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Problem converting from Tetrahedral to Polyhedral Mesh

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Old   March 24, 2020, 11:34
Default Problem converting from Tetrahedral to Polyhedral Mesh
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Iakov Bobrov
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Hi everyone,

I am trying to run my firts ever 3D simulation in Fluent - over a supersonic rocket, and I would like to ask for help with the following.

I understood that I would need to implement polyhedral mesh to save time and cut the cells number to be below 500,000 (uni license). For some reason, I do not always get the polyhedral mesh. Often, surface of the rocket is meshed with the polyhedrals but with the lines inside (or triangles?), see below.
pic1.PNG
pic2.PNG

So this way the number of cells adjacent to the rocket actually increases!

I noticed that this does not happen when I get rid of the inflation layer, see below.
pic3.png

Also, attaching a picture with the tetrahedral cells, before converting them.
pic5.png

Could someone point to me what am I doing wrong, please?

And the second question is, I found it really hard to mesh the nosecone, which is a sharp geometry. And the resultant mesh does not represent the geometry - I thought the reason could be that the mesh is not fine enough. Then, is it simply adding as many cells as possible near the sharp tip? I have tried face sizing and the sphere of influence - are there better strategies?
pic4.PNG
Or is it not to do with the number of cells?

Thank you!

Last edited by iakov1703; March 24, 2020 at 14:19.
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Old   March 25, 2020, 05:51
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There are two aspects of what you are doing and observing. First, Fluent will NOT convert hexahedral cells to polyhedral. So, if the boundary layer mesh is made of hex cells, those will be maintained as it is. If those are made of triangular prisms, then those will be converted. However, that's where the second aspect comes into play. Since you are looking at the mesh on the surfaces, inflation layer will always show rectangles. You have to mark a few cells near the boundary and then display the register. You can use Adapt > Boundary to mark cells. Do not click on Adapt. Just choose the wall of the nose cone, use 1 or 2 cell distance and then click on Mark. Go to Manage > Display and this will display 3D cells. You may have to enable options for displaying it in 3D under the options. All these are accessible from Adapt > Manage.
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Old   March 25, 2020, 09:09
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Quote:
Originally Posted by vinerm View Post
There are two aspects of what you are doing and observing. First, Fluent will NOT convert hexahedral cells to polyhedral. So, if the boundary layer mesh is made of hex cells, those will be maintained as it is. If those are made of triangular prisms, then those will be converted. However, that's where the second aspect comes into play. Since you are looking at the mesh on the surfaces, inflation layer will always show rectangles. You have to mark a few cells near the boundary and then display the register. You can use Adapt > Boundary to mark cells. Do not click on Adapt. Just choose the wall of the nose cone, use 1 or 2 cell distance and then click on Mark. Go to Manage > Display and this will display 3D cells. You may have to enable options for displaying it in 3D under the options. All these are accessible from Adapt > Manage.
Thank you a lot for you reply! "You have to mark a few cells near the boundary..." - I am sorry, why should I mark the cells? If this will allow me to see the cells in 3D, how will it help me or what can I conclude from it?

It seems like in Fluent 2019 the option to Mark cells disappeared and I can't even try it now!
pict.PNG

Since I am inserting the inflation layer on the surfaces, like you said the boundary layer mesh will be made of hex cells, and therefore I will always see rectangles instead of the polyhedral mesh on the rocket surface. Is that correct, please tell me if I misunderstood you?
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Old   March 25, 2020, 09:37
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Cell marking is just to help you in seeing the shape of the cells.

Inflation layers, whether made up of hexahedral or triangular prisms, will only show rectangles on the the surfaces. You have to create a surface of revolution, that is shaped like the object itself but is slightly offset from its surface so that you could display the mesh on this surface.

But as you can observe from the mesh on the surface of the object, all faces are polygons implying the inflation layer created in mesh is made up of triangular prisms and not hexahedral cells. If inflation layer would have'd been hexahedral, it would show quadrilaterals on surface of the object. But, as mentioned above, sides are always rectangular.
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Old   March 26, 2020, 07:11
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Quote:
Originally Posted by vinerm View Post
Cell marking is just to help you in seeing the shape of the cells.

Inflation layers, whether made up of hexahedral or triangular prisms, will only show rectangles on the the surfaces. You have to create a surface of revolution, that is shaped like the object itself but is slightly offset from its surface so that you could display the mesh on this surface.

But as you can observe from the mesh on the surface of the object, all faces are polygons implying the inflation layer created in mesh is made up of triangular prisms and not hexahedral cells. If inflation layer would have'd been hexahedral, it would show quadrilaterals on surface of the object. But, as mentioned above, sides are always rectangular.
I understand! Thanks a lot!
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