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Old   March 6, 2013, 06:49
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I've been moving vertices and changing nodes number, negative volumes change in number and distribution (see pics) but they are always there.
I've been checking http://www.youtube.com/watch?v=tYrbScUH9RE for some blocking ideas but every blocking strategy I try to imitate gives negatives volumes for me.
Is there any base problem?
Isn't it a common mistake in Icem practice? Cause I'm facing it too often!
Any suggestion? Thanks!
Attached Images
File Type: png NACA_Cylinder_blocking_1.png (13.1 KB, 11 views)
File Type: jpg NACA cylinder blocking 1 neg vol.jpg (45.6 KB, 10 views)
File Type: png Naca_cylinder_blocking 2.png (24.4 KB, 10 views)
File Type: jpg Naca_cylinder_blocking_2_neg_vol.jpg (51.1 KB, 16 views)
Attached Files
File Type: zip NACA cylinder 06 03 2013 collapsed.zip (8.5 KB, 1 views)
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Old   March 6, 2013, 06:55
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please attach .tin
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Old   March 6, 2013, 07:06
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Here you are
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Old   March 6, 2013, 08:37
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blocking 1 is perfect. No problem of negative volume
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Old   March 6, 2013, 08:46
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I've converted one of the meshes to unstructured and then exported it to fluent and it doesn't detect any negative volumes! I had tried that before with no success but now it works...
How does the pre-mesh to unstructured step work?
Apparently Negative Volume indicator doesn't mean too much...
Which quality parameters should I pay attention to? Quality, angle, volume change?

Thanks!
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File Type: jpg fluent_neg_vol.jpg (34.7 KB, 9 views)
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Old   March 6, 2013, 09:03
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Qaulity, min angle and volume change. Also dihedral angle (should be less than 165)
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Old   March 7, 2013, 06:01
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Quote:
Originally Posted by Far View Post
Qaulity, min angle and volume change. Also dihedral angle (should be less than 165)
Which values would be appropiate for that? I've read min quality > 0.2, min angle >18 and max volume change??
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Old   March 7, 2013, 06:04
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Min quality > 0.3, min angle > 18 amd volume change <20
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Old   March 7, 2013, 06:08
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Quote:
Originally Posted by Bollonga View Post
I've converted one of the meshes to unstructured and then exported it to fluent and it doesn't detect any negative volumes! I had tried that before with no success but now it works...
How does the pre-mesh to unstructured step work?
Apparently Negative Volume indicator doesn't mean too much...
Which quality parameters should I pay attention to? Quality, angle, volume change?

Thanks!
You won't see negative volume unless you do a check mesh on the unstructured mesh. But if you look at the quality, you would find a lot of elements with a quality around 0. Those elements are your negative volume elements

About the values of quality, Far is right, but quality, > 0.2 is good too.
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Old   March 7, 2013, 06:37
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Quote:
Originally Posted by BrolY View Post
You won't see negative volume unless you do a check mesh on the unstructured mesh. But if you look at the quality, you would find a lot of elements with a quality around 0. Those elements are your negative volume elements
For the same mesh I get negative volumes at the pre-mesh and quality over 0.6 at the unstructured mesh. I don't understand why. (attached files)

Then in fluent it doesn't show any negative volumes.

However, for this mesh, my fluent case diverges from the beginning. I'm using 33.87 m/s, k-ep standard with TI=1% and TLS=1e-4m at the inlet. Adaptive time-step starting at 1e-8. I've tried reducing under-relax factors one order of magnitude but it doesn't avoid divergence.
Attached Files
File Type: zip NACA wing 07 03 reduced.zip (29.9 KB, 2 views)
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Old   March 7, 2013, 06:42
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No negative volume in this project, the pre-mesh quality > 0.8.
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Old   March 7, 2013, 06:51
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But if I select Volume in the quality menu, select histogram form -1 to 0 you can see there are negative volumes. In the image there's the histogram from 1e-15 to 0 showing many elements, even if quality histogram is >0.6...
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File Type: jpg quality_060_neg_vol.jpg (57.0 KB, 9 views)
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Old   March 7, 2013, 06:53
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check " only visible index range"
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Old   March 7, 2013, 06:55
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Quote:
Originally Posted by Far View Post
check " only visible index range"
Nothing changes
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Old   March 7, 2013, 07:30
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I've discovered that just changing the min distance of the edge near the wall from 1e-5 to 5e-5 it improves quality a lot, from having near zero quality elements to a minimum quality of 0.6. It's pretty sensitive to node distribution!
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Old   March 7, 2013, 10:11
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Okay, so negative volumes is solved so far. Now I want to make some test with the mesh but it diverges no matter what I do.

Quote:
Originally Posted by Bollonga View Post
However, for this mesh, my fluent case diverges from the beginning. I'm using 33.87 m/s, k-ep standard with TI=1% and TLS=1e-4m at the inlet. Adaptive time-step starting at 1e-8. I've tried reducing under-relax factors one order of magnitude but it doesn't avoid divergence.
I've tried ke-epsilon standard (enhanced wall treatment) and k-om SST aswell for steady and transient case. Mesh quality is good, volume change is not that good I guess (see histogram and element distribution in picture 2).

Could it be the problem for not reaching convergence?
Could I reach convergence with a coarser mesh?

Thanks!
Attached Images
File Type: jpg mesh_naca_wing_volume_change.jpg (97.3 KB, 20 views)
File Type: jpg volume_change_naca_wing.jpg (50.0 KB, 15 views)
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Old   March 7, 2013, 11:16
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Quote:
Originally Posted by Bollonga View Post
volume change is not that good I guess (see histogram and element distribution in picture 2).

Could it be the problem for not reaching convergence?
Could I reach convergence with a coarser mesh?

Thanks!
I thought an appropiarte volume change was 20% so I understood that higher than 1.2 was not good. But as I've read in the user guide, less than 20 means 20 times the maximum neighbour volume. So my volume change dsitribution is pretty good! So I don't know what can be wrong
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Old   March 7, 2013, 11:20
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Quote:
Originally Posted by Bollonga View Post
I thought an appropiarte volume change was 20% so I understood that higher than 1.2 was not good. But as I've read in the user guide, less than 20 means 20 times the maximum neighbour volume. So my volume change dsitribution is pretty good! So I don't know what can be wrong

Just few thoughts:

1. Open up mesh in far field. In other words do not carry forward mesh sizing and growth ratio in farfield. Instead make spacing uniform and use match edges.

2. check domain extents
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Old   March 7, 2013, 14:13
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Quote:
Originally Posted by Far View Post
1. Open up mesh in far field. In other words do not carry forward mesh sizing and growth ratio in farfield. Instead make spacing uniform and use match edges.
I haven't understood it completely. You mean I shouldn't keep edges parallel until far field but to open them?

And use uniform spacing instead of hyperbolic or geometric or whichever?

Which edges should I matche to which ones?

Thanks!
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Old   March 8, 2013, 05:26
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Download file from first post in following thread and look at the meshing in farfield.

Fundamental mistake in mesh
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