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VOF boundary conditions

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Old   March 4, 2020, 10:37
Default VOF boundary conditions
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Hello everybody,
i have a question about boundary conditions usiong VOF.
i am simulating the air inlet in a sort of tank filled with water. in this way the only bounderies should be:
-air inlet for bubbles;
-wall everywhere (with slip or no slip condition shouldn't be an issue)

The Model has the following features:

-Both fluids have density constant;
-air inlet velocity 0.5 m/s;
- water is still;
-turbulence model is laminar (Re about 200).

my question is: if i have rigid wall where it will go the water pushed by air?
i checked during the simulation that the density of the mixture (or the density) is decreasing therefore i think that the water in some manner have to leave the domain but i am not able to understand how.
is the water substiuted by air or leave the domain?
thank you in advance at all

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Old   March 4, 2020, 10:44
Default The reality
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It depends on what happens in reality. If the tank is closed, then air injected will compress the water a little and get itself compressed. So, you have to model air as compressible gas. If tank is fully filled with water, then even water has to be modeled as compressible liquid. If there is some space, then water can be left as incompressible.

But all this has to be in correspondence with the real scenario you are trying to emulate.
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Old   March 4, 2020, 10:57
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Vinerm thank you for your reply.
I am trying to simulate air bubbles formation from an inlet of air.
The real scenario is the bubble formation from a pipe filled with air at the bottom of the sea (bubbles curtain). Even if i did not catched the correct model (probably it is like this) i would like to know in my case what is happening to the water: it disappears from the domain? how it disappears? is it a bug of the code?
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Old   March 4, 2020, 11:06
Default Fixed Volume
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If both, air and water, are being modeled as incompressible, then the volume available can only accommodate a fixed mass, given by

V_{water}*\rho_{water} + V_{air}*\rho_{air}

If air starts to come in, water has to disappear. This is the outcome of the approach used to model primary phase. If water is the primary phase in your simulation, its volume fraction is given by the following equation

\alpha_{water} + \alpha_{air} = 1

As the volume of the gas increases, volume fraction of the water has to reduce. If the system is incompressible, then its actual volume has to reduce, which is happening in your case. So, you have to assume that water can go out of the domain, just like it can go somewhere within the sea.
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Old   March 4, 2020, 11:20
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Vinerm thank you again for your explanation.
Ok i understood, but thi is not sense (not your explanation but how the model behaves).
Ad absurdum if i run for a very long time the simulation the water will diseappear completely and it will be substituted by air, correct? in this way it is not sense

In fact during the simulation i saw the density of the mixture to decrease (and the total mass) and i did not understand how. This is, let me pass the concept, a bug of the code for this unreal situation.

thank you

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Old   March 4, 2020, 12:46
Default Code Behavior
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Let's consider an example from childhood. Take a glass tumbler, ensure that it is vertically held, upside-down, and then push it in a water tank. Do you expect tumbler to get filled with water?

Another experiment you may do is with a straw. Put it in water so that it is fully or partially filled with water. Then close it from top with your finger and take the straw out, keeping it as vertically aligned as possible. Do you expect water to fall down?

Multiphase model behaves in same way.
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Old   March 5, 2020, 03:26
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Vinerm you are right, my doubt is non sense.
Thank you again for your reply

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Old   March 5, 2020, 03:35
Default VOF Simulation
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You are welcome. Let me know if the simulation does not work as expected. Or in case you need help with defining the problem and setup.
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