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Volume fraction at outlet not known

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Old   September 4, 2017, 05:41
Default Volume fraction at outlet not known
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Kamal Bisht
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Hello,

I am simulating a two phase (Gas & liquid) problem (see figure).

Problem statement: Liquid is flowing in a rectangular channel which has an opening at the top. The flow is driven by a motor at the outlet. As outlet flow rate increases, pressure in the channel drops, subsequently the free surface of the liquid at the opening collapsed and ingested air in the from of bubbles. The frequency of the ingested bubbles depends on the outlet flow rate.

The known boundary conditions are the pressure at the inlet recorded with
sensor and the outlet mass flow rate.

I am using pressure inlet and velocity outlet BC as mass flow outlet is not supported with VOF. At velocity boundary, I need to define the Volume Fraction of the two phases which is not known in advance for air (bubble).

When I defined vf of liquid as 1.0 at outlet, the simulation works fine until the bubble reached to the outlet boundary, where it couldn't pass through the outlet due to volume fraction of air defined as zero.

I would like to know how to proceed in this case, the vf of air is unknown. Am I using the correct BC for the problem?
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Old   September 4, 2017, 05:48
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Cees Haringa
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why aren't you using a pressure outlet b.c.? That doesn't discriminate between phases and allows both gas and liquid to flow out.

Edit: I see the rationale between your choice of b.c.; however, if you do know the inlet volume fractions, then it may be easier to use velocity inlet and pressure outlet - the net flow is the same at in and outlet due to conservation of mass anyway.
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Old   September 4, 2017, 06:45
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Hi Cees,

At inlet the volume fraction of liquid is always 1.0, the pressure is also always constant regulated through some other means. The mass flow rate at the outlet is known which is controlled by a pump placed in the downstream.

The problem is that air ingested through an opening at the top when the flow rate is higher than a specific value. As you can see I don't know the bubble size or the frequency of the bubble ingestion beforehand in this case.

So, at inlet its always liquid and at outlet its liquid with intermittent bubbles.
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Old   September 4, 2017, 07:04
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But currently there is no opening at the top, it's a symmetry. Should't there be a pressure inlet condition or so at the top to draw air from?
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Old   September 4, 2017, 07:24
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Ah sorry for the vague representation of the image. While initializing the flow field I patched the top box with VF of air as 1 (image attached)
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Old   September 4, 2017, 07:55
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I see, but how is the air being drawn from the top chamber? Is it being replaced by water somehow then? Or being replenished by air from another location - in the latter case, you will need an inlet at the top too, right?
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Old   September 4, 2017, 08:08
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That is my other concern, which I am currently investigating. But yeah you are right the top should be a Pressure inlet to replicate atmosphere.

However, how to handle the bubble at outlet? The mass flow rate should be satisfied at outlet, which can be done by Velocity or Mass flow Boundaries.

The current situation is that velocity boundary can handle only one phase, Mass flow outlet is not available with VOF.
Can it be done by using mass flow inlet at outlet?
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Old   September 4, 2017, 09:43
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You are dealing with a rather unique system in your conditions; I don't suppose you can consider the liquid inflow to be constant, right - it is really affected by what happens at the outlet? From what I gather, it seems you probably need to look into a custom UDF boundary condition, that maintains a constant total flux through the boundary with a variable composition, based on the volume fractions of the neighboring cells.
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Old   September 5, 2017, 02:38
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Yeah inflow depends on the outlet condition. I will look into the UDF manual to deal with this issue.

Thanks for your help.
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