FDS Alignement Problems for Meshes of Different Sizes

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FDS Alignement Problems for Meshes of Different Sizes

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Topic: FDS Alignement Problems for Meshes of Different Sizes (Read 118 times)

gabferrara

Position: New Member

Posts: 7

« on: June 21, 2010, 07:10:44 AM »

Hello.
I''m trying to exploit the Pyrosim capability of creating abutting meshes of different size.
I created 5 meshes with the schematic of the attached pdf file.
Each mesh perfectly adheres each other and the cells of the finer mesh are integer multiple of the cells of the coarser meshes.
Pyrosim is telling me that everything is fine with the mesh alignment test (all are "passed").
But when I run FDS, it tells me that Mesh-1 cannot align with Mesh-3 etc... and the simulation cannot start.
Would you please give me some help.

------------------------------- Mesh-Test.pdf (21.12 KB - downloaded 17 times.)

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Charlie Thornton

Group: Thunderhead

Posts: 207

« Reply #1 on: June 21, 2010, 07:41:38 AM »

We''ve tried to reproduce the same kind of alignment tests in PyroSim that are run by FDS, but it''s possible that we''ve either missed a case or there''s a difference in tolerance.

The question is if your mesh #3 has the alignment problem shown in the attached image (from figure 6.2 of the FDS manual) when compared with any of the other meshes. Could you post your &MESH FDS input records so I could see the numbers?

I really want to add an option that lets you enter a bounding box and do an automated mesh decomposition and refinement (split a single mesh into 9). I think that would make cases like this a lot more straightforward.

------------------------------- mesh.png (4.15 KB, 495x135 - viewed 9 times.)

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gabferrara

Position: New Member

Posts: 7

« Reply #2 on: June 21, 2010, 07:54:23 AM »

First of all many thanks for yor ready reply.
Please find the psm file attached.

Gabriele

------------------------------- mesh-test.psm (29.62 KB - downloaded 14 times.)

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Charlie Thornton

Group: Thunderhead

Posts: 207

« Reply #3 on: June 21, 2010, 10:54:49 AM »

There appears to be a limit to how big of a cell size change FDS will tolerate. If the central column is finer by a factor of up to 19 it runs, at 20 it fails the alignment test.

Here is the FDS code that checks:

Code:

MM=>MESHES(NOM)
ALIGNED = .TRUE.
IF ( (ABS(IOR)==2 .OR. ABS(IOR)==3) .AND. MM%DX(IIO_MIN)<=M%DX(I) .AND. &
ABS( ((IIO_MAX-IIO_MIN+1)*MM%DX(IIO_MIN)-M%DX(I)) / MM%DX(IIO_MIN))>0.05 ) ALIGNED = .FALSE.
IF ( (ABS(IOR)==1 .OR. ABS(IOR)==3) .AND. MM%DY(JJO_MIN)<=M%DY(J) .AND. &
ABS( ((JJO_MAX-JJO_MIN+1)*MM%DY(JJO_MIN)-M%DY(J)) / MM%DY(JJO_MIN))>0.05 ) ALIGNED = .FALSE.
IF ( (ABS(IOR)==1 .OR. ABS(IOR)==2) .AND. MM%DZ(KKO_MIN)<=M%DZ(K) .AND. &
ABS( ((KKO_MAX-KKO_MIN+1)*MM%DZ(KKO_MIN)-M%DZ(K)) / MM%DZ(KKO_MIN))>0.05 ) ALIGNED = .FALSE.
IF (.NOT.ALIGNED) THEN
WRITE(LU_ERR,''(A,I3,A,I3)'') ''ERROR: MESH '',NM,'' is out of alignment with MESH '',NOM
PROCESS_STOP_STATUS = SETUP_STOP
IERR = 1
RETURN
ENDIF

I didn''t bother to track down all those variables, but that tolerance of 0.05 matches pretty well with the 1/20th I mentioned above. It looks like you can''t have a cell size difference of more than a factor of 19 in any direction.

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gabferrara

Position: New Member

Posts: 7

« Reply #4 on: June 22, 2010, 01:40:00 AM »

Yes you''re right: it works with a smaller cell size ratio.
This is very powerful for my applications to pool fires in the open.
I need fine grids where the fire is burning and coarser where the gradients have smeared out.
Thank you again.

Gabriele

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