Simcenter STAR-CCM+ What are some meshing guidelines to follow to reach a good and healthy convergence?

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What are the best practices / requirements for the mesh when we want to reach a good primal convergence in order to run the adjoint solver


How to reach a good and healthy convergence for any simulation or even for shape optimization with adjoint? To get a good convergence, setting correctly the physics models, boundary conditions, and the solver parameters is paramount. However, these settings must also be applied to a high-quality mesh.
The following recommendations are valid for polyhedral and trimmed mesh. After the mesh generation, the mesh must be checked and improved according to the following quality metrics, to which the solver is sensitive:
  1. Volume change
  2. Aspect ratio
  3. Chevron cells
  4. Prism layer retraction
  5. "Potato chips" cells
  6. Other recommendations

The above cell quality metrics are listed and defined in  Pre-Processing > Meshing > Meshing Field Functions Reference
You can identify cells based on these metrics using the technique illustrated in Pre-Processing > Meshing > Checking the Volume Mesh > Identifying And Visualizing Bad Cells
The volume change is the ratio between a cell and its biggest neighbor. In general the volume change is bigger at the transition between prism layer and core mesh when you use a Low Reynolds meshing approach, as illustrated below:
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Try to avoid having a volume change less than 10e-2. See also: Pre-Processing > Meshing > Checking the Volume Mesh > Mesh Quality
To avoid higher volume changes than 10e-2, set the mesh size on boundaries carefully. 
If you set mesh sizes that are too different on boundaries that are close one to another, you would have to change the default values for  Mesh Density in order to get a smooth transition between the larger size cells and smaller ones. See also: Setting Mesh Tet/Poly Density
In addition, you can reduce the Surface Growth Rate to improve the volume change and the mesh quality. See also: How to use the surface curvature and growth rate meshing options
In order to avoid high volume change between prism layer cells and core mesh cells, you can use the Near Core Layer Aspect Ratio option in the Prism layer mesher model. Set the value to for example 0.5. See also: How to use the Prism Layer setting Near Core Layer Aspect Ratio (NCLAR)
For Trimmer mesher, you can also use the option Maximum Core/Prism Transition Ratio in the default values of your automated mesh operation. See also: Pre-Processing > Meshing > Volume Meshers > Prism Layer Mesher > Global Controls > Maximum Core Prism Transition Ratio
A prism layer must be used on all walls. For a Low Reynolds meshing approach, a fine grid should be used in the boundary layer in order to resolve it correctly. This practice in turn means that you can have high aspect ratio cells.

When using the coupled solver, you must avoid high aspect ratio cells (higher than 50-75). 

For example, the following mesh is not recommended:
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In order to avoid high aspect ratio cells, choose carefully the prism layer parameters.
Chevron cells are pairs of thin slender cells that share a common face whose angle is such that the line joining the cell centers does not go through the common face. An example is shown below:
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In that case, refine the mesh at the sharp edge. See also: How to control the prism layer behavior around a sharp trailing edge? 
When two surfaces are close, the prism layer can retract and some layers can be removed.
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In order to avoid reduction of the prism layer, you can reduce, as below, the Minimum Thickness Percentage and Layer Reduction Percentage options. See also: What are the implications of changing the "Minimum Thickness Percentage" of the prism layer? and How does the Prism Layer Reduction affect the prism layer mesh?

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5. "Potato chips" cells
On curved surfaces with thin prism layer cells, depending on the mesh parameters, one can get highly concave and thin cells, also called potato chips, as illustrated below:
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To improve the mesh quality of this type of cells, you can either decrease the prism layer stretching ratio in order to increase the thickness of the first cell, or increase the surface mesh density.
See also: Meshing > Volume Meshers > Prism Layer Mesher > Prism Layer Model Properties > Working with the Stretching Function

6. Other recommendations
Below, some others tips & tricks about mesh which will help you to improve its quality.
  • Surface mesh quality
In order to improve the volume mesh, you should improve the surface mesh. 
Increase the surface Minimum Quality to 0.2 (maximum value) and deactivate the Aligned Mesh option if it is necessary. See also: Meshing > Surface Meshers > Surface Mesher Controls and Values
  • Polyhedral Optimizer
            In order to improve your mesh is it also recommended to change the default values for the Polyhedral Mesher Optimizer as below (for versions before 2019.1):
  • 3 Optimization Cycles instead of 1
  • 0.7 for the Quality Threshold instead of 0.4.
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For the version 2019.1 and later, these properties of the optimizer are available in the node default values of your automated mesh operation in the sub-node called Core Mesh Optimization​​​​​​. See also: Pre-Processing > Meshing > Volume Meshers > Volume Mesher Controls and Values
  • Polyhedral & Trimmer Post Optimizer
    From 13.02, you can also enable the option Optimize Boundary Vertices in Polyhedral or Trimmer expert properties.
From 2019.1, this optimizer is enabled via the expert properties of the Polyhedral/Trimmer model named Run Post Mesher Optimizer. Thus, you can enabled two sub-options in the sub-node of default values named: Post Mesh Optimization. The two sub-options are Optimize Boundary Vertices and Optimize Cell Topology. See also: Pre-Processing > Meshing > Volume Meshers > Volume Mesher Controls and Values
  • Mesh quality
In order to check the mesh quality, you can use the Mesh Diagnostics tool. See also: Pre-Processing > Meshing > Checking the Volume Mesh > Running Mesh Diagnostics
In addition, you can use the Field Function Bad Cell Indicator. This Field Function shows the cells where the Cell Quality Remediation would be activated, so where the mesh quality is insufficient for the coupled solver. The Cell Quality Remediation model must not be used to solve the primal solution or the adjoint flow solver, but only for mesh quality post-processing. To use it, select the Cell Quality Remediation model in your Physics Continuum. Initialize the solution and create a threshold to extract the cells which have a value equal to 1  or 3 for the field function Bad Cell Indicator. If the threshold contains many cells, continue improving your mesh. Once the mesh improvements are complete, deselect Cell Quality Remediation in your Physics Continuum.
See also: Simulating Physics > Remedying Cell Quality > Cell Quality Remediation Field Functions Reference
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