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Skewness in ANSA not matching Fluent

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Old   January 1, 2019, 09:41
Default Skewness in ANSA not matching Fluent
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I am having a problem where I have created a model from a 3D scan. I've cleaned up the model a lot and generated a volume mesh with prism layers. When I switch to "hidden" I see no elements with the title OFF (after changing fluent skewness to 0.85, and going through an iteration of fixing quality).

I then export to fluent and read in. I check quality and it says I have 450 elements with skewness above 0.85.

This seems to have baffled support. Anyone seen anything like this before?
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Old   January 10, 2019, 09:25
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Vangelis Skaperdas
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Hi MachZero

Have in mind that Fluent Skewness (Equi-Volume skewness) applies only to tetras.
Your mesh however also contains prisms as you have created layers.
If you want to check their quality too, you should activate Fluent ANGLE in F11 window for solids.
Then you will also be calculating Fluent EquiAngular skewness which applies for pentas, hexas and pyramids.
Then I am sure ANSA will show you the violating prisms that Fluent is identifying.
Hope this helps
Vangelis
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Old   February 14, 2019, 20:24
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Thank you for your reply. I didn't even see this reply until now.

You are correct about the angles metric, however even when I use this metric (which is by default on and set to 0.92) it says my. Each does not have any violating elements.

I followed this up with ANSA support and they told me that this is a bug and that they need to update how they are calculating this quantity. They said hey have no idea when it will be implemented.

So if this is the case, is there effectively no way to get a good quality mesh on a complicated geometry, since what I am being told is that their mesh metrics don't work. I'm concerned I simply can't use this as a meshing tool.

Please advise!
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Old   February 15, 2019, 04:21
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Hi there
Indeed in extreme cases where you have prism elements with very warped facets (warp angle>50deg) then the calculation of Fluent Equiangular Skewness of ANSA may not agree to that of Fluent.

However in most cases if you have a good mesh and not extreme warp angles of the quad facets of the prisms, then both ANSA and Fluent will report the same equiangular skewness.

I would not be worried about this to the extend of not being able to use ANSA for Fluent meshing, as it is something that has worked very well for many years.
ANSA can generate very good quality meshes. Try to use Batch Mesh tool for surface, layers and volume meshing. Ensure your surface mesh does not have skewness violations (Fluent EquiArea skew should be below 0.5).
Ensure you do not have extreme sharp angles, you can find this in Checks>Surface Mesh> Sharp Edges, especially if they are concave.

Ensure the layers are generated in a good manner with proper length values.

The default quality criteria of ANSA should always lead to a good Fluent mesh,

Best regards
Vangelis
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Old   February 15, 2019, 09:40
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Thanks you for your continued replays and advice. I sincerely appreciate it!

My geometry is very complicated and honestly it would take months to manually inspect all surfaces, which is why I am relying on the mesh metrics within ANSA to tell me where to look.

I have gone through as created a good surface mesh that has a surface skewness of less than 0.5. I don't believe I used the batch mesher because this was orgininally an stl. I did use the reshape and reconstruct features in ANSA which generate a nice surface mesh, at least in all the parts I have inspected. I then use the batch mesher for the layers and volume mesher.

Focusing on the layers for a second, since that is where the problem lies, my problem is that (after several iterations of manually fixing the surface mesh) ANSA tells me I have a skewness of 0.85 and a fluent angle of less than 0.85 for the solids. Then when I take it to fluent it says I have a skewness of 1.0.

Because the model is so complicated and tortuous I can believe there are other areas that I need to fix, I just need to find them, and I can't do it manually. That's why I'm looking to their quality metrics to tell me where.

So with that being said, I'm looking for something to tell me where I need to repair. It looks like you are indicating jay maybe I should look at the warp on my cells before I generate my layers. Is that correct?

The concavity is something that also concerns me but I'm not sure how to address this one at a global scale.

Any continued advice is appreciated!
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Old   February 15, 2019, 10:13
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Hello again
If you have a complicated geometry it may be that if you aim for skew<0.85 you are very strict.
If you could relax your threshold values to say 0.9, I believe things will get better.

When you look for suitable quality criteria values, I suggest that you batch mesh the layers and volume mesh without any imposed quality criteria.
Then when the mesh is complete, set your values in F11 window and switch to Hidden mode to see how many and WHERE you have bad elements.

It may be in some proximities, in some concave angles.

When you then use FixQual, ANSA will try to fix these elements with small nodal movements. However if these elements are too many and are actually packed together in clusters, FixQual will not be able to save you and it may start to move the nodes in ways that do not help.

What I would consider as reasonable criteria for Fluent are
Fluent skew 0.92 to 0.96
Fluent angle 0.92 to 0.96
Jacobian 0.4
Left Handed
Negative
and maybe Layers Quality 1 to 1.1

And as you say you can also add Warp say 45

Have in mind though that
1) The more active quality criteria you put the harder for ANSA to fix
2) The more strict quality threshold values you put, the harder again
3) Before running Fix Qual check how many and where you have the violations

Can you give me some indicative numbers? For example how many cells and how many OFF before you Fix Qual. Also what kind of mesh do you have? Trias on surface and then prisms and tets? Poly mesh?

Best regards
Vangelis
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