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-   -   [ICEM] Meshing of a parabolic trough (http://www.cfd-online.com/Forums/ansys-meshing/102882-meshing-parabolic-trough.html)

Bazinga June 5, 2012 11:31

Meshing of a parabolic trough
 
2 Attachment(s)
Hello everybody,

I am working on this case now for quite a long time and haven't found the solution for my problem so i thought maybe you can help me out. I am a beginner at ICEM so maybe i just lack of knowledge and my problem isn't that difficult to solve.

We want to create a Hexa mesh around a parabolic trough. I have to make a numerical simulation for three different angles of the trough. You can see two of them in the pictures.

First I tried to create a block structured mesh with an O-Grid around the trough. But since the trough is very close to the bottom i wasn't able to create a high quality mesh (picture 1 shows the trough and the bottom).

After that I tried some tetra meshing using the octree method with prism layer around the mesh and at the bottom of the wind tunnel. This mesh was the best to this point but since my task is to create a hexa mesh I can't use it. It's still not perfect because i quit at some point when i understood how to create a tetra mesh in general.

At the moment i try to create a unstructured hexa mesh using the BFCart mesher. It took me quite some time while testing on how to create an unstructured hexa mesh to realize that the mesh has to be very dense around the trough. The mesh always went through the trough before. But since the trough has a very small thickness even very small hexas around the trough are still to big (see picture 2).

So once again, I am new to ICEM and this Forum so I hope you do understand my problem.

Thanks in advance!

Far June 5, 2012 13:38

create an ogrid around geometry

Bazinga June 6, 2012 05:40

1 Attachment(s)
Hello Far,

thank you for your answer.

I already tried an O-Grid and it didn't work out that great.
I uploaded a picture of my O-Grid so you can see where i have trouble.
I'd be very happy for any advice to improve this grid :)

edit: Of course this grid needs some refinement etc. but my main problem the transition between the O-Grid and the surrounding mesh especially in the bottom area remains the same I think.

Far June 6, 2012 05:48

you are almost there. just more splits and move vertices. Also reduce the size of o-block (go to edit block> rescale oblock).

You may need some advance topology strategy. See the video tutorial series (famous 3 part YouTube video on air-foil meshing by Simon) where he has explained how to handle high staggered air-foil with advance blocking.

Bazinga June 6, 2012 05:55

Quote:

Originally Posted by Far (Post 364970)
you are almost there. just more splits and move vertices. Also reduce the size of o-block (go to edit block> rescale oblock).

You may need some advance topology strategy. See the video tutorial series (famous 3 part YouTube video on air-foil meshing by Simon) where he has explained how to handle high staggered air-foil with advance blocking.

Thanks for your quick response!
I'll work on this mesh again after watching your suggested series.

edit: The series I found is for a 2D modell. Mine is 3D. Is it the one you were talking about? http://www.youtube.com/watch?v=tYrbScUH9RE

Far June 6, 2012 06:04

I cannot open link from my office (Youtube is banned). Here is the Google search revealed same videos I was referring http://www.google.com.pk/search?suge...+meshing+3part

http://www.veengle.com/s/ICEM-CFD/2.html

Bazinga June 6, 2012 12:35

1 Attachment(s)
I worked on this basically the whole day and wasn't able to get it done.
I tried the rescaling of the O-Grid and got the error "No rescale direction specified." I don't know where I can specify this and I am not sure how this rescaling option works generally.

I watched the Tutorial. It was very interesting but I couldn't get too much out of it concerning my Problem.

While the mesh was mostly OK my main problem is still the bottom part of the O-Grid where the mesh becomes very dense (as shown in the attached picture). I wasn't able to fix this problem with my little ICEM knowledge.

Far June 6, 2012 12:46

send files

Bazinga June 6, 2012 13:43

1 Attachment(s)
Quote:

Originally Posted by Far (Post 365080)
send files

There you go.

I hope my work is not too bad.

The farfield is huge in both x-directions because we wanna use this mesh to for two calculations to simulate different angles of the trough. Your welcome to comment on this also ;)

PSYMN June 6, 2012 16:50

Better Topology
 
5 Attachment(s)
Your topology is not great...

Instead of thinking of this as some box at an incline, think of it as 1/4 of a circle (imagine the ogrid creating a box around the theoretical center) or as a fillet (imagine the surfaces extending up stream and up towards the top of the box...

Here I will show how to capture the latter...

Start with two splits, one below and one behind the curve...
Attachment 13622

Then put in a quarter Ogrid (one block with 4 faces) to capture the curvature.
Attachment 13623

I rescaled my Ogrid (0.3) to be closer to fitting the model...
Attachment 13624

Then I split out to fit the blade... Split upstream, downstream, upper surface, lower surface to box it out.
Attachment 13625

Other verts could be adjusted also to improve the angles, etc. Then use "Align verts" to get it nice and crisp.


I don't have time to complete it for you, but this should get you started.

Also, here is a 30 second start on the other option, the circle option... Here is the basic blocking after 4 splits and an Ogrid thru the model (one block, 2 faces). From here, you would split out the airfoil, etc.
Attachment 13626

diamondx June 6, 2012 19:55

Here is what i got... i don't know why winrar couldn't compress it in zip format, so i made it in my dropbox, here is the link:
https://dl.dropbox.com/u/35161486/parabolic.rar



https://dl.dropbox.com/u/35161486/cfdonline.JPG

diamondx June 6, 2012 20:02

picture is huge :o sorry about that

PSYMN June 6, 2012 22:50

Yes, now the question is if that resolution is sufficient... (clearly it is not yet, but I assume you plan to keep working on it).

I hadn't really taken a close look at the shape of your airfoil before... Quality on the curved "corners" won't be ideal. You could probably also place a small Ogrid around the airfoil blocks for an extra refined boundary layer...

diamondx June 6, 2012 23:34

true... because refining on a laptop requires a bit more of memory... but i added and o-grid around. hope it's correct
here is new link :
https://dl.dropbox.com/u/35161486/barabolic2.rar

I don't know why this pictures are so big !!!

https://dl.dropbox.com/u/35161486/cfdonline2.JPG

https://dl.dropbox.com/u/35161486/cfdonline3.JPG

diamondx June 6, 2012 23:41

of course quality can be improved more when playing with vertices...i'll play with it more in lab. PCs are faster there.
thanks Simon for this blocking strategy and the advices .

PSYMN June 7, 2012 00:33

On the right track...
 
Yea, I think you are on the right track now...

The topology is done, and you just need to spend some time fine-tuning the vertex placement and working out the edge distributions...

Make sure you learn how to use "Align Vertex" and "Set Location", both of these are under "Move Vertex" and can really help you align everything for maximum quality... You control the extent of their influence with the index control, so hopefully you have figured out how to use that also.

Bazinga June 7, 2012 03:54

Wow, these are great ideas.

Thank you very much everybody.

I'll try to do it in a similar way on my own and post again when I am done.

Far June 7, 2012 05:14

1 Attachment(s)
I have managed to make blocking as Simon suggested. Still internal o-block is remaining and also some splits on wing or whatever ;)

Far June 7, 2012 09:04

Quote:

Originally Posted by PSYMN (Post 365113)

Start with two splits, one below and one behind the curve...

Horizontal split is necessary?

Quote:

Then put in a quarter Ogrid (one block with 4 faces) to capture the curvature.
Should we crate the y-blcok at the corner where two splits meet?


Quote:

the circle option... Here is the basic blocking after 4 splits and an Ogrid thru the model (one block, 2 faces). From here, you would split out the airfoil, etc.
Which option is better, this one or quarter Ogrid?

PSYMN June 7, 2012 14:31

@ Far

Quote:

Horizontal split is necessary?
The horizontal split assumes that there is a floor in this model... I assumed there is or the model would have centered the scoop. If there is a floor and we are considering viscous effects... It is nice to keep that boundary layer clean. Without that lower horizontal split, the boundary layer would peel away from the wall and shoot straight up, wasting mesh in the volume, creating volume jump problems and leaving no boundary layer for downstream...

Quote:

Should we crate the y-blcok at the corner where two splits meet?
No idea what you mean about the Y block... But I don't think anything else is necessary. Simpler is usually better, even if the quality is just a little worse. This blocking suggestion should give more than adequate quality.

Quote:

Which option is better, this one (circle) or quarter Ogrid?
That depends on what the flow looks like (or what is expected). If the flow is fast and this scoop sends a large portion upward for a long distance, the Quarter Ogrid will be good because you can get higher resolution in that area above (downstream) of the scoop. You could even come back and adjust the blocking slightly to align the vertical portion of the Ogrid with the flow and get better resolution.

But if the flow is lower speed and not much happens between the scoop and the top of the airfoil, then that mesh refinement is wasted in the quarter Ogrid and the circle method may be better... The circle method also has the advantage of being somewhat self contained... In other words, it doesn't propagate and cause difficulty elsewhere in the model. That is not a concern for this particular case, but imagine if we had many such scoops and all the quarter ogrids started intersecting, etc. It could become a mess.


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