CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > OpenFOAM

limited precision of checkMesh?

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

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   May 17, 2010, 15:42
Default limited precision of checkMesh?
  #1
New Member
 
Robert Langner
Join Date: Dec 2009
Location: Freiburg, Germany
Posts: 27
Rep Power: 16
Robat is on a distinguished road
Hi foamers!

I'm working with "natural"(smooth) shaped junctions in micro-scale (at least I try to). My problem already occurs by scaling down my mesh from meter to micrometer (with OFs "transformPoints-tool").
I ran checkMesh before scaling and afterwards: The results on meter-scale were OK. The shape caused a moderate nonOrthogonality to my pure hex-mesh.
But at a certain scaling point (where the cell size drops below 1µm) checkMesh detected some "wrong oriented faces"

But an analysis of this so-called-damaged mesh in postPro(VisIt) showed nothing of such problems, just a medium quality mesh.

So should I expect an impact on my calculation or are there some limits in checkMesh's pecision?

Regards,
Robert
Robat is offline   Reply With Quote

Old   May 18, 2010, 05:46
Default
  #2
New Member
 
Robert Langner
Join Date: Dec 2009
Location: Freiburg, Germany
Posts: 27
Rep Power: 16
Robat is on a distinguished road
I've made another attempt: this time with just a simple box and equidistant hex-cells. I produced the mesh size on three different ways:
- scaled by "transformPoints"-utility
- scaled by my own written scaling script
- and build the mesh immediately in the right µm size (in Salome)

Everytime the same result: checkMesh detecting weird things as soon as the cell size drops below 1µm.
Have you ever seen ideal hexahedrons with a skewness of 160 ?

I'm completely confused and can't even see a reason for this error.
Please could anyone give me an advise.


PS: I'm working with OpenFOAM 1.5

Regards,
Robert
Robat is offline   Reply With Quote

Old   May 18, 2010, 06:20
Default
  #3
Member
 
Cedric Van Holsbeke
Join Date: Dec 2009
Location: Belgium
Posts: 81
Rep Power: 16
CedricVH is on a distinguished road
Your findings are very helpfull for everybody simulating small biological structures! This should be in a bug report.

Is it only a checkMesh issue? Or do the solvers also have problems with these scaled cases (crashing, giving unphysical results, ...)?
CedricVH is offline   Reply With Quote

Old   May 18, 2010, 06:33
Default
  #4
ngj
Senior Member
 
Niels Gjoel Jacobsen
Join Date: Mar 2009
Location: Copenhagen, Denmark
Posts: 1,900
Rep Power: 37
ngj will become famous soon enoughngj will become famous soon enough
Hi Robert

I have seen similar issues, however they originated from the value of writePrecision in controlDict. Try, whether or not checkMesh behaves well if you use e.g. writePrecision 14 during the transformation process.
writePrecision controls the precision of all written data, i.e. also your transformed mesh.

Hope it helps,

Niels
ngj is offline   Reply With Quote

Old   May 18, 2010, 07:22
Default
  #5
New Member
 
Robert Langner
Join Date: Dec 2009
Location: Freiburg, Germany
Posts: 27
Rep Power: 16
Robat is on a distinguished road
Thank you Niels,

this advise is quiet helpful, but in this caes it not solved the problem.
I set write precision to "17" and there was an influence but the skewness of my equdistant test cube still occurs in checkMesh (at 10µm cell size).
There must be solution: I saw cases in this forum with even smaller cells like in this thead: "interfoam behavior in micro-dimensions".

Thanks for your help
Regards, Robert
Attached Files
File Type: zip SimpleCube10x10x10xx10.unv.zip (31.4 KB, 2 views)
Robat is offline   Reply With Quote

Old   May 18, 2010, 07:28
Default
  #6
Senior Member
 
Hrvoje Jasak
Join Date: Mar 2009
Location: London, England
Posts: 1,904
Rep Power: 33
hjasak will become famous soon enough
Don't forget - in double precision you have only 14 significant digits; the rest will be garbage.

Hrv
__________________
Hrvoje Jasak
Providing commercial FOAM/OpenFOAM and CFD Consulting: http://wikki.co.uk
hjasak is offline   Reply With Quote

Old   May 18, 2010, 07:48
Default
  #7
New Member
 
Robert Langner
Join Date: Dec 2009
Location: Freiburg, Germany
Posts: 27
Rep Power: 16
Robat is on a distinguished road
May be the source of my problem comes from preProcessing.
I used Salome to build the mesh. Salome gots a precision of 1e-7. The purpose of my scaling actions is to increase this accuracy with the scaling factor.
Precision(1e-7) * scaling(1e-6) should be accurate enough or do I misinterpreting something?
Robat is offline   Reply With Quote

Reply

Tags
checkmesh

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Switch from single precision to double precision kk81 OpenFOAM Bugs 0 March 11, 2010 07:14
Number of elements x double precision matheusguzella ANSYS Meshing & Geometry 2 October 7, 2009 11:15
CFX Pre/Double Precision Scott Nordsen CFX 4 July 22, 2009 19:45
what's wrong about my code for 2d burgers equation morxio Main CFD Forum 3 April 27, 2007 11:38
double precision assigning in FRTRAN R.A.Khurram Main CFD Forum 12 October 2, 2001 12:00


All times are GMT -4. The time now is 05:02.