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Old   August 17, 2011, 04:20
Default Wall zone separation
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Mohsin Mukhtar
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hello

I have 2 cylinders of different diameters (One bigger and the other smaller). both are of same height. The flow of air is from the annular region between these two cylinders. When I mesh these 2 cylinders and export them into Fluent, then, Gambit automatically gives default wall boundary and fluid zones. But the face which separates the two cylinders is not recognized as "wall" by GAMBIt. If i manually give this face as "wall" then, after export, fluent creates "shadow wall' and if i give this region as "solid" continuum entity then, after export, fluent creates different wall zones as shown:

building mesh....
separting wall zone 4 into zones 4 and 6...

Making the inner cylinder "solid" instead of "fluid" creates extra zones. Why is this so and how can I avoid this problem.

Note: I tried to merge the extra zones (wall and wall:08) in FLUENT (Merge----> Zones) but it gives this message "unable to merge, Zones do not match".
Thank you.

Last edited by Mohsin; August 17, 2011 at 07:01.
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Old   August 17, 2011, 07:18
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Maxime Perelli
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post picture.
In Gambit when you created the 2 cylinders, you have to use split or substract tools for getting only the annulus region.
To be sure, enable shaded mode on, and check that you volume contains only what you need.
Else connect/face/all, to be sure that all surfaces are connected
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Old   August 17, 2011, 08:47
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Max Thank you for your reply.

Picture.1, shows that isometric view of the geometry. There are 2 cylinders. The cylinder shown in red (with vanes attached) is defined as 'solid region" by me. The outer cylinder is defined as "fluid". Air comes from the top and hits the vanes (attached to the smaller cylinder) and gets a swirl motion.

The problem is that when i define the inner cylinder (including vanes) as "Solid" and export it in FLUENT then FLUENT automatically creates some extra wall zones. As soon as it reads the mesh it displays the following message:

building mesh....
separting wall zone 4 into zones 4 and 6...

Note:
1. The geometry is connected.
2. As u said, for annular regions the inner cylinder is usually subrtacted from the outer cylinder, however, If i subtract the inner cylinder from the bigger cylinder then how can i connect the vanes to the inner cylinder. (I m only using the inner cylinder for connection of vanes, there is no heat transfer or flow in the inner cylinder)

Please guide.
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Old   August 17, 2011, 08:48
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Max Thank you for your reply.

Picture.1, shows that isometric view of the geometry. There are 2 cylinders. The cylinder shown in red (with vanes attached) is defined as 'solid region" by me. The outer cylinder is defined as "fluid". Air comes from the top and hits the vanes (attached to the smaller cylinder) and gets a swirl motion.

The problem is that when i define the inner cylinder (including vanes) as "Solid" and export it in FLUENT then FLUENT automatically creates some extra wall zones. As soon as it reads the mesh it displays the following message:

building mesh....
separting wall zone 4 into zones 4 and 6...

Note:
1. The geometry is connected as is shown in Picture.2 by shades.
2. As u said, for annular regions the inner cylinder is usually subrtacted from the outer cylinder, however, If i subtract the inner cylinder from the bigger cylinder then how can i connect the vanes to the inner cylinder. (I m only using the inner cylinder for connection of vanes, there is no heat transfer or flow in the inner cylinder)

Please guide.
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Old   August 17, 2011, 08:51
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substract blue volume with red volume
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Old   August 22, 2011, 20:21
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Max, thank you for your time

I have three questions, I'll be grateful if u could answer these question:

1. When u subtract the inner cylinder from outer cylinder then u can still mesh the upper face of the subtracted region (only face not volume). It's a little confusing, why is it meshing the face if there is no surface there?

2. Secondly, for the above volume(Picture shown in my previous post), apparently cooper scheme can't be applied so tetrahedral meshing would be done on that volume but is it ok if i mesh the upper face as Quad-pave for a tetrahedral volume mesh? usually, for a better quality tetrahedral mesh Do i need to mesh all the faces with Tri_pave?

3. Thirdly, what do u recommend, can somehow cooper rmeshing scheme be applied on this geometry?

Thank you so much
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Old   August 23, 2011, 01:37
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1. Substract, then you will see what surfaces are available (don t forget to delete the volume you don't need after substracting)

2. 3. yes you can apply cooper on this geometry, but you need to decompose (split) it
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Old   August 23, 2011, 02:05
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Thank you Max

How to split the vanes attached to the volume? Could you please elaborate on how can i split it to make it cooperable?
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Old   August 23, 2011, 05:52
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from the geometry you sent me, create one plane (100 0 100), rotate it with (0 0 30).
Rotate this surface now with (0 0 90) by enabling copy option.
You can split your volume with those 2 surfaces, and you can isolate one slice (periodicity).
If you want full domain, you will be able to copy and rotate your your mesh (linking faces may help).
From this slice you can do some splits for having cooper mesh. (quite basic)
you have lots of possibilty to mesh it with hexa.
check tutorials:
http://my.fit.edu/itresources/manual...uide/tgtoc.htm
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