[cyw]

Hi researchers,

Anyone simulate a bubble in still water without gravity using interFoam? If so, how is the result should be?

Best Regards
CYW

[BlnPhoenix]

Without gravity you will probably get a solution that is heavily corrupted by parasitic currents.

[cyw]

Originally Posted by BlnPhoenix

Without gravity you will probably get a solution that is heavily corrupted by parasitic currents.

Thank you for your reply. I have simulated such a case. But the bubble evolves in a weird direction. I do not know the reason. What is " parasitic currents"? Is that to say, interFoam cannot simulate such a case?

[BlnPhoenix]

Originally Posted by cyw

Thank you for your reply. I have simulated such a case. But the bubble evolves in a weird direction. I do not know the reason. What is " parasitic currents"? Is that to say, interFoam cannot simulate such a case?

Parasitic currents are a numerical problem that occur at the bubble interface where the pressure difference is high. As far as i know there is no general solution to this problem yet in VOF type solvers, even commercial codes like FLUENT. You can search this forum for more. What you see in your simulation is probably weird behaviour because of this. When you activate gravity these parastic currents get overlapped by the rising velocity of the bubble. Don't ask me how this works numerically but you can get a meaningful and accurate description of the rising behaviour (velocity, shape) nevertheless.

[cyw]

Originally Posted by BlnPhoenix

Parasitic currents are a numerical problem that occur at the bubble interface where the pressure difference is high. As far as i know there is no general solution to this problem yet in VOF type solvers, even commercial codes like FLUENT. You can search this forum for more. What you see in your simulation is probably weird behaviour because of this. When you activate gravity these parastic currents get overlapped by the rising velocity of the bubble. Don't ask me how this works numerically but you can get a meaningful and accurate description of the rising behaviour (velocity, shape) nevertheless.

Thank you for your reply. I read a paper about this peoblem. https://journal.austms.org.au/V46/CT.../Har1/Har1.pdf


Also this thread helps me a lot. parasitic currents

Thank you for your kind reply

[BlnPhoenix]

Originally Posted by cyw

Thank you for your reply. I read a paper about this peoblem. https://journal.austms.org.au/V46/CT.../Har1/Har1.pdf


Also this thread helps me a lot. parasitic currents

Thank you for your kind reply

No problem.

[cyw]

Originally Posted by BlnPhoenix

No problem.

Hello, again

When I simulate this case with interFoam, do I need to set the pressure difference between the bubble and water (2*sigma/radius) at t=0?

[BlnPhoenix]

Originally Posted by cyw

Hello, again

When I simulate this case with interFoam, do I need to set the pressure difference between the bubble and water (2*sigma/radius) at t=0?

Hm, can you do this in OpenFoam? How?

I've never done it, since my assumption was that the pressure field is calculated with the first time step.

It would be interesting to know how we can set a pressure Field in OpenFoam, i guess with setFieldsDict...

[cyw]

Originally Posted by BlnPhoenix

Hm, can you do this in OpenFoam? How?

I've never done it, since my assumption was that the pressure field is calculated with the first time step.

It would be interesting to know how we can set a pressure Field in OpenFoam, i guess with setFieldsDict...

That is true. I set different pressure in air and water phase with setFieldsDict. But the result did not get better...

[BlnPhoenix]

Originally Posted by cyw

That is true. I set different pressure in air and water phase with setFieldsDict. But the result did not get better...

Can you post the setFieldsDict for pressure? I'm curious to see.

[cyw]

Originally Posted by BlnPhoenix

Can you post the setFieldsDict for pressure? I'm curious to see.

Here is it.

/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.3.0 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
location "system";
object setFieldsDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

defaultFieldValues
(
volScalarFieldValue alpha.water 1
volScalarFieldValue p_rgh 1e5
);

regions
(
sphereToCell
{
centre (0 0 0.3);
radius 0.15;

fieldValues
(
volScalarFieldValue alpha.water 0
volScalarFieldValue p_rgh 2e5
);
}
);

// ************************************************** *********************** //

[BlnPhoenix]

Originally Posted by cyw

Here is it.

/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \\ / O peration | Version: 2.3.0 |
| \\ / A nd | Web: www.OpenFOAM.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
location "system";
object setFieldsDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

defaultFieldValues
(
volScalarFieldValue alpha.water 1
volScalarFieldValue p_rgh 1e5
);

regions
(
sphereToCell
{
centre (0 0 0.3);
radius 0.15;

fieldValues
(
volScalarFieldValue alpha.water 0
volScalarFieldValue p_rgh 2e5
);
}
);

// ************************************************** *********************** //

Yep, i guess that's it. Thanks.

Shame it does not help with the parasitic currents... May i ask what you need the stagnant bubble for? If you have a bubble that is perfectly shaped (depending on diameter and surface tension) you can simulate under gravity a few time steps and the bubble should not deform, than this time step should not contain parasitic currents anymore (it does contain rising velocity filed though..)

[cyw]

Originally Posted by BlnPhoenix

Yep, i guess that's it. Thanks.

Shame it does not help with the parasitic currents... May i ask what you need the stagnant bubble for? If you have a bubble that is perfectly shaped (depending on diameter and surface tension) you can simulate under gravity a few time steps and the bubble should not deform, than this time step should not contain parasitic currents anymore (it does contain rising velocity filed though..)

Thank you for your help. The parasitic currents really worked for me. I changed a few things in interFoam and the result is better.
This is a preliminary research for my PhD work. My final goal is to simulate bubble condensation. Later compressibleInterFoam will be used. Do you know how to set alphaSgs file and what is it?

[BlnPhoenix]

Originally Posted by cyw

Thank you for your help. The parasitic currents really worked for me. I changed a few things in interFoam and the result is better.
This is a preliminary research for my PhD work. My final goal is to simulate bubble condensation. Later compressibleInterFoam will be used. Do you know how to set alphaSgs file and what is it?

This seems to be related to a LES turbulence model... Not sure though..