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variableHeightFlowRateInletVelocity bubbles at the inlet

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Old   October 7, 2019, 11:16
Default variableHeightFlowRateInletVelocity bubbles at the inlet
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Grivalszki Péter
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Hi,

I have a multiphase interIsoFoam simulation with variableHeightFlowRateInletVelocity inlet boundary condition for U.

When I watched the results I've noticed inflow bubbles at the inlet. It looks quite random.

Why are these bubbles? Are they normal? Can I do something against them? I'm afraid that these bubbles may affect my final results.

Here is a photo of the problem (but a video would be more effective I know...)

Thank you in advance!

Griva
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File Type: jpg jelenseg1.jpg (92.0 KB, 30 views)
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Old   October 8, 2019, 04:45
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Sita Drost
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Hi Griva,

I happened to come across exactly this in the Applied OpenFOAM course (CFD-Direct) I recently attended. The bubbles are a result of air being trapped at the inlet when the water level is rising at the start of the simulation. The good news is that this can be fixed, the sad news is that I can't remember exactly how... I think it was done by setting the initial volume fraction field not only in a boxToCell region, but also in a boxToFace region, to set the volume fraction on the inlet faces:

Code:
/*--------------------------------*- C++ -*----------------------------------*\
  =========                 |
  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
   \\    /   O peration     | Website:  https://openfoam.org
    \\  /    A nd           | Version:  7
     \\/     M anipulation  |
\*---------------------------------------------------------------------------*/
FoamFile
{
    version     2.0;
    format      ascii;
    class       dictionary;
    object      setFieldsDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

defaultFieldValues
(
    volScalarFieldValue alpha.water 0
);

regions
(
    boxToCell
    {
        box     (-5 -1 -1) (0.5 1 1.3);
        fieldValues
        (
            volScalarFieldValue alpha.water 1
        );
    }
    
    boxToFace
    {
        box     (-5 -1 -1) (0.5 1 1.3);
        fieldValues
        (
            volScalarFieldValue alpha.water 1
        );
    }    
);
Hope this helps,
Sita
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Old   October 8, 2019, 12:54
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Unfortunately it didn't work... Can you please ask for the solution among your coursemates?

Thank you!

Griva
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Old   October 8, 2019, 15:30
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Hi Griva,

Ah, that's too bad, I'm fairly sure that this was the solution that was used during the course. If you want I can send you the case we did in a PM, so you can compare and try things for yourself.

Sita

P.S. I'm not sure I can see those bubbles in the images you attached. Wouldn't a phase fraction plot be more suitable here?
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Old   October 9, 2019, 04:11
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I would be really grateful, thank you!
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Old   October 9, 2019, 08:02
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Hi Griva,

The private messaging tool is throwing errors, so I put the case on Dropbox instead:

https://www.dropbox.com/s/j39gpxb4tx...rflow.zip?dl=0

I'm not sure whether the problem we prevented in this case is the same one you're encountering now, but hopefully it's useful to you anyway.

Good luck,
Sita

P.S. Please let me know when you've downloaded the case, so I can remove it from my Dropbox
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Old   October 10, 2019, 07:55
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Thank you so much, I've downloaded it!

Griva
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Old   November 7, 2019, 11:14
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It didn't work
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Old   November 8, 2019, 06:30
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Ah, that's too bad, sorry to hear that! I'm afraid I can't help you any further in that case.
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