[Saideep]

Hi guys;

I, as always have a strange question though could be skipped in many cases but I am really curious on what is causing the following problem.

I have a rectangular capillary tube through which i have a capillary flow. {inlet & outlet pressure bc = 0Pa and U bc at inlet and outlet = zeroGradient, c.a. specified as wetting}. Flow is only taking place due to capillary force. Further density and viscosity ratios are =1.

I used the following rectangular model.
See the attached figure regarding the dimensions.
fig1.png

I have a spatial discritization of d/dx = 10. {where d is the height/ length of the channel}.

I am using a 2D model with "empty" bc on the 3rd dimension. The third dimension has always 1 cell and varied the value of width between 1e-6 and 1e-4.

From my understanding,
as this is a 2D case, the 3rd dimension(width of channel) value shall not effect the result in any way. {3rd dimension is just a projection of what is happening on the other 2 dimensions}.

But from the figures you can see when i have a larger 3rd dimension I have a sharper interface
fig2.jpg
and when i scale down to e-6 on the 3rd dimension I have a smeared out interface.
fig3.jpg

Any idea why this is occurring? I cannot find a valid reason why this is happening.

Is there some sort of a limit upto which volume of fluid can capture the interface and below that VOF fails to capture interface accurately? {I was just thinking that could be true if i was solving a 3d case but see this effect in a 2d case also}.

Thanks and happy Easter;
Saideep

[Saideep]

Hi guys;

As nobody answered this thread I took some courage to report this issue as a bug. Henry was quick to reply that "VoF surface tension cant handle high curvatures" when the length scale of the channel is extremely small. But what i see is that the curvature is completely independent along the 3rd axis (when empty b.c assigned).

Additionally I have the following questions.
1. How does "empty b.c." work? Is it just considering a velocity in the empty axis as 0 and trying the match pressure gradient with surface tension force (interFoam solver) else solving the NSE along 2 axis only not considering the empty axis?

2. Getting back to my main question again, Why do results vary upon changing the dimension of the 3rd axis assigned to be an "empty b.c"? I expect the behavior to be same whether length of axis defined by empty b.c is e-3m or e-20m. This isn't the case. Where am I misunderstanding concepts?

Attached is a small group of test cases I ran to understand this concept. Dimensions, length scales and interface behavior reported there.
Just to mention, I tried to see if the effect of "empty" b.c causes any deviation from what we expect for a single phase flow {tested with pimpleFoam} and it seems to work perfectly. Flow velocity matches Poisuielle flow and works for any length scale along the empty b.c. axis.

Saideep

[wyldckat]

Hi Saideep,

I got here from the PM you sent me. I saw your bug report the other day and your insistence on the topic.

I haven't managed to look into your questions yet, but based on Henry's answers, I'm guessing you need to study a bit more on the topic of multiphase with surface tension issues. Here are a few thread I can remember about:

• http://www.cfd-online.com/Forums/ope...interfoam.html
• With some indirect help from Kent Wardle:

  Originally Posted by kwardle

  Here is a related topic with some info.
  http://www.cfd-online.com/Forums/ope...tml#post576093

I'll try to look into your other questions as soon as I can... which might still be a few weekends away


Best regards,
Bruno

[Saideep]

Hi Bruno;

Thanks for the answer. I will go through the directed thread in detail.

As I mentioned in the bug report, if interface curvature was only the reason, I went to the extent of having a contact angle of 80 with a surface tension of 1e-4. I am just not able to blame it on interface curvature.

Meanwhile, I just tested the same case on the extended version(3.1) and it works perfectly. Perfectly in the sense I don't have any smearing of interface at very small length scales.

Will keep updated on progress.

Thanks;
Saideep

[Saideep]

Hi guys;
Wanted to update on issue related to interface smearing.

Will refer to normal OpenFOAM(V-2.3/3.0) as OF and foamExtend(V-3.1) as FE.

Fig1, shows the capillary rise model that I have been experimenting on. I varied the third dimension value where "empty b.c" was specified.
fig1.png
Fig2, shows the comparison between OF and FE.
fig2.png
Fig3, shows the VOF indicator function advection. {make a note of the alpha value in legend}.
fig3.png
Fig4, shows all cases representing the meniscus location.
fig4.png

I didn't dig much into this issue wrt the code but a general take is that the external structure and implementation of the code remains similar in both versions and somehow think that MULES(FE) and CMULES(OF) is causing this difference.

Will update once i come across the solution but any advice, idea is appreciated.

Saideep

[Tobi]

Hi all & dear Bruno,


the interesting fact here is, that with OpenFOAM-2.2.0 you will not get the smearing of the interface. The question is why? I checked the MULES implementation of OF2.2 and 2.3 it is identical. Also the calculation of alpha is similar. I also had a discussion with my colleague. He also said this came from interface motion in small meshes but then the Question is, why is FOAM2.2 handle this problem in a more accurate way.

See here: http://www.cfd-online.com/Forums/ope...-0-higher.html

Bruno could you answer this?
And Saideep can you give me the bug report link?

[Saideep]

hi Tobi and others...

Bug link:
http://bugs.openfoam.org/print_bug_page.php?bug_id=2047

Just to mention, the author of following paper also observed this strange behaviour and I came across a similar thread to mine in mantis few months ago but the answer was the same i guess. (Just not able to find that specific thread).

https://www.researchgate.net/publica...f_Fluid_method

Of-course switching to Extend version seems to be a solution, but knowing the reason for this behaviour is worth finding and addressing for OF community.

Thanks for the support guys

[mehdiataei]

Hi,

Could you find out what is causing the problem?