Guidelines and Recommended Practices

Criteria File

When defining the Target element size, it is recommend to use either the Minimal height or the Minimal normalized height methods. These methods do not affect the quality of quads, and for the same mesh only the tria quality will be reduced with the minimum height method. Moreover, the use of the Shortest edge method may result in the creation of rhombus-like quads, which are only useful for special kinds of meshes.
After the Target element size, the most important aspects are the Min Size and Max Size. These have the highest impact on the final mesh output.
- The Min Size should not be too big with respect to the Target element size. The recommended size is 33% of the Target element size. On the upper end, a value of 40%-50% of the Target element size is also acceptable. A larger than recommended Min Size may trigger intensive cleanup that disrupts the mesh flow.
- The Max Size should not be too small. A value of 175% of the Target element size is reasonable.
- The range between Min Size and Max Size should not be too tight, otherwise there is limited ability to improve the mesh result.
A Warpage value between 20°-25° is recommended. A value of 15° may be too tight, over constraining the mesh generation. With a value of 15° or lower, violating quads may have some nodes be moved away from the geometry, or may be split into two trias, depending on the parameter file settings.
Skew, Min Angle, and Max Angle are dependent on each other. The Jacobian value also has an effect on the element shape. An element with a small Jacobian value, for example, will have poor angles. Calculating the Jacobian at the corner points is a more strict setting.
Taper is used for orthogonality. In some way, it controls the same outcome that angles and Jacobian also account for. Taper is most useful for solvers that have a direct requirement for it.
The reduction of tria elements is performed automatically by the Batchmesher, even when the criterion % of Trias is not enabled. Therefore, you will not see a large difference when changing this value with respect to the default Batchmesher result.
Careful consideration should always be given when changing criteria values. As many criteria are interdependent, over-constrained values can lead to poor mesh flow and mesh adherence to the geometry is likely to be disturbed.

Parameters File

Midsurface Extraction

The skin offset method is recommended whenever constant thickness parts without ribs/t-connections require mid-surface generation. This method is the fastest and most efficient for such models.

Geometry Cleanup - Surface Hole Recognition

For improved mesh flow, an even number of elements should be specified. Usually, this value is set to comply with the min and max element size requirements. While not recommended, it is permissible to set this so that the element size violates the min or max element size restrictions. The use of the auto setting is not recommended for holes with washers.
- The following formula can be used to calculate the resulting element size S given a hole radius R and number of elements around the hole N:
  $S = 2 R \sin (\frac{180}{N})$
  
  For example, a hole with a 2.5 mm radius and 6 elements around the hole will result in a mesh size of 2.5 mm.
- To not have elements smaller than the min element size around holes, use a radius threshold for removal > = 0.708*min element size.
Incompatible settings for holes with washers can lead to the creation of failed elements and/or outer washer ring elements with lengths significantly different from the target mesh size. This can lead to bad mesh flow and bad transitions to the regular mesh.
- It is not recommended to set the number of elements for a hole with washers to less than 6.
- Using minimal mode for # elems and auto mode for washers is recommended whenever possible, as it gives the most flexibility. With these settings, the outer washer element size is close to the target element size, and the washer elements comply with the quality criteria. However, washers will not be built if it is not possible to create elements of permissible quality, or the outer washer ring element size appears significantly larger than the target element size. Another side effect is that as a last resort, a washer may be built with an odd number of elements in order to build a good quality washer.
- When specifying an exact number of elements for holes and/or a fixed number of elements, or radius fraction, for washers, consider the implication to the element quality, aspect ratio, min/max size, Jacobian, and the outer washer ring element size, using the formula as above.
- It is possible to create washers with failed elements and/or with an outer washer element size different from the target element size. This is done by defining washers as a fixed number or as a radius fraction. Use exact mode for # elems if it is expected that the outer ring element size is more than 140% or less than 60% of the target element size. In addition, when a hole is set to high priority, washer elements are not modified to correct for failed element quality. If a hole is set to normal priority, washer nodes are allowed to move to correct the quality.
Use the Attempt to maintain narrow slots rounded ends using >= 6 elements option only if absolutely necessary. Meshing of small areas produced by this option will often have a negative impact on the mesh flow.

Geometry Cleanup - Solid Hole Recognition

This should be disabled for models having only shell geometry, as the recognition of the solid holes is potentially time consuming.
Define even numbers of elements around holes, trying to comply with the min element size criterion and trying to have the element size as close to the target element size as possible.

Geometry Cleanup - Surface Fillet Recognition

The following formulas define how the fillet geometry parameters and the mesh density are related:
$W = R α; W_{c h} = 2 R \sin \frac{α}{2}; S = 2 R \sin \frac{α}{2 N}; D = R (1 - \cos \frac{α}{2 N});$

W

Fillet arc width

R

Fillet radius

$α$

Fillet arc angle (radians)

W_ch

Fillet chordal width

S

Element size across the fillet

N

Element density across the fillet

D

Chordal deviation
Enabling the Minimize transitions option is highly recommended. However, in very rare occasions it may create rows of strongly skewed elements. This may happen for fillets having multiple irregular fixed points on the fillet sides, or multiple intersections of those edges. For such cases, it may be disabled to improve the result.

Geometry Cleanup - Flange Recognition

It is recommended to use a minimum of two elements across the flange width. The actual number of elements is determined internally by accounting for restrictions on the min and max element sizes, as well as the aspect ratio.
A reasonable upper limit for the maximum flange width is around (N + 1.5) * element size, where N is the number of elements across the flange. Similarly, a reasonable lower limit is two times the min element size.

Geometry Cleanup - Bead Options

The Suppress beads with height < value is dependent on specific modeling requirements. Typically, this is set around 20% of the target element size, but not larger than the criteria minimal element size.

Geometry Cleanup - Logo Options

For logo removal, a Concavity factor value of 2.0 is recommended for logos with complex lettering. For logos with simple shapes, though, a value of 1.0 or even 0.0 may be used. In this case, to reduce the danger of removing valid features, assign height < a value no greater than the Suppress beads with height < value.

Geometry Cleanup - Other Options

The Remove edge fillet with radius < recommended upper limit is between the min element size and the target element size.
Flat feature suppression level is recommended to be set at “low” with “element size” as Feature character size.
Suppress edges by proximity value is generally kept less than minimum element size considering node movement tolerance.

Meshing Cleanup

For most applications, it is recommended to use the mixed type for Create mesh with element type.
Note: When the quad type is set, a specific algorithm is used which may have negative effects on the mesh flow and may create undesirable mesh patterns, for example, three quads sharing a node.
- Enabling both the align and size mesh flow options is highly recommended, as they have minimal negative side effects.
All the feature edges kept after geometry cleanup are respected during meshing and post meshing cleanup. Nodes are moved only for the failed elements considering allowable limit defined in parameters controlling the node movement (across shared and free edges).
The method chosen for Apply optimized smoothing effects the capabilities of all post-mesh element cleanup operations. The across edges mode is set by default and is strongly recommended, as it allows for the most flexibility in correcting the mesh. Other options are likely to result in a high rate of failed elements because:
- Geometry cleanup and defeaturing cannot address all problems.
- The across edges mode attempts cleanup steps in the following order:
  - Move nodes only along surface edges.
  - Move nodes only across non-feature edges.
Remaining failed elements are fixed using small constrained node movements for feature edges, including strong feature edges. The only exception is that user preserved edges are excluded from such movement. The Correct features parameters provide control over these movements.
- For Move nodes across shared edges, max dist <, the suggested value is 10% of the target element size.
- For Move nodes across free edges, max dist <, the suggested value is 5% of the target element size.
For correcting warpage of quad elements, it is recommended to enable the Offset nodes from surfs, max dist <, and Divide quads into trias options.
- The recommended value for the offset distance is 10% of the target element size.
- When both options are enabled, warped elements are first fixed by nodal movements normal to the geometry, followed by splitting any remaining failed quads.
The recommended value for Feature angle during elements cleanup is 20-30. A lower value may help with better feature representation, but a value lower than 15 may significantly increase the rate of failed elements.