Icem cfd - hexa meshing
 

i am trying to mesh a torpedo kind of geometry the problem comes when i create pre- Mesh, some negative mesh generating at the area of fin. how can i get rid of it???...

The pictures shows where the problem

Aspect ratio...
 

That mesh has poor aspect ratio...

The solution is with adjusting the edge parameters behind the torpedo... Ideally, you would match edges (edge distributions) behind the torpedo to match the edge distribution of the trailing edge.

A bigger quality problem is at the nose of your torpedo where you have wide open (high max angle) quads. You can sort this out with an Ogrid topology.

i cleared the nose problem, the tailing edge of the torpedo is just touching the end circle so its hard to match the edges, if i increase the length of the torpedo at the end it may work na ???

Just click on the edge behind the torpedo and match it to the edge at the back of the torpedo... You can even do this at the far field if you like... Once that aspect ratio is reduced, quality will appear better.

Assuming that those elements were highlighted by the aspect ratio metric...

I would have to see a zoom in on the problem cells, maybe even a scan plane thru the area, to make a better suggestion.

initial impressions...
 

Taking a closer look at your actual model, I see that your fins have sharp trailing edges. This means you need to collapse the blocks behind these edges... (under Merge Vertices, you can learn more about it with the pipe fin example or the airfoil example up on youtube) You will then need to collapse the blocks beside those collapsed blocks all the way out to the far field extents.

I also suggest you put this block inside an ogrid...

Actually, I suggest working on a quarter of the model an using a quarter ogrid...

I will make a quick go at it later...

Actually i tried that method to collapse blocks,but i have a confusion regarding which edges i have to select when doing it , for a trail i randomly selected different edges but didn't get the right collapse ..i will try that again. pipe fin example is available in you tube???

The procedure i followed for making o grid is difine the blocks which we have to remove as 'dead' then make o grid around it ..is that the right way ?

Done...
 

For collapse blocks, you select the direction to collapse... This is the edge you want to go away... And then you select all the blocks you want to collapse in that direction.

I did if for you and sent a replay file...

Here are some images for everyone else... (I just did a quick blocking without spending much time on matching edges, etc.)

I used a quarter ogrid to wrap the torpedo and I used collapse to capture the sharp trailing edge on the fins...
Attachment 8448

Then I put a final quarter Ogrid in the wake of the torpedo
Attachment 8449

Pretty quick to do with ICEM CFD... If I spent more time, i would match edges, etc for smooth transitions between the sections...

This would make a nice little demo if I could use the file publically.

As often happens...
 

As often happens, your first try at a particular topology (such as a torpedo) is not really your best. As soon as you look back on it, you can think of a better topology.

If I did this one again, I would not have the fins extend to the edge of the original OGrid... I would split that ogrid so the fins fell within it... That would probably help with convergence around the tips of the fins... In ICEM CFD, the extra effort would be a single split operation (across the Ogrid) to capture both fins...

it was really helpful all your advice , and thanks a lot for all the effort you taken during your busy schedule.. i am not that expert just learning things and i come to know experience is the only on thing that make u master of ICEM , every day i learning some thing different ... the mesh is so good as it is, if we know how to do O grding effectively we can improvise in many area na ?

once again i am thanking you... i learned so many things from you iam hoping help from you in future also

Thankfully

Arun

Great info by Simon.