contents of the inlet. You can also use a simple ramp function in your expressions to avoid any sudden changes.