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Velocity at end of porous region

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Old   August 13, 2015, 10:57
Default Velocity at end of porous region
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carsten fuetterer
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Hi,

I am trying to solve the compressible flow through a catalyst which is model as a porous medium. The solver is rhoPorousSimpleFoam and run implicit. The porous section (cell zone ) is in the middle of the extruded mesh.

In the last cells of the porous medium, the velocity suddenly drops, which you can see in the attached images. A similar behavior is also seen in the sample of open foam (compressible, implicit).

Does somebody knows how to avoid this? Which further informations do you need?.

with best regards

Carsten
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File Type: jpg img1.jpg (15.6 KB, 49 views)
File Type: jpg img2.jpg (83.1 KB, 46 views)
File Type: jpg sample.jpg (12.3 KB, 46 views)
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Old   August 13, 2015, 12:23
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maybe the reason can be found also in the drop of the turbulent viscosity?
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Old   August 16, 2015, 02:06
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hi
I think it is because of your mesh. the way of mesh generation and the mesh geometry
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Old   August 17, 2015, 04:53
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ok, its a structured mesh of the inlet part. This mesh is then extruded with the openfoam extruder. So I think the mesh is good. Here is the output of checkMesh:

Code:
Mesh stats
    points:           477594
    faces:            1409320
    internal faces:   1386680
    cells:            466000
    faces per cell:   6
    boundary patches: 4
    point zones:      0
    face zones:       0
    cell zones:       1

Overall number of cells of each type:
    hexahedra:     466000
    prisms:        0
    wedges:        0
    pyramids:      0
    tet wedges:    0
    tetrahedra:    0
    polyhedra:     0

Checking topology...
    Boundary definition OK.
    Cell to face addressing OK.
    Point usage OK.
    Upper triangular ordering OK.
    Face vertices OK.
    Number of regions: 1 (OK).

Checking patch topology for multiply connected surfaces...
    Patch               Faces    Points   Surface topology                  
    wall                6640     6720     ok (non-closed singly connected)  
    outlet              2000     2041     ok (non-closed singly connected)  
    inlet               2000     2041     ok (non-closed singly connected)  
    porosityWall        12000    12080    ok (non-closed singly connected)  

Checking geometry...
    Overall domain bounding box (-0.254958 -0.0725438 -0.122602) (0.0726735 0.126725 0.2)
    Mesh (non-empty, non-wedge) directions (1 1 1)
    Mesh (non-empty) directions (1 1 1)
    Boundary openness (-2.65156e-16 -2.05682e-16 6.89138e-17) OK.
    Max cell openness = 2.16877e-15 OK.
    Max aspect ratio = 32.9462 OK.
    Minimum face area = 2.72631e-08. Maximum face area = 7.19034e-05.  Face area magnitudes OK.
    Min volume = 3.78169e-11. Max volume = 7.76384e-08.  Total volume = 0.00481471.  Cell volumes OK.
    Mesh non-orthogonality Max: 54.5986 average: 8.51888
    Non-orthogonality check OK.
    Face pyramids OK.
    Max skewness = 1.02173 OK.
    Coupled point location match (average 0) OK.

Mesh OK.
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Old   September 3, 2015, 01:09
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Quote:
Originally Posted by carstenf View Post
Hi,

I am trying to solve the compressible flow through a catalyst which is model as a porous medium. The solver is rhoPorousSimpleFoam and run implicit. The porous section (cell zone ) is in the middle of the extruded mesh.

In the last cells of the porous medium, the velocity suddenly drops, which you can see in the attached images. A similar behavior is also seen in the sample of open foam (compressible, implicit).

Does somebody knows how to avoid this? Which further informations do you need?.

with best regards

Carsten
Hi

I wish your problem have been solved after this long period answering!!!

but if not

to better understand your problem, is it the sudden failure of the velocity?
in the pictures where and what is the exact problem you have seen? could you bold and highlight it?

I think at the junction of two blocks, the adjacent boundary can not pass the right condition from a side to the other one. I have not ever used rhoPorousSimpleFoam but from the picture tell the statement.
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Old   September 3, 2015, 07:03
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thanks for your reply. Sadly I did not solved the problem, which why I skipped this project. But I have to solve it. So any help would be really nice. I could also share the project files with you if you like.

I attached an image which marks the problem cells.

Yes I think its somehow a mistake in diskretising between two regions. I did not defined an extra boundary between these regions. So there must be internally a problem.

best regards

Carsten
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Old   September 3, 2015, 13:54
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Quote:
Originally Posted by carstenf View Post
thanks for your reply. Sadly I did not solved the problem, which why I skipped this project. But I have to solve it. So any help would be really nice. I could also share the project files with you if you like.

I attached an image which marks the problem cells.

Yes I think its somehow a mistake in diskretising between two regions. I did not defined an extra boundary between these regions. So there must be internally a problem.

best regards

Carsten
Hey Carsten
I'm afraid that don't have a solution or even misunderstand the problem but do not worry at last you will do it.

2 question
does the velocity drops in the mentioned cell and then go up again?(as seen in picture if it is velocity contour)

can't you neglect or ignore it in results or it affects the results?
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Old   September 3, 2015, 13:58
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yes, the weird thing is, that the velocity goes up again. But I don't know if it effects the solution. I can not neglect this, since it is not just a visualization problem

cheers
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Old   September 3, 2015, 16:42
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Hi,

I am faceing the same artefacts. But from experience and comparisons with other commercial software, I can tell you that this artefact is not influenceing your results. So you can use the results.

Kind regards
Chrisi
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Old   September 4, 2015, 03:00
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Hey
according to picture and now Chrisi's experience I think in the same way, although wish you could fix it
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Old   September 7, 2015, 16:17
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Hi,

please have a look to post #7 on this
I deleted pictures from my dropbox, but summarizing, I made a test case of a rectangular channel (2D) and I set the pressure drop data (experimental) to have alpha & beta.
I had cfd results aligned with experimental.

I performed some other test cases, alpha & beta given by a velocity-pressure drop tendency line drawn by myself and I found that fixed coeffcient model performs better than Darcy-Forcheimer model.
I can't confirm at all, and I can't tell you why I had these results.

Anyway, you could give a try...

Cheers.
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Old   September 7, 2015, 16:48
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Hi, thanks for this advice. I will try this out this week.
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