VOF - Water Filling Air Vol - Velocity Field Issue
 

Please see the attached PDF file with snapshots illustrating the simulation results.

Pressure and density distributions are Good, however the velocity field is NOT ok, it is as if the VOF in FLUENT is using a “compressible mixture phase” of constant compressibility all over the domain regardless of the volume fractions of water and air ?

Are you having the same issue ?

Flow Field
 

Not just the velocity field, the pressure field is wrong as well. If the top is closed, it means that the air should get compressed. But it appears that you are using constant density air. You need to model air using ideal-gas, with gravity enabled in correct direction, setting up operating pressure and operating density to 0.

The question is why the velocity field is wrong ?
 

Vinerm, thanks for your feedback. Two questions:
1) Will "everything" gets fixed, including the velocity field in the water, when we change the air model to ideal gas ?
2) Why do you think the water velocity is wrong as it is now ?
Thanks,

Setup
 

It depends on what everything means. But in any case where volume is supposed to change without any changes in the mass, the only option is change in density. Since this is most likely an isothermal case, density change has to be brought by pressure alone, hence, ideal-gas is a must. With constant velocity, flow-field is not really converged, rather adjusting to the conditions in an unphysical manner. If you keep running, it will diverge. Until and unless case is converged, solution is wrong. After convergence, there is a high likelihood that the solution is correct.