CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > OpenFOAM Running, Solving & CFD

empty boundary condition behavior

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree3Likes
  • 1 Post By Saideep
  • 1 Post By Saideep
  • 1 Post By Saideep

Reply
 
LinkBack Thread Tools Display Modes
Old   March 24, 2016, 14:28
Default empty boundary condition behavior
  #1
Senior Member
 
Saideep
Join Date: Apr 2015
Location: INDIA
Posts: 174
Rep Power: 4
Saideep is on a distinguished road
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 is offline   Reply With Quote

Old   April 13, 2016, 08:13
Default
  #2
Senior Member
 
Saideep
Join Date: Apr 2015
Location: INDIA
Posts: 174
Rep Power: 4
Saideep is on a distinguished road
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
Attached Files
File Type: docx interface report..docx (87.7 KB, 11 views)

Last edited by Saideep; April 13, 2016 at 10:05. Reason: Additional information.
Saideep is offline   Reply With Quote

Old   April 15, 2016, 18:07
Default
  #3
Super Moderator
 
Bruno Santos
Join Date: Mar 2009
Location: Lisbon, Portugal
Posts: 10,020
Blog Entries: 39
Rep Power: 109
wyldckat is a glorious beacon of lightwyldckat is a glorious beacon of lightwyldckat is a glorious beacon of lightwyldckat is a glorious beacon of lightwyldckat is a glorious beacon of lightwyldckat is a glorious beacon of light
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:


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
__________________
wyldckat is offline   Reply With Quote

Old   April 16, 2016, 09:58
Default
  #4
Senior Member
 
Saideep
Join Date: Apr 2015
Location: INDIA
Posts: 174
Rep Power: 4
Saideep is on a distinguished road
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
wyldckat likes this.
Saideep is offline   Reply With Quote

Old   May 31, 2016, 08:38
Default Update related to the difference between normal OpenFOAM and FoamExtend versions.
  #5
Senior Member
 
Saideep
Join Date: Apr 2015
Location: INDIA
Posts: 174
Rep Power: 4
Saideep is on a distinguished road
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 likes this.
Saideep is offline   Reply With Quote

Old   July 1, 2016, 04:09
Default
  #6
Super Moderator
 
Tobi's Avatar
 
Tobias Holzmann
Join Date: Oct 2010
Location: Leoben (Austria)
Posts: 1,872
Blog Entries: 6
Rep Power: 32
Tobi has a spectacular aura aboutTobi has a spectacular aura about
Send a message via ICQ to Tobi Send a message via Skype™ to 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?
__________________
Keep foaming,
Tobias Holzmann
Tobi is offline   Reply With Quote

Old   July 1, 2016, 04:29
Default
  #7
Senior Member
 
Saideep
Join Date: Apr 2015
Location: INDIA
Posts: 174
Rep Power: 4
Saideep is on a distinguished road
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
rasool_soofi likes this.
Saideep is offline   Reply With Quote

Old   July 16, 2017, 22:09
Default
  #8
New Member
 
Mehdi
Join Date: Jan 2017
Posts: 17
Rep Power: 2
mehdiataei is on a distinguished road
Hi,

Could you find out what is causing the problem?
mehdiataei is offline   Reply With Quote

Reply

Tags
empty boundaries

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Wrong flow in ratating domain problem Sanyo CFX 17 August 15, 2015 06:20
Question about heat transfer coefficient setting for CFX Anna Tian CFX 1 June 16, 2013 06:28
Low Mixing time Problem Mavier CFX 5 April 29, 2013 00:00
Velocity profile boundary condition Tuca FLOW-3D 1 April 23, 2013 12:02
Opening Boundary Condition andreachan Main CFD Forum 11 March 19, 2013 17:46


All times are GMT -4. The time now is 02:26.