[Chander]

How to interpret the pre-mesh quality criteria in ICEM.
For example, what are the minimum allowable values of criteria like Angle, 2X2X2 determinant, Skewness, etc. ?
Is there somewhere a table of such values which the user must ensure that the mesh satisfies?

[pipolaki]

Hello,

Unfortunatelly, there is no answer on this old topic... ... and I am very interressted by this problem of quality criteria. Does anyone has a personnal experience or any information on this point :
- list of important parameter to monitor ;
- value of the criteria to never exceed ;
- range of "good" value for this criteria ;

Thank you for any help that will be provided.

Originally Posted by Chander

How to interpret the pre-mesh quality criteria in ICEM.
For example, what are the minimum allowable values of criteria like Angle, 2X2X2 determinant, Skewness, etc. ?
Is there somewhere a table of such values which the user must ensure that the mesh satisfies?

[BrolY]

Determinant and min angle are the most important criteria.
You can also include aspect ratio, but from my point of view, it's more related to refinnement than quality.

Also, those critera depends a lot on your solver. I've read that skewness is very important for Fluent.

Usually, you want a determinant > 0,2 at least, and a min angle > 18°. But Fluent or CFX works with a min angle of 9°, or even lower ! Just check your results in the area where your min angle is lower than 18° to see if everything is ok.

[Far]

First of all these values are relative. There are many values which are not acceptable to Fluent but CFX. For example fluent is more tolerant of angle issue than the CFX. But Fluent does not like the pyramid shape and CFX does not mind it because of its internal working.

Keeping in view they have (meshing guys) have devised some metrices with factor of safety. Following are the most important quality metrics with thier range.

Angle : Must be greater than 18 deg (But some times Fluent sometimes can work as low as 9 deg)

Quality : Must be greater than 0.3. But quality of 0.2 or greater can be tried and if solver doesn't mind it than carry on.

Expansion rate: Change of cell volume with respect to neighbouring cells. Should be less than 10 (or 20 check the CFX of Fluent manual).

Skewness : (Fluent) Must be less than 0.8. Up to 0.95 is acceptable.

Aspect Ratio : Must be less than 100 for single precision solver and 1000 for double precision solver. But due to implementation of better algorithms, I have tested and found that AR up to 8000 is OK in boundary layer with no impact on the solution. Moreover it is the characteristics of boundary layer to have the smaller cell size in the normal direction where flow gradients are steep and have bigger cell size in stream wise direction as flow gradients are not sharp. This approach is not valid for the transition prediction where you requires ten time more mesh points in the vicinity of transition.

Important thing to remember:

If you are using Fluent then make the mesh which has orthogonal quality greater than 0.01. To ensure this you need to have the

1. Min quality greater than 0.3
2. Angle greater than 18
3. Smooth cell size change


For CFX, you need to ensure this in ICEM:

1. Min quality greater than 0.3
2. Min angle 18
3. Smooth cell size change.

[pipolaki]

Originally Posted by BrolY

Also, those critera depends a lot on your solver. I've read that skewness is very important for Fluent.

First of all, thanks for all your answers.

I totally agree with you, the question of the solver is important and I should have mentionned it. In my case, its Fluent and I also heard that skewness is important. But... which skewness are we talking about ? In the quality histogram of ICEM I can see there are three kinds of "skewness" :
- skew ;
- TGrid skew ;
- Equiangle skewness.

I guess we are talking about TGrid skew which have an inverted scale (1 indicate a degenerated element so having cells over 0.8 is not good).
Sorry if this question seems very basic for you but I'm still perplex.

[pipolaki]

I think I have an answer to my previous question.

I red the ICEM Help manual and I found p 404 the definition of the Equiangle Skew. I also red the Fluent user's Guide and I found p 1550 that the definition is different. In fact we have :

Equiangle skew (ICEM) = 1 - Equiangle skew (Fluent).

So my first impression was wrong, TGrid skew is not the skewness we are talking about. I think that the value of skewness computed in Fluent and which should not exceed 0,8 is, in fact, related to the equiangle skew in the ICEM quality histogram.

[energy382]

Originally Posted by Far

First of all these values are relative. There are many values which are not acceptable to Fluent but CFX. For example fluent is more tolerant of angle issue than the CFX. But Fluent does not like the pyramid shape and CFX does not mind it because of its internal working.

Keeping in view they have (meshing guys) have devised some metrices with factor of safety. Following are the most important quality metrics with thier range.

Angle : Must be greater than 18 deg (But some times Fluent sometimes can work as low as 9 deg)

Quality : Must be greater than 0.3. But quality of 0.2 or greater can be tried and if solver doesn't mind it than carry on.

Expansion rate: Change of cell volume with respect to neighbouring cells. Should be less than 10 (or 20 check the CFX of Fluent manual).

Skewness : (Fluent) Must be less than 0.8. Up to 0.95 is acceptable.

Aspect Ratio : Must be less than 100 for single precision solver and 1000 for double precision solver. But due to implementation of better algorithms, I have tested and found that AR up to 8000 is OK in boundary layer with no impact on the solution. Moreover it is the characteristics of boundary layer to have the smaller cell size in the normal direction where flow gradients are steep and have bigger cell size in stream wise direction as flow gradients are not sharp. This approach is not valid for the transition prediction where you requires ten time more mesh points in the vicinity of transition.

Important thing to remember:

If you are using Fluent then make the mesh which has orthogonal quality greater than 0.01. To ensure this you need to have the

1. Min quality greater than 0.3
2. Angle greater than 18
3. Smooth cell size change


For CFX, you need to ensure this in ICEM:

1. Min quality greater than 0.3
2. Min angle 18
3. Smooth cell size change.

Based on my experience:

1. I never take quality as a criterion. Most important criterions are cell angle/skewness

2. CFX can handle angles >9° in almost all cases (even if Ansys guys always say, that it have to be at least 18° )

[attiquejavaid08]

Originally Posted by Far

Important thing to remember:

If you are using Fluent then make the mesh which has orthogonal quality greater than 0.01. To ensure this you need to have the

1. Min quality greater than 0.3
2. Angle greater than 18
3. Smooth cell size change

What could be the reason for this? any idea/suggestion please:

In ICEM
Minimum quality for:
Determinant 3x3x3: 0.462
Equiangle Skewness: 0.304
angle: 27.8
Quality: 0.459

after converting into unstructrued:

In FLUENT when i read mesh,

Mesh Quality:
Minimum Orthogonal Quality = 1.62236e-03
(Orthogonal Quality ranges from 0 to 1, where values close to 0 correspond to low quality.)
Warning: minimum Orthogonal Quality below 0.01.
Maximum Ortho Skew = 9.98378e-01
(Ortho Skew ranges from 0 to 1, where values close to 1 correspond to low quality.)

If Equiangle skew (ICEM) = 1 - Equiangle skew (Fluent).
then maximum orhto skew is supposed to be around 0.7 in FLUENT. correct me please if i'm wrong.

EDIT: so actually, ortho skew = 1 - orthogonal quality
it means, all the skewed elements in domain under my consideration are at wall. because i've very low orthogonal quality when i try to reach for certain y+ by setting first cell height. i'm stuck

[Lilly]

Dear all,

I have got some questions which are pretty close to that of attiquejavaid08. I just meshed quite a complex geometry and managed to reach the pre-mesh limits which are mentioned above.
My final pre-mesh has a quality of

• min det 3x3x3: 0.314
• min angle: 19.17
• min equiangle skewness: 0.213
• min quality: 0.314

After converting the mesh into an unstructured mesh in ICEM I get really bad results for some quality parameters:

• min quality: -0.439
• min Determinant: -0.439

Is there any explanation for this? I think i once read, that the quality calculation for the unstructured mesh at ICEM is not that credible, but I cannot find it anymore.

Can somedbody comment on that or does it really mean that my grid gets that bad after the conversion?

By the way, the ortho quality and the aspect ratio of that unstructured grid are the same for ICEM and at Fluent.


After reading this mesh into Fluent I get the following quality:

• min ortho quality: 0.214
• max cell squish: 0.786
• max aspect ratio: 52.9
• min expansion ratio: 0.157

I hope this is sufficient good again? I just found that the ortho quality should be larger than 0.01 in Fluent, but are there any more limits for the quality parameters in Fluent (I couldn't found any at the manual)?
How can I calculate the orthogonal skewness with Fluent (or is it equal to the cell squish index?)? Even if I set "/mesh/check-verbosity 2" I only get the quality parameters mentioned above.



Thanks a million for any help in advance!
Lilly

[HAYALL]

hi guys
for me my solver will be DES so any recommendation about which criteria for the mesh equality should I consider?
Regards
HAYDER

[cfdesf1990]

The skewness in ICEM should be close to 0 or 1? I am confused because of the following statement in ICEM guide. Generally, it is mentioned that skewness should be near zero but here I think it is the opposite

Eriksson Skewness
This is an empirical criterion, obtained for a hexahedral element by dividing the volume of the closest
parallelepiped by the product of its edges. It measures the shear of the parallelepiped closest to the
current element using least squares approximation. The default range of values is 0–1. Generally acceptable elements have skewness ranging from 0.5 to 1

Does any body have any idea?

[Far]

that value is opposite in ICEM CFD as compared to Fluent and not directly comparable!!!

For Fluent : 0 = Best, 1 = Worst. Should be less than 0.93 for good accuracy and convergence.

For ICEM CFD :
From help :

The skew is
defined as one minus the ratio of the shorter diagonal over the longer
diagonal. Thus, 0 is perfectly rectangular, and 1 represents maximum
skewness.

Eriksson Skewness

This is an empirical criterion, obtained for a hexahedral element by dividing the volume of the closest parallelepiped by the product of its edges. It measures the shear of the parallelepiped closest to the current element using least squares approximation. The default range of values is 0–1. Generally acceptable elements have skewness ranging from 0.5 to 1.

Important:

Note1 : ICEM CFD: Please concentrate on min angle (should be greater than 18) and determinant 2*2*2 (should be greater than 0.3 for volume mesh and 0.6 for surface mesh)

Note2: Fluent: Look for orthogonal quality and it should be greater than 0.01. 1 is the best. It combines all quality metrics in one metric.

[cfdesf1990]

Thanks for your good response.
Actually, I am using CFX for my simulation, and the problem is with the negative elements since the case in moving mesh. Based on other threads, one possibility is the bad skewness. I reduced the mesh size and time step, but neither of them were helpful.

Do you have any suggestion?

[Far]

further redcue the time step and study other parameters. Can you show the mesh pic and some info which zone is moving in relative manner, sometimes mesh is just fine and it is the way you divide the domain.

Can you try in Fluent which has re meshing algorithm.

This was done in CFX
https://www.youtube.com/watch?v=uZeqHBJuM1o