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-   -   SnappyHexMesh Refinement nCellsBetweenLevels (http://www.cfd-online.com/Forums/openfoam-meshing-snappyhexmesh/92039-snappyhexmesh-refinement-ncellsbetweenlevels.html)

laymelek August 31, 2011 05:29

SnappyHexMesh Refinement nCellsBetweenLevels
 
2 Attachment(s)
Hi all,
I am trying to formulate effect of nCellsBetweenLevels parameter. I have meshed one case with different value.

I was thinking that when nCellsBetweenLevels=10 ( which i thought that is a very high value) there will be 10 cells between 2 levels of refinement in each direction. But as u can see from the screenshots its not like that at all. When i increase the value to 50, i get hardly 6 cells between two levels.

Does anyone can explain me what does the value of nCellsBetweenLevels exactly refers to?

nCellsBetweenLevels =10
Attachment 9028
nCellsBetweenLevels =50
Attachment 9029

wyldckat August 16, 2014 15:21

Greetings to all!

@laymelek: Sorry for the reaaaally late reply. That looks like a bug that existed on that older version of OpenFOAM :(.

I'm answering to this thread so that questions related to this parameter "nCellsBetweenLevels" this can be discussed in a single thread.

At the request of the forum member CFDUser_, I've added a new section "nCellsBetweenLevels" at openfoamwiki.net in the page respective to snappyHexMesh: http://openfoamwiki.net/index.php/Sn...sBetweenLevels

If anyone has questions about this, feel free to ask!

Best regards,
Bruno


PS: In case anyone is wondering: the CFD-Online forums prefer that discussions on a particular topic are kept in a single thread.

CFDUser_ August 17, 2014 01:52

Dear Bruno,

What happened when you set nCellsBetweenLayers to 0? can you upload that picture too?
I have some suggestions if you may consider them as valid point.
1. When you consider a case by default please specify OpenFOAM version so that one can understand which version can reproduce the same results.
2. Since I have observed the definition of some parameters changed with different versions, please give us Dict files so that one can have clear idea about other parameters too.

Thanks a lot for your time and efforts. :)

Best Regards,
CFDUser_

Quote:

Originally Posted by wyldckat (Post 506284)
Greetings to all!

@laymelek: Sorry for the reaaaally late reply. That looks like a bug that existed on that older version of OpenFOAM :(.

I'm answering to this thread so that questions related to this parameter "nCellsBetweenLevels" this can be discussed in a single thread.

At the request of the forum member CFDUser_, I've added a new section "nCellsBetweenLevels" at openfoamwiki.net in the page respective to snappyHexMesh: http://openfoamwiki.net/index.php/Sn...sBetweenLevels

If anyone has questions about this, feel free to ask!

Best regards,
Bruno


PS: In case anyone is wondering: the CFD-Online forums prefer that discussions on a particular topic are kept in a single thread.


wyldckat August 17, 2014 10:55

Dear CFDUser_,

Thanks for the feedback!

Quote:

Originally Posted by CFDUser_ (Post 506310)
What happened when you set nCellsBetweenLayers to 0? can you upload that picture too?

Ooops, I forgot to document the valid values. I've updated the subsection with an entry "Valid values": http://openfoamwiki.net/index.php/Sn...sBetweenLevels

Quote:

Originally Posted by CFDUser_ (Post 506310)
I have some suggestions if you may consider them as valid point.
1. When you consider a case by default please specify OpenFOAM version so that one can understand which version can reproduce the same results.

Good reminder, I forgot to indicate which versions that analysis applies to. I've updated the subsection accordingly: http://openfoamwiki.net/index.php/Sn...sBetweenLevels

Quote:

Originally Posted by CFDUser_ (Post 506310)
2. Since I have observed the definition of some parameters changed with different versions, please give us Dict files so that one can have clear idea about other parameters too.

Strange, I thought that the wiki page already had an indication of where examples could be found. I've added a new section "Examples" on how they can be found: http://openfoamwiki.net/index.php/Sn...xMesh#Examples

Beyond this, feel free to edit the wiki page and improve it!

Best regards,
Bruno

CFDUser_ August 18, 2014 00:14

Dear Bruno,

Good day :). Thanks once again for your explanations.
what is gapLevelIncrement in refine surfaces sub-dict?


Thanks & Regards,
CFDUser_

wyldckat August 18, 2014 15:51

Hi CFDUser_,

If you document in that wiki page one or more parameter(s) with which you are already familiar with, I can try next weekend to figure out and document "gapLevelIncrement".

Best regards,
Bruno

CFDUser_ August 19, 2014 07:01

Dear Bruno,

Coming 2-3 months are like hectic time for me. I will upload some of the valid cases in feature. My analysis are problem specific, so its not good to update in wiki pages. I will find out better way to post as I cant post my current cases as well.

Thanks & Regards,
CFDUser_


Quote:

Originally Posted by wyldckat (Post 506513)
Hi CFDUser_,

If you document in that wiki page one or more parameter(s) with which you are already familiar with, I can try next weekend to figure out and document "gapLevelIncrement".

Best regards,
Bruno


wyldckat August 30, 2014 08:39

Hi CFDUser_,

Quote:

Originally Posted by CFDUser_ (Post 506602)
My analysis are problem specific, so its not good to update in wiki pages. I will find out better way to post as I cant post my current cases as well.

Well, the reason why I used one of the cases from the snappyWiki was because it was a very simple case in which certain features can be tested.

It's very likely that if you extract only the problematic location of your geometry and create a base mesh only for that location, you'll be able to test a lot faster and a lot more possible workarounds for the problems you are getting with the whole geometry on that particular location.
Furthermore, those small test cases will then be so unrelated to the real geometry, that it will probably be almost impossible to deduce what it was from.

Beyond this, I'm quoting some feedback you sent me over PM back on August 26th, because this way it'll be easier for me to keep track of the things I still want to do in the near future, and because this way it's kept a public record of who discovered what:
Quote:

Originally Posted by CFDUser_
If you see in your documentation for nCellsBetweenLevels, you said "it's an error of 1 missing layer at level 1" for level 2 to 3. Actual reason is level0 is not exactly equal to base mesh cell size. Its equal to a factor times base mesh level 0 size. That factor is defined to balance number of faces in two different levels fall adjacent to each other. Since for nCellsBetweenLevels 2 the ratio is very high, for the next level (means nCellsBetweenLevels = 3), it won't choose cells in level0 to refine. Instead of the it will refine mode cells in level1.
This explanation will not improve confidence to choose right value for nCellsBetweenLevels but it will give some in-depth understanding for it.

Update if you are fully convinced with reason.

It's the end of the week for me and my brain isn't very cooperative today :( I'll keep a note to look into this as soon as my brain is rested and able to figure this out ;)

Best regards,
Bruno


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