Splitting the blocks
 

I’ve just started exploring Hex meshing with ICEM CFD and I had a question regarding the split up of blocks. In ICEM I see that a split can be confined to a particular block alone instead of running it through all the blocks. But I learnt that this can lead to problems with the meshing later on and that it is always preferred to have continuous splits.

So can someone please elucidate the general points to bear in mind while making a split , with some images

Thanks in advance

Propagate splits...
 

Sorry, no time for pics, but I can give the jist of it.

When you make a split thru a single block, you have a number of nodes on either side of the split edge. in order to have a pure "structured" hexa mesh, these must propagate thru even if the split does not. Therefore, if you split a single block, the adjacent blocks get implied splits... You can't see these splits, but if you go to the split edge and adjust the number of nodes, you will see that this must propagate thru. The problem is that without the split, it is harder to control the distribution on the other edges. It can be done with "link edges", but it is a hassle.

More importantly, if you later split one of these "unsplit" edges, the new split also has implied splits that must go back past other real splits and implied splits. This can cause quite a control mess.

My suggestion is that it is usually better to split thru all the blocks so you can retain this control. The down side is visual clutter, but this can be countered with other means such as turning of surfaces if you don't need them, reducing what you are looking at with index control (don't forget that index control can limit splits), etc. The other issue is that structured solver users may want fewer blocks at the end; this can be handled with merge output blocks after you have sorted out all your edge distributions, etc. This "output blocks" option allows you to keep the control of the edges, but merges them on output.

Have fun with it.

Here are some images to illustrate what Simon said.

SPLIT_BLOCK_geom.jpg is the geometry (as a simple example)
SPLIT_BLOCK_mesh_3.jpg is the result of the mesh. If you need to refine the mesh at the middle of the geometry, you can see the number of nodes of your middle edge is propagating to the top and bottom edges.
SPLIT_BLOCK_mesh_5.jpg is the same mesh than the previous one but with the linked bunching option activated. So the mesh is more uniform.

But imagine that with more splits and edges, and you can guess how difficult it would be to have an uniform and nice mesh.
So, as SImon said, it's better to propagate your splits everywhere, even if it would require more associations (so you would need to create more curves and points to fully associate your blocking).

Hope it helps,
Good luck !

Thanks for the replies Simon and Boles. Simon, Can you please share some ppt that covers Advanced hex meshing techniques , tips and tricks for hex meshing. I've done a search on the customer portal but couldn't find anything as such (or) if you think that I missed out something from the portal , Please suggest me upon finding it

And can you tell me the acceptable quality range for pyramids in ICEM CFD if the solver is either CFX (or) Fluent

A bit more information about merging blocks
 

Hi, I'm learning about splits and I found Simon's old post very useful. However, I'm not sure of understanding this last comment about merging blocks.

If we merge blocks, do all the splits (real and implicit) merge? How does this work? :confused:

And about the lines concerning "output blocks", maybe it's a problem of language (I'm not an English speaker) but I don't understand what does Simon mean by "output" block and merge them "on output"... Could someone develop or explain a bit more this idea?

Last but not least, saisanthoshm88, did you ever find more information/tutorials about hex meshing techniques?

Thanks in advance!!:D

1. He meant to use "output blocks" option and did not mean to physically merge them. see attached pics

2. You can many tutorial about hexa in icem cfd help and youtube.