[JuPa]

I’d like to create a guide for CFX users who use ICEM. I’d like some clarification from ICEM power users to help clear this age old issue.

CFX Recommendations for Mesh Quality

In the CFX modelling guide, the suggested measures of mesh quality are:

1. Mesh orthogonality
2. Mesh expansion
3. Mesh Aspect ratio

Mesh Orthogonality

The concept of mesh orthogonality relates to how close the angles between adjacent element faces or adjacent element edges are to some optimal angle (for example, 90º for quadrilateral faced elements and 60º for triangular faces elements). The most relevant measure of mesh orthogonality for the CFX-Solver is illustrated below. It involves the angle between the vector that joins two mesh (or control volume) nodes (s) and the normal vector for each integration point surface (n) associated with that edge. Significant orthogonality and non-orthogonality are illustrated at ip1 and ip2, respectively.



There are 6 orthogonality measures:

1. Orthogonality Angle
   
   Large values indicate good orthogonality, and this must be more than 20º.
   
   In ICEM I think this is called
   • Min ortho
   • Max ortho
   
   
2. Orthogonality Factor
   
   Area weighted average of all integration point surface scalar products associated with each control volume.
   Must be more than 1/3.
   
   Unclear what the ICEM equivalent is
   
   
3. Orthogonality Angle Minimum
   
   Same as point 1, but the minimum value. Must be more than 10º
   
   
4. Orthogonality Factor Minimum
   
   Same as point 2, but the minimum value. Must be more than 1/6.
   
   Unclear what the ICEM equivalent is
   
   
5. Minimum/ Maximum Face Angle
   
   CFD-Post only. Must be between 10 and 170º
   Must check your mesh in CFD Post to get this value.
   
   I don’t know if ICEM has an equivalent criterion.
   
   
6. Minimum/ Maximum Dihedral Angle (ANSYS ICEM CFD)
   
   Must be between 10 and 170º
   This is the only criteria where the CFX modelling guide refers directly to a mesh criteria in ICEM
   

Notes: the criteria for points 5 and 6 are the same – so it’s not too far-fetched to assume CFD-Posts Minimum/ Maximum Face Angle and ICEM’s Minimum/ Maximum Dihedral Angle are the same.

====================================

Mesh Expansion

The concept of mesh expansion relates to rate of change in the magnitude of adjacent element face areas or volumes.
The most relevant measure of mesh expansion for the CFX-Solver is illustrated below. It involves the ratio of the maximum to minimum distance between the control volume node and the control volume boundaries. Because this measure is relatively expensive to calculate for arbitrarily shaped control volumes, an alternative formulation, the ratio of maximum to minimum sector volumes, is used.



This measure and its acceptable values are tabulated below, along with a measure that is available through the CFD-Post post-processor. Values outside of the suggested acceptable range will increase sources of error that are due to the discretization of transient and body force terms.

There are 2 mesh expansion measures:

1. Mesh Expansion Factor
   
   Ratio of largest to smallest sector volumes for each control volume.
   Must be less than 20.
   
   In ICEM I think this is called:
   • Mesh expansion factor
   
   
2. Element Volume Ratio
   
   CFD-Post only. Must be less than 20.
   
   I don’t know if ICEM has an equivalent criterion.
   

====================================

Mesh Aspect Ratio

The concept of the mesh aspect ratio relates to the degree that mesh elements are stretched. The most relevant measure of aspect ratio for the CFX-Solver is illustrated below. It involves the ratio of the maximum to minimum integration point surface areas in all elements. Nodal (that is, control volume) values are calculated as the maximum of all element aspect ratios that are adjacent to the node.



The area based measure is tabulated below along with another measure that is available through the CFD-Post post-processor. Values outside of the suggested acceptable range will lead to round-off errors and associated difficulties converging the discretized equations.

1. Aspect Ratio
   
   Largest ratio of maximum to minimum integration point surface areas for all elements adjacent to a node.
   Must be less than 100 for single precision solver, or less than 1000 for double precision solver.
   
   Not clear what this is in ICEM.
   
   
2. Edge Length Ratio (CFD-Post)
   
   Same as point 1, but in CFD-Post.
   
   I don’t know if ICEM has an equivalent criterion.
   

====================================

Closing remarks:
If anyone has any input for this, or suggestions to look at any other mesh criterion for CFX then I’m all ears.
Credit to Ansys CFX Modelling guide for the pictures and literature.

[JuPa]

Any opinions anyone? I'm sure of this was ICEM guide for Fluent we's be on page 100 by now

[Hossam Alaa]

please, Is there any reference for these values (CFD-POST)?

[JuPa]

Yes, like I mentioned in the first post the reference is the CFX modelling guide. It explains what CFX requires to have a good mesh. Unfortunately the criteria CFX suggests does not match perfectly with the criteria that ICEM produces.

Both products are made by Ansys so I'm not sure why there's any disparity.

[JuPa]

I've done some more investigating.

I've concluded the most reliable way to determine mesh criteria for CFX jobs are to view the mesh in CFD Post.

[Hossam Alaa]

Yes I think it is the best way to check mech quality. Even you can make one iteration only then check the mesh

[JuPa]

That's what I have concluded. After generating a mesh in ICEM, I load the mesh into a new CFX setup. I don't change any settings at all, except for the maximum number of iterations (1 iteration).

I then post process the results in CFD Post to determine the mesh statistics. It's a bit long winded (takes perhaps an extra 5 minutes to do) but it's the only way to be sure.

I'm not sure why there is a huge disparity between what CFX asks for and what ICEM gives you.


There should be a configuration in ICEM, like there is in Ansys Meshing where you select your desired solver (CFX or Fluent) and it provides you with the necessary details:
1. Inflation: stair step (CFX) or layer compression (Fluent).
2. Orthogonal quality of the mesh based on your solver.
3. Skewness of your mesh based on your solver.
4. Max aspect ratio and cell expansion based on your solver.

[Hossam Alaa]

I am using Ansys meshing, but how can I select my desired solver (cfx or fluent)

[JuPa]

Sure:



Just change Fluent to CFX.

[Batis]

Hi,

It won't add much value to the main thread but one comment from my side. To check the mesh in CFX-Post you don't need to run simulation at all. Definition file can be opened directly in post processor.

[JuPa]

Fantastic I didn't know that. I was running 1 iteration to get a CFX .res file and then opening up in CFD Post.

[chiragsvnit]

This is indeed useful information and thank you.
Do you know what is the difference between "Orthogonality Angle Minimum" and "Orthogonality Angle" in cfx/CFD-post?
Regards,
Chirag.