[sh yang]

Hi,everybody!

I am trying to use fluent-vof solver to simuliate a problem that a drop impacts onto a film. I am a green hand in this area, and not very confident of the result.
The speed of air turns out very large, which seems not right. The Momentum of air seems to be raised up numerically by the motion of the drop in a closed box, insistently with a large dissipation near the interface, which is embodied in the distortion of the interface.
1.Could it result from the two-phase-vof method that fluent uses? Will true-vof work?
2.Is it so-called spurious currents? (I use CSF model to calculate surface tension.)
3.Some papers say that higher density ratio would lead to stronger numerical instability at the interface, does it have something to do with the false velocity of the air near the interface?

Anyone gets any idea?
Thanks for comments!

And sorry about my English

-Yang

[vinerm]

Did you setup gravity and reference density under operating conditions properly? This should not show any spurious results.

[sh yang]

Originally Posted by vinerm

Did you setup gravity and reference density under operating conditions properly? This should not show any spurious results.

Thank you for your reply, Vinerm!
I didn't set gravity. I set reference density as 1000 kg/m3. Did I do something wrong?

[vinerm]

Density has to be density of lightest phase. Gravity is must for VOF and should be enabled in correct direction

[sh yang]

Originally Posted by vinerm

Density has to be density of lightest phase. Gravity is must for VOF and should be enabled in correct direction

I thought the reference value just used for postprocessing? I didn't found any request of gravity on VOF method in the help document. Could you give me some clue to this point？
Thanks again!

[vinerm]

Reference values are only for post processing. That's why I mentioned reference density under operating conditions, essentially operating density. No multiphase model works without gravity. You may ignore in some cases where density ratio is small, otherwise, this is very important for momentum exchange

[sh yang]

Do you mean the gravity source (rho-rho0)g should balance the pressure so that the hydrostatic pressure remains zero? But physically, gravity is ignorable in drop impact problem since the Froude number and the Bond number is far greater than one; and numerically, if I don't activate gravity, will it make errors? You reminded me of that there might be something wrong in the calculation of the pressure of air. I am now checking the operating pressure, and I think mayby this is the point.

[vinerm]

Without gravity, drop has no reason for impact until there is an initial velocity given to it.

[vinerm]

And Bond number greater than one implies gravity cannot be ignored, surface tension can be ignored though

[sh yang]

You are right, I give the drop a 5m/s initial speed without gravity in calculation.

[sh yang]

sorry, bond number far smaller than one.

[vinerm]

Still, I'd recommend enabling gravity of density ratio between phases is high and using density of lighter phase as operating density. Location of operating pressure plays role in offsetting hydrostatic pressure.

[sh yang]

Thank you for the advice! I am goint to add it. I just notice the importance of the location of operating pressure as you said. You helped me a lot!