transient VOF simulation with discontinuity at interface
 

Hi,

I have been solving a crank lubrication problem with a static inlet, and rotating crank (mesh motion). I have done this steady state (frame rotation) and transient (mesh motion) with success.

I have been looking into the effects of air entrapment with a VOF model (0.1 volu fract at the inlet). And now i am having issues with convergence. The model typically starts fine, and then after a few hundred time steps will start diverging either due to turb / molecular viscosity limit, or courant number exceeding 250.

I am using a pressure inlet and pressure outlet boundary condition, and recently started ramping up the pressure linearly with time, and ramping up the speed linearly with time (offset from the pressure, so that speed doesn't start ramping until pressure is fully on) to try and overcome these convergence problems.

I look back into my recently stored databases, and find that there is a big pressure gradient across my interface. I post process the flow through the interface and it seems to line up properly, but nonetheless there is a big step change in pressure across the interface. Since i've been using the VOF model i've played a lot with the p-v coupling, under-relaxation, etc and so i am not sure if it is common for there to be discrepancies across the interface, or if my under-relaxation between pressure and momentum are out of whack or something.

Has anyone had any experience like this? Ultimately i do not make it to full pressure before the solution diverges (and so mesh motion is ON, but rotating at 0.0001 rad/s, it never begins ramping). When I solve this model steady state (in absence of the VOF) my y+ / y* are <5 everywhere in the model.

and FYI when i solve this model steady state / transient (without VOF) there is no pressure discontinuity across the interface.

Any guidance would be greatly appreciated!

VOF setup:

/define/models/viscous/ke-rng? yes
/define/models/viscous/near-wall-treatment/enhanced-wall-treatment yes
/define/models/multiphase/model vof number-of-phases 2 body-force-formulation yes volume-fraction-parameters explicit 0.25 1E-6

relaxation / discretization:

/solve/set/p-v-coupling 22
/solve/set/discretization-scheme mom 0
/solve/set/discretization-scheme epsilon 0
/solve/set/discretization-scheme k 0
/solve/set/discretization-scheme pressure 13
/solve/set/discretization-scheme mp 16
/solve/set/under-relaxation pressure 0.6
/solve/set/under-relaxation mp 0.5
/solve/set/under-relaxation turb-viscosity 0.5
/solve/set/under-relaxation epsilon 0.5
/solve/set/under-relaxation mom 0.75