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brissyben June 1, 2009 03:13

C Gridding and removing the triangular blocks
1 Attachment(s)

I am currently working ona generic submarine hull form, I am fairly happy with the mesh so far however i have one area where i can not find a solution.

I have used a c grid approach to block the appendages (fins and sail) however when i have c gridded the block left for the appendages has a diamond/triangular shape, thus creating a region of mesh that has poor angles and skewness.

1. Is c gridding the best approach for the appendages (fins and sail)?
2. Is there method to remove this triangluar block inside the fins?

I have atatched a couple of pictures of my mesh and the blocking stratergy.

Thanks for any help,


p.s. I have tried merging vertices, merging blocks and all either distort the surronding blocks or create negative volumes.

brissyben June 1, 2009 03:14

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next picture

brissyben June 1, 2009 03:16

2 Attachment(s)
Pictures 3 and 4

xamo June 3, 2009 03:27

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if you have a geometry like a triangle (or a geometry with a small angle) it is usually better to do the blocking like in the attatched picture.


PSYMN June 3, 2009 09:45

XAMO is showing a Y block configuration, but I don’t know that it will help you for 2 reasons. First, the end will still be as sharp, and second, you will need to collapse the propagation thru the volume and that will cause more problems…

I think you are on the right track, but have some options…

At the front, you have created a chevron shaped block… You definitely need to move that inner corner vertex further back to have all angles less than 180 degrees… This won’t hurt the quality of the blocks behind it… You should probably do the same for the rear pointing chevron.

Also, you have created an OGRID (this is not a CGrid) inside your fin (hopefully passing out to the far field), but you should have started one layer out from there so that the Fin is inside the Ogrid and then has another Ogrid inside that (basically a 2 index ogrid where 0 is the outside of a boundary layer around the OGrid, 1 is at the surface of the Fin and 2 is inside the fin, coming out of the tip. THis OGrid Tube would pass from inside the submarine to the far field (face on each end).

To solve your issue you have some choices, but they depend on your solver. What is you solver? Many structured solvers can actually handle those sharp (<2 degrees) trailing edge blocks. If you move the verts around, you should be able to make everything positive. If your solver can handle unstructured meshes with prism elements or perhaps degenerate structured blocks (wedges), you could merge those trailing edge blocks into wedges. You would need to be careful and extend that merge out to the far field. You could also replace those wedges with Y-Blocks (use the create Y blocks from 6 verts option) and get something similar to what Xamo suggested.

Another option is to start your fin more like a box with a finite leading and trailing edge… This will require some finite trailing edge on your sub, does it have any trailing edge thickness? If so, your Ogrid would have a different effect near the trailing edge and give you dense mesh, but better angles. To start this concept, just block it in a more straight forward way with a split ahead of and behind the fin, and then another above and below… Within this block, from the sub to the ff, put an OGRID tube. Fit the inside of this ogrid tube to the edges of the fin, with the flat back fit to the small trailing edge. Then reduce your index to just the inner OGrid tube and put another Ogrid tube inside that… This next layer forms the inner layer and will have better angles…

have Fun...

brissyben June 3, 2009 20:06

Thank you both for your help, I have spent many hours being frustrated with this problem, so I appreciate the help very much

I will try and give it a go today and let you know how I go,


brissyben June 3, 2009 20:52

PSYM, I am using CFX as my solver

brissyben June 4, 2009 02:16

2 Attachment(s)
Quater grids seem to be working well, I have attached a picture of my first fin that i have used the quater grid on, I will now extend this to the FF and the other appendages,

Thanks again

PSYMN June 4, 2009 11:13

Great, I just thought you might run into problems further out...

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