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How to do a proper SnappyHexMesh?

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Old   March 27, 2024, 21:26
Default How to do a proper SnappyHexMesh?
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Alberto Campuzano
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Dear all.


I am quite new using OpenFoam and of course snappyHexMesh. i have a geometry which is not quite simple, is a stack of material they holes arranged in zigzag from plate to plate (material to material), the liquid must pass through those holes, hence i made my geometry in Salome having inlet.stl, outlet.stl, and plate.stl, this last one is composed by the plates and the holes, hence doing my blockMesh everything is fine but after applying the SnappyHexMesh only one plate is meshed and the rest it thrown away. What did i do wrong?


In attache file you will find the snappyHexMesh file and also the geometry.


Finally i am planning to use incompressibleVOFluid solver
Attached Files
File Type: zip geometry_snappy.zip (121.1 KB, 2 views)
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Old   March 28, 2024, 03:28
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Yann
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Hello,

We will need to have more information to be able to help you.
Check here How to give enough info to get help

Can you show screenshots of the mesh you get?
What is the cell size of your initial mesh (blockMesh) ?

My first thought is that your mesh is too coarse compared to the size of the holes in your plate, so snappy doesn't "see" the holes and keeps only the plate corresponding to your locationInMesh point.
Of course this is a total blind guess as I didn't see your mesh and I have no idea how you defined your blockMesh.

Regards,
Yann
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Old   March 29, 2024, 16:36
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Alberto Campuzano
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Quote:
Originally Posted by Yann View Post
Hello,

We will need to have more information to be able to help you.
Check here How to give enough info to get help

Can you show screenshots of the mesh you get?
What is the cell size of your initial mesh (blockMesh) ?

My first thought is that your mesh is too coarse compared to the size of the holes in your plate, so snappy doesn't "see" the holes and keeps only the plate corresponding to your locationInMesh point.
Of course this is a total blind guess as I didn't see your mesh and I have no idea how you defined your blockMesh.

Regards,
Yann

First of all sorry for the late reply, i saw your answer and i decided to take a deeper look at my blockMeshDict. Well in attached file you will see the screen shot of the blockMeshDict, look, (the cells are not highly visible but you will more or less see s simplegrading), such a block belongs just to one plate, exactly at
Code:
locationInMesh (0.0 0.0 -0.00045);
and the rest of the cells were removed, hence i refined everything in blockMeshDict but i notice that 20000000 cells where created, something i didn't expect. Hence, what i did was a
Code:
simpleGrading
and of course the cells were reduced but not too much, also the holes where "considered" but not all of them, this was your idea and it worked. Well i know that i refining the mesh will help, but that will increase the amount of cells considerably. This brings up, three questions.


What can i do in order to get a proper mesh but not having too many cells?.


the inlet and outlet are not considered after performing the snappyHexMesh, just the internal Mesh and of course the plate, do i have to increase a bit the size of the initial mesh?



If a simplification of the geometry is needed, hence ill remove the upper and bottom plates but ill have to consider the upper holes "cylinders" as inlets, (many inlets), and the bottom holes as outlets (many outlets). hence, how to do that? because ill use the solver
Code:
incompressibleVoF
and those inlets will have continuous flow.


Finally you will see the screenshot and also the
Code:
blockMeshdict
Attached Files
File Type: zip info.zip (165.6 KB, 2 views)

Last edited by baezacaljo; March 29, 2024 at 16:43. Reason: Forgot more info
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Old   April 2, 2024, 05:04
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Yann
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Hello,

It is difficult to see on your screenshot, but snappy often struggles to properly snap to geometry when there is "high" aspect ratio on the cells in the background mesh. It behaves way better with an aspect ratio of 1.
Long and thing geometries like your are always tricky to mesh with snappy due to the way snappy works.

You mentioned blockMesh, but you didn't post your snappyHexMeshDict file. How did you define the surface refinement levels for the holes, inlet and outlet?

Few recommendations :
  • Keep an aspect ratio of 1 for the background mesh in the area of the holes (but this comes with constraints on the overall mesh size)
  • In snappyHexMeshDict, specify surface refinement levels fine enough to properly capture the holes

Regards,
Yann
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Old   April 12, 2024, 15:05
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Alberto Campuzano
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Quote:
Originally Posted by Yann View Post
Hello,

It is difficult to see on your screenshot, but snappy often struggles to properly snap to geometry when there is "high" aspect ratio on the cells in the background mesh. It behaves way better with an aspect ratio of 1.
Long and thing geometries like your are always tricky to mesh with snappy due to the way snappy works.

You mentioned blockMesh, but you didn't post your snappyHexMeshDict file. How did you define the surface refinement levels for the holes, inlet and outlet?

Few recommendations :
  • Keep an aspect ratio of 1 for the background mesh in the area of the holes (but this comes with constraints on the overall mesh size)
  • In snappyHexMeshDict, specify surface refinement levels fine enough to properly capture the holes

Regards,
Yann

Dear Yann


First of all sorry for the late reply, i have been testing several things on my mesh and geometry



Thank you very much for your help, now everything is getting easier and better, however, after several test as i mentioned before we decide to change to 2D. of course the mesh is easier and faster i even i already did a simulation and everything was as i expected.


After all that, i have a question, look as i already mentioned the geometry will be in 2D hence, i took a "tiny slice" of my geometry because OpenFoam needs 3D, then after performing the
Code:
blockMesh
i got several cell in X and Y direction and just 1 cell in the other direction, which is right because at the end i only want 2D. Then after doing
Code:
snappyHexMesh
i got 2 cells, i dont want that, because my cells increases by double. Hence how to keep 1 cell?. In attached file you will find the blockMeshDict, snappyHexMeshDict, and the geometry


thanks in advance
Attached Files
File Type: zip question.zip (6.2 KB, 2 views)
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Old   April 13, 2024, 04:54
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Yann
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Hello Alberto,

You are right, you should keep only one cell in the 3rd direction (and use the empty type on the faces normal to the 3rd direction)

Snappy cannot do that due to the way it refines the cells (octree structure).
However, you can use the extrudeMesh utility to create a one cell thickness mesh from the mesh generated with snappy.

You can type this to get some information and examples:

Code:
foamInfo extrudeMesh
Cheers,
Yann
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