CFD Online Discussion Forums - reduce 3D to 2D mesh

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reduce 3D to 2D mesh

Hi everybody,

I have a rather strange problem, namely, I have a 3D geometry but I want to perform a 2D simulation based on it. It would be a start if I could use the setSet or a similar utility to extract only my boundary faces (at one side) and use that as my 2D mesh, but I really don't know how to start - has anyone done this before? Or is there another way to get a 2D geometry?

Thanks a lot,
Natalie

how do you create your mesh before. in snappyhexmesh you can for example also create a 2d mesh when the front and back faces are empty and nly 1 cell in the depth.

Hi!

I export my mesh from Altair Hypermesh. It provides the possibility to export geometry in an OF format, but, as far as I know, for this option I need a tetrahedral volume mesh.

SnappyHexMesh exports also a 2D geometry to every 3D geometry it creates? Maybe I should write to the Hypermesh support and ask whether this option is also available in Hypermesh.

Kind regards,
Natalie

if you want to run a 2d simulation and want you use snappyhexmesh for meshing. you only have to be sure that your patches are defined the right way. so you can run snappy hexmesh and get no refinement in the third dimensiobns. what does your mesh in your third dimension looks like? if you only have 1 element you should run a 2d simulation by setting the front and back face to "empty"

Hi Natalie,

Potentially, you could use the utility "extrudeMesh", which are located in

Code:

<OpenFOAM>/applications/utilities/mesh/generation/extrudeMesh

and the description reads:

Code:

    Extrude mesh from existing patch (by default outwards facing normals;

    optional flips faces) or from patch read from file.



    Note: Merges close points so be careful.



    Type of extrusion prescribed by run-time selectable model.

Best regards

Niels

Actually, I have only 1 element in the third direction and I've tried just setting the front and back face to empty.. seems I got it wrong somehow, I will have a look at the tutorials.

@A Wendy:
Can I import my 3D geometry into snappyHexMesh and get somehow a 2D mesh by extracting patches? Or what do you mean?

Thanks a lot,
Natalie

Quote:

Originally Posted by Natalie2210 (Post 422515)

hi sorry i mixed something. if you want to create a 2d mesh with snappyhex mesh you have a 3d mexh first and can use extrudeMesh to create a mesh with 1 element in the depth. but if you do not set the front and back in the blockMeshdict snappyhexmesh will not create a clean front face.
you can have a look a at this example I created
http://ubuntuone.com/722FQ4i7HA8A6KVDsVbYs4

best wishes

Andy

Hi!

I'm afraid my browser does not find the homepage you posted a link to .. could you copy the example somehow into this forum?

Thanks,
Natalie

Quote:

Originally Posted by Natalie2210 (Post 424556)

Hi!

I'm afraid my browser does not find the homepage you posted a link to .. could you copy the example somehow into this forum?

Thanks,
Natalie

hi,

i cleaned up my webspace. here is the new link

http://ubuntuone.com/3kPBj7ob4yfD8bcHMzZyFK

Andy

Quote:

Originally Posted by A.Wendy (Post 424569)

hi,

i cleaned up my webspace. here is the new link

http://ubuntuone.com/3kPBj7ob4yfD8bcHMzZyFK

Andy

Hi,
my browser does not find the link you posted .. could you repost it please?

aglios

Mesh quality issues after using extrudeMesh

Hi everyone,

I am having some difficulties with the extrudeMesh utility and was wondering if anyone could give me some assistance.

I have generated a 3D mesh around an STL file with snappyHexMesh. After meshing completes, I run checkMesh and the utility reports "Mesh OK". I then run extrudeMesh and createPatch to generate a 2D mesh from one of the 3D mesh walls. When I run checkMesh on this result, I get several skewFace errors.

I gather from reading other forum posts that extrudeMesh might combine nearby points, thereby creating very thin cells (particularly near the STL surface). I have tried using the mergeTol = 0 setting in the extrudeMeshDict, to no avail. I have also tried setting the writeFormat in controlDict to "binary" as recommended in other forum posts, but this has not helped my specific case either.

I have attached the relevant dict files here, including the checkMesh logs for the 3D and 2D cases. Any help resolving this issue would be greatly appreciated.

Thanks,

Ben

Solved: Mesh quality issues after using extrudeMesh

Hi everyone,

I thought I would post an update to my last post about extrudeMesh creating mesh quality issues. I have been playing around with some different blockMesh and snappyHexMesh settings, and seem to have stumbled on something that works.

I had set up my blockMeshDict to have 0.25 m^3 cells initially. The mesh was 68 m x 0.25 m x 4 m, e.g.,

Code:

scale 1;



vertices

(

    ( 19 23 -3 ) // 0

    ( 87 23 -3 ) // 1

    ( 87 23.25 -3 ) // 2

    ( 19 23.25 -3 ) // 3

    ( 19 23 1 ) // 4

    ( 87 23 1 ) // 5

    ( 87 23.25 1 ) // 6

    ( 19 23.25 1 ) // 7



);



blocks

(

    hex (0 1 2 3 4 5 6 7) (272 1 16) simpleGrading (1 1 1)

);

When I ran snappyHexMesh to add my STL surface and refine the grid to Level 3, checkMesh reported no errors. But, as I said before, running this 3D grid through extrudeMesh created some skewfaces that were causing my multiphase simulation to crash.

The solution for me was to simply increase the number of cells in x-coordinate of the blockMeshDict by a small amount, e.g.,

Code:

hex (0 1 2 3 4 5 6 7) (280 1 16) simpleGrading (1 1 1)

and now both snappy and extrude report no checkMesh errors. I believe this behavior arises from extrudeMesh, as it does some kind of spatial averaging of nearby cells near boundaries like my STL surface. Small cells get distorted, causing skewface or orthogonality issues.

Hopefully this will help anyone who is having similar issues. I did not find that increasing the writePrecision or changing the writeFormat from ascii to binary, helped at all in my case.