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ship meshing in hex elements

  • Tomislav Maric
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14 years 11 months ago #3326 by Tomislav Maric
ship meshing in hex elements was created by Tomislav Maric
Hello!

I have gone through all the tutorials, but I've learned most of the stuff by playing around int geom. or mesh. modules and by reading the TUI and GUI docs - it's really strait forward. Thank you developers for such a powerful, intuitive and free software.

I've come to the point where I want to create a mesh of a ship within a bounding box. I have imported the .iges of a ship, exploded it to edges and faces (need them for sub-meshing later) and composed a solid.

Around the ship I have built a bounding box and have made a solid out of it also.

I have created my final volume by doing a boolean operation on both volumes with the ship as the tool (carrying over all the face/edge information from the geometry).

I have meshed the surface (2D) of the ship's hull in quad elements using submeshing (I've refined the mesh in the area I expect the water level to be), and now I want to make a 3D mesh and run the simulation.

I have selected hexahedral (i,j,k) algorithm for 3D, wireframe meshing for 2D with number of nodes defined. When I calculate the mesh, I get an error that tells me the ship is made out of 3 faces and that's a bad thing for the hex mesh.

Now, how can I mesh everything in hex elements, then use the ship's surface mesh as a submesh? There's no way I can create aditional faces on the ship that make sense. Why can I mesh a three sided volume with spatial faces using hex elements, and not the volume itself in 3D hex elements?

Thank you in advance,

:huh:

Tomislav

P.S.
I promise to write a tutorial about everything I've learn so far in exchange for some help. B)
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14 years 11 months ago #3333 by johannes ackva
Replied by johannes ackva on topic Re:ship meshing in hex elements
Hi Tomislav
solids must have a certain, very limited topology to be HEX-meshable. I attach a file with 3 examples. With complicated solid-geometry it is much easier to make TET-meshes. If You really want to make HEX-meshes, You must subdivide a complicated geometry in such a way that all subsolids are hex-meshable. This can be extremly labourous!

The topic was also treated extensively here:

caelinux.com/CMS/index.php?option=com_jo...&catid=4#msg2752

Good luck
mit freundlichen Grüßen,

Johannes Ackva

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Ingenieurbüro für Mechanik 708152
Dr.-Ing. Johannes Ackva
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www.code-aster.de [file]

Post edited by: johannes ackva, at: 2009/08/10 17:34<br /><br />Post edited by: johannes ackva, at: 2009/08/10 17:35
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14 years 11 months ago #3334 by johannes ackva
Replied by johannes ackva on topic Re:ship meshing in hex elements

Attachment Hex_meshable_Solids-f6af109d7b39a38a04f0a6d80d6d5d35.pdf not found

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14 years 11 months ago #3335 by johannes ackva
Replied by johannes ackva on topic Re:ship meshing in hex elements
If I remember well, Salome's HEX-meshing possiblities are still more restricted than in the pdf-file which I posted. I think, for Salome, the number of lateral faces must be 4. J.
  • Tomislav Maric
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14 years 11 months ago #3336 by Tomislav Maric
Replied by Tomislav Maric on topic Re:ship meshing in hex elements
Vielen dank Johannes für die Antworten! :cheer:

I have been reading the thread about hex meshing in three dimensions and I am worried. If meshing a wind blade was so complicated, I can't imagine what it would be like to mesh a ship's hull, because of the extremely sharp edges and thin parts of the solid.

I have heard from one engineer that uses Salome and Engrid for meshing external flows. He told me to mesh all the surfaces (ship's hull and the faces of the surrounding box) with tet elements and then use Engrid to create a prismatic boundary layer around the ship. That's why I have decided to go that way, at least to try, because it seems less complicated and proven for ships. :)

Now I am having problems with the generation of 2D tet mesh for the ship hull. This is my first meshing experience and only a first step in detailed numerical study of ship motion on waves, so excuse me for asking so many questions.

I really need advice on meshing this geometry. I have managed to create a fair mesh, but at the sharp edges on the top surface of ship's bow and stern I get really messed up triangles. Should I correct them manualy?<br /><br />Post edited by: Tomislav Maric, at: 2009/08/11 14:01
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14 years 11 months ago #3337 by Tomislav Maric
Replied by Tomislav Maric on topic Re:ship meshing in hex elements
I can't believe this. When I use Netgen 1D-2D algorithm it creates more or less a nice mesh, but I can't export it to .stl.

All I get is this:

begin solid
end solid

in the .stl file. But, if I go sequentially, choose Netgen 2D, Wireframe discretisation and then Node distance for 0D, two faces of the hull are great, but I get an error for the &quot;deck&quot; face of the ship.

Of course, THIS i can export to .stl to work on in engrid! :(

Can someone please help me mesh this? I'll send the .hdf file if needed.<br /><br />Post edited by: Tomislav Maric, at: 2009/08/11 15:06
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