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-   -   [ICEM] ICEM number of cells (http://www.cfd-online.com/Forums/ansys-meshing/115038-icem-number-cells.html)

kannstmasehn March 22, 2013 07:33

ICEM number of cells
 
Hello,

I want to simulate a cyclone in Star CCM+. I have a ICEM .prj file with .atr, .blk, .fbc, .par and .tin. I extracted a mesh with 800k cells. How can I change the number of cells? The rest should stay the same. Is that possible or do I need a new icem project? I tried to reduce the size of the cells but that didn't really work. Can you help me?


Thank you very much!

diamondx March 22, 2013 08:06

you need to generate a new mesh. that should not take longtime because you have the blk. open the geometry, open the blocking and reduce the number of node of your edges. then output it again

kannstmasehn March 22, 2013 08:33

do i have to select each edge or can I increase the number of nodes for the whole geometry? can you take a screenshot how to do it? or which tab do I need to use? i want to increase the number of cells but decrease would be also good

diamondx March 22, 2013 11:01

yes, each edge. you will find that in the blocking tab. i suppose you are not familiar with icem cfd and you don't have time to pass trough the tutorial. then just take some minutes in youtube to learn how to do that.

asal March 28, 2013 20:28

If you have equal spacing in all or most of the edges you can easily change the spacing in the part mesh setup and then update it blocking => pre-mesh params.
Then you don't need to assign new spacing for each edge.

kannstmasehn March 29, 2013 05:51

Thank you very much. I created more and less cells, that worked well. Now I need to add some prism layers. I think the mesh doesn't have any. Can I easily add some oder maybe how can I refine the cell number just near to the wall?

Another problem is in Star CCM+. The SDR does not converge. What does that mean? And how do I fix that problem?
I am always reading about k-epsilon and RSM. Why does no one goes with k-omega? Can you explain that to me? Is that model not working with cyclones? I think it's a little bit better than k-epsilon. Or is it not?

asal March 29, 2013 17:12

In the mesh, global mesh setup, prism, set the number of layers and other parameters for you prism. In mesh part setup, mark the parts which you want to have prism. In the compute mesh, prism, choose the existing mesh and then click compute!
ICEM with add prism layer in the mesh.

kannstmasehn April 1, 2013 06:40

okay, thank you. I'll have to go through every step, right?
Could it be that no prism layers are created because the cells are already very small?
I just have an inlet, an outlet and a wall. To select the parts for the prism layers I just cannot select 'wall', I need to create new parts, because I don't want to have prism layers everywhere, is that right?

asal April 1, 2013 07:27

Quote:

Originally Posted by kannstmasehn (Post 417532)
okay, thank you. I'll have to go through every step, right? Could it be that no prism layers are created because the cells are already very small?

No


Quote:

Originally Posted by kannstmasehn (Post 417532)
I just have an inlet, an outlet and a wall. To select the parts for the prism layers I just cannot select 'wall', I need to create new parts, because I don't want to have prism layers everywhere, is that right?

Yes!

kannstmasehn April 1, 2013 07:37

The prism layers worked. But it seems that icem did not create the number of prism layers I wanted to have. Or am I wrong? Here is a picture of my actually mesh:

http://s7.directupload.net/file/d/3212/kf8m6qw8_jpg.htm

The next problem with the mesh is that star ccm says the mesh is invalid:

Quote:

-> MESH VALIDITY:
Mesh is INVALID:
# Negative Volume Cells: 55
# Unclosed Cells: 426
In icem I run in 'edit mesh' --> 'check mesh' and there was no problem. I went throug Blocking --> Block Checks --> "Run Check/ Fix" and "Fix inverted blocks". But nothing really happened. It just asked if I want to remesh and I said yes. How can I fix that?

Quote:

Num. of inverted Hexa elements = 268
Fixing inverted hexas by diagonal smoothing...
Num. of inverted Hexa elements = 252
Num. of inverted Hexa elements = 127
Num. of inverted Hexa elements = 107
Num. of inverted Hexa elements = 108
Warning: could not fix all iverted hexas
This appears while creating the prism layers.

asal April 1, 2013 09:02

Do you generate hexa by using blocking?!!!!
Then why inflation? you should use the spacing to fine the mesh close to the walls. I think you want to generate tetra.
See the figures!


kannstmasehn April 1, 2013 10:09

Quote:

Originally Posted by asal (Post 417572)
Do you generate hexa by using blocking?!!!!

what do you mean by that? how I create the mesh? As I said, I did not create the icem-project myself. I just want to extract some meshes with different cell numbers and for y+, I want to make the mesh finer or coarser near the wall.

My procedure to get a .msh file out of the icem project was first to right-click on 'pre-mesh' and then 'convert to unstructered mesh', then on output I selected the wall, velocity inlet and pressure outlet. For increasing/ decreasing the cell numbers I changed the number of nodes for every edge. If this way is wrong, please tell me, I'm not an expert in icem.
Finally I want to have a hexa mesh structured as it is on your picture. I want to change the number of cells and the refinement near the wall for different y+ values. Do you understand? I just need the mesh in some varieties.

It would be great if you can help me with that!

asal April 1, 2013 11:03

To be honest, I cannot understand you!!
What do you mean by this:
"then on output I selected the wall, velocity inlet and pressure outlet."!!!

y+ is a function of velocity and kinematic viscosity and not just function of the mesh.
See below:
http://www.cfd-online.com/Wiki/Dimen...tance_(y_plus)

You should first use the blocking and then as I said assign the correct spacing to fine/course the mesh near the walls, if you want to generate hexa meshing. Prism is used just for tetra meshing!

try some tutorial same as below:

http://www.youtube.com/watch?v=9h97FvWJPcY

kannstmasehn April 1, 2013 12:22

Quote:

Originally Posted by asal (Post 417604)
To be honest, I cannot understand you!!
What do you mean by this:
"then on output I selected the wall, velocity inlet and pressure outlet."!!!

To extract a .msh I right-clicked as it is seen in the video you posted at 9:20 on 'pre-mesh' and then on the tab 'output' at the top. There I assigned my parts to 'wall' and so on and then extractet the .msh file. I just wanted you to explain how my was to get the .msh-file out of the project, maybe there is a different or better way.


Quote:

You should first use the blocking
Within the blocking I can change with the number of nodes of every edge and as a consequence the number of cells. Do you want to say that? If not I do not know what you want to say.

Quote:

and then as I said assign the correct spacing to fine/course the mesh near the walls, if you want to generate hexa meshing.
Yes, I understand that. I need a certain heigth for the cell near the wall, can I just control the cellheigth with the spacing and the number of nodes, if the length of the edge is given?

Quote:

Prism is used just for tetra meshing!
That was my mistake. Thank you!

asal April 1, 2013 12:30

Quote:

Originally Posted by kannstmasehn (Post 417622)
Within the blocking I can change with the number of nodes of every edge and as a consequence the number of cells. Do you want to say that? If not I do not know what you want to say.

Yes!


Quote:

Originally Posted by kannstmasehn (Post 417622)
Yes, I understand that. I need a certain heigth for the cell near the wall, can I just control the cellheigth with the spacing and the number of nodes, if the length of the edge is given?

Yes!

kannstmasehn April 1, 2013 13:04

great!

Is there an easy way to change the measurements of the mesh? I think of diameteres and something like that. Do I have to scale every edge or which way should I go for that?


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