Week 5.2 - Multi Level Meshing

AIM

The objective of the project is to create the multi level meshing to resolve CAD objects in the Icepak model and reduce the original mesh elements.

TASK

• Create a non-conformal mesh assembly and use the mesh separately option with slack values.
• Use set uniform parameters and use average on 1 assembly, in the multi-level meshing tab allow multi-level meshing up to level 2 for the objects.
• Generate mesh & observe mesh reduction.
• Do a visual inspection & check for quality.

Model

File --> unpack project --> hanging node.tzr - This opens the hanging node model and click on View --> default shading --> solid.

This model has two CAD models for which multi-level meshing is done.

i) sheet metal HS

ii) Guide sweep

The mesh refinement around CAD objects needs to be high in order to capture the model accurately. The model is first mesh using coarse settings. The model results in about 700,000 elements. Note that this mesh count results from using the non-conformal meshing technique, which reduces mesh bleeding and mesh count.

Mesh Control

Sheet metal HS

Guide sweep

The circular regions are the area in which the model is not properly captured.

The cut plane view of the meshing around the Sheet metal HS and Guide sweep objects.

In order to capture the CAD model the mesh is too refined around the CAD objects where refinement of mesh causes increase in the simulation time. So Multi level meshing is used mesh elements reduction.

Different methods to improve mesh resolution

i) Multi-level meshing

It enables local refinement from a starting Cartesian mesh generated using the max sizes you specified. Multi-level meshing is useful if you have a large variety of length scales in your model. The refinement can be controlled such that the initial mesh is refined in an area where the mesh needs to be finer in order to resolve some geometrical features or gaps. This will significantly reduce the overall mesh count while providing adequate mesh resolution where it is needed. Multilevel meshing applies to 2D and 3D cut cells as well as stair-stepped meshing.

• Max levels is the maximum number of levels that may be required to achieve the desired mesh refinement based upon curvature/proximity. For a given coarse mesh the desired proximity/curvature-based refinement may be achieved even before reaching the max levels.

• Edit levels enable you to specify refinement levels for selected objects. Per-object meshing levels can be used in conjunction with the max level and size functions.

When Enable per-object size function is enabled, the size functions (proximity and/or curvature) are used to subdivide the mesh around the object up to the max level specified for the object. When Enable per-object size function is disabled, the mesh is subdivided to the max-level everywhere on the object.

The objects around the Fan guide are grouped into individual assemblies for further reduction in the mesh elements.

ii) Mesh assemblies separately

It indicates whether or not ANSYS Icepak should generate a non-conformal mesh for the assemblies that have the Mesh separately option turned on. This option enables you to turn off (or on) all defined non-conformal meshing for the entire model.

A separate assembly for the objects around the Fan guide is created.

Sheet metal HS assy

The Mesh separately option is selected for this assembly and the following data are provided.

i) Slack setting

Min X - 2 mm.

Max X - 2 mm.

Min Y - 2 mm.

Max Y - 2 mm.

Min Z - 2mm.

Max Z - 2mm.

ii) Mesh type

Max element size X - 4mm.

Max element size Y - 1mm.

Max element size Z - 4 mm.

Min Gap X - 0.06 mm.

Min Gap Y - 0.06 mm.

Min Gap Z - 0.06 mm.

The Multi-level mesh option is selected and max levels are set to 2 under the options set unifrom mesh parameters is selected & use average is selected.

Fan guide

The Mesh sepaartely option is selected for this assembly and the folowing data are provided.

i) Slack setting

Min X - 2 mm.

Max X - 2 mm.

Min Y - 2 mm.

Max Y - 0.

Min Z - 2 mm.

Max Z - 3 mm.

ii) Mesh type

Max element size X - 5 mm.

Max element size Y - 2 mm.

Max element size Z - 4 mm.

Min Gap X - 0.01 mm.

Min Gap Y - 0.01 mm.

Min Gap Z - 0.01 mm.

The Multi-level mesh option is selected and the max levels is set to 2 and under options set unifrom mesh parameters is selected & use average is selected.

Mesh Settings

All the parameters are kept unchanged and the model is meshed by clicking on generate option.

The number of elements in the model has significantly reduced to 516225 elements (i.e) a reduction of 2 lakhs elements is done by creating multi-level mesh.

Visual Inspection

Sheet metal HS

Guide sweep

The mesh refinement has captured the CAD model more accurately. The Cut plane of the model around the CAD objects is provided below.

Mesh along X plane

Mesh along Y plane

Mesh along Z plane

Mesh Verification

Face alignment

• It is the measure of mesh quality defined by face alignment index = C0C1.f
• C0 & C1 are the centroids of two adjacent elements that are normal vectors to the face between the two elements.
• Range of face alignment 0 (bad) to 1 (good) & value greater than 0.05 (preferred) & value greater than 0.15 provide better results.

The min value of the model is 0.0676 and in order to get better results min value must be greater than 0.15 & in our model, the 15 elements were under 0.15 value.

Volume

• The preferable volume of mesh to be greater than 1e-13 for single precision solver & greater than 1e-15 for double precision.
• Max / Min cell volume should be less than 10^7 for single precision & must be less than 10^12 for double precision since a very small volume create divergence of the solution.

The volume of the mesh is greater than 10^-13 & Max / Min cell ratio is around 50000 which is in the required range & single precision is used for this model.

Skewness

It determines how close to the ideal & is based on the equilateral volume. A value greater than 0.25 provides fair cell quality.

The min value of the skewness is 0.005 and the quality below 0.25 have around 1000 elements this is due to the sharp curves around the guide sweep and sheet metal HS and max multi-level mesh is kept at 2 but further refinement will reduce the skewed elements around the corner of the objects.

Conclusion

The multi-level meshing technique is used in the model to capture the CAD model and also reduce in 2 lakhs mesh elements the generated mesh is visually inspected and also the mesh quality is checked using three criteria - face alignment, skewness, and volume. From these data, it is concluded the better resolution of CAD objects and reduction in mesh count is achieved using a multi-level meshing technique.