Tutorial and comparison of manual meshing and automatic meshing methods (Casting and Batch mesh) in ANSA

Objective:

The objective of this project is to do preprocessing, i.e., geometry cleanup, midsurface extraction, and shell meshing for a sheet metal component using the manual and automatic methods in ANSA. The component will be meshed by extracting the mid-surface using the Skin option, Casting, and also using the Batch mesh option to understand the pros and cons of all methods and compare them based on their performance, feature capturing, and mesh quality.

Mesh generated with all three methods needs to follow the quality criteria given below,

Introduction:

The component on which we are going to work in this project is a thin-walled sheet metal component, i.e., one dimension is very small compared to the other two dimensions. For such components, it is preferred to create a 2D mesh rather than a 3D mesh for FEA. By doing so, we save a considerable amount of time in processing & still get similarly accurate results as from 3D mesh. This is achieved by extracting the mid-surface of the component & assigning thickness to it to capture the original thickness. And then the mesh is created for that mid-surface.

Sheet metal components are extensively used in the automotive and aerospace industry. And time is of the essence in industries to design these components. Therefore, it is the responsibility of the FEA engineer to choose a method that gives the best result while minimizing the time to finish the task. As for preprocessing, i.e., midsurface extraction and 2D meshing, ANSA provides three methods, one manual, and two automatic methods. And, the best method must be used based on the given job.

Therefore, in this project, the midmeshing of a sheet metal component is demonstrated with all three methods based on the same quality criteria to understand and compare them based on their performance, feature capturing, and mesh quality.

Procedure:

First, the given model is imported into ANSA. The image below shows the model that needs to be meshed,

After importing the model, the first step in preprocessing is to perform geometry cleanup and ensure that the geometry is error-free. This step is common to all three methods. To ensure error-free geometry, first, the Topo tool is used by going to Topo>Faces>Topo, and the complete model is selected. This tool ensures that all the edges are stitched properly. Next, geometry check is done, by going to Tools>Checks>Geometry. There were a few errors that were fixed automatically.

The next step is to extract the midsurface and mesh it, which is demonstrated with the three different methods below.

1. Manual method

For manual meshing, midsurface can be extracted manually using the offset tool or automatically using the skin tool. Here it is extracted using the offset tool by going to Topo>Faces>Offset>Faces and all the surfaces on either face of the component are selected. The thickness of the component is 1mm which can be checked using the measure tool by going to Utilities>Measure. So, 0.5mm is given as the offset value and the complete face is offset to the middle. The extracted midsurface is assigned to a new PID by going to Topo>Faces>Set PID and a new PID is created. By default, the thickness of a new PID is 1mm. The image below shows the extracted midsurface.

Next, the extracted midsurface is checked properly for any error and none were found. The next step is to prepare the midsurfaces for meshing. For that, switch to the Mesh module. First, the shell element length is altered by going to Perimeters>Length and all the CONS are selected and target element length 3 is fed as length value. The same is done for all the Macros (surfaces).

Next, all the areas are checked for minimum element length and altered where elements might fail for minimum length using the Macros>Cut. Similarly, CONS which are not essential to define a feature are removed by joining the surfaces using the Macros>Join tool. This component has a lot of complex features. Some of such regions where the element would fail for minimum length and modifications done are shown below,

Next, all the holes were aligned parallel to the nearby edge using the slide tool available at Mesh>Perimeters>Slide, as shown below,

Before meshing, mesh parameters and quality parameters are fed into ANSA. Mesh parameters are configured by going to Utilities>Mesh parameters and quality parameters are configured by going to Utilities>Quality criteria. Configured parameters are shown in images below,

The next step is to mesh the midsurface. Meshing is done surface by surface using the Best tool available under Mesh>Mesh generation>Best and, if required surfaces were further divided strategically to obtain a good mesh flow. After, generating mesh, the mesh is reconstructed by going to Mesh>Shell mesh>Reconstruct tool wherever required to improve the mesh and eliminate any off (failing) elements. After reconstructing the mesh smooth tool is used to improve mesh flow and eliminate any off elements. 

It is ensured that there are no rotating quads. Also, trias are kept as low as possible. While dealing with trias, the following things are ensured.

• No back to back trias touching each other
• No opposite trias
• No trias touching the edges
• No trias on a feature
• Low concentration of trias in a region

To deal with trias, the following tools are used,

• Grid>Paste, to paste the nodes and move tria backward
• Elements>Split, to split an element and move tria forward
• Elements>Swap, to move the tria left or right
• Elements>Join, to join two trias to make a quad

The complete mesh was generated with 2536 elements with just 30 trias and zero off elements. All the features are also captured properly. The image below shows the final mesh generated.

2. Casting (Automatic method)

The next method with which the component is shell meshed is using the Casting tool available at Topo>Faces>Mid Surface>Casting. This is a completely automatic tool that generates the midmesh in a single shot without extracting a midsurface first. Therefore, in the final result, we only get a FE model of midmesh and not the geometry (surfaces). This completely automatic function is used to create FE shell elements that represent the middle skin of a solid model. The process to generate the midmesh with the casting tool is as follows:

• Activate the Casting tool by going to Topo>Faces>Mid Surface>Casting
• Select all the faces that compose the solid model
• The Automatic Middle-Surface Mesh Generator window pops up with the following parameters,
  • Minimum Thickness: Give the minimum thickness of the solid
  • Element Type: Choose the element type for the FE mesh.
  • Target element length: Define the Target element length.
  • Collapse perimeters distance: Choose the distance under which perimeters will collapse. It can be given as an absolute value or as a percentage of the target length. 
  • Collapse junctions distance: Choose the distance under which triple bounds will collapse. It can be given as an absolute value or as a percentage of the target length. 
  • Sharpen fillet with radius <: Enabling this feature will defeature all the fillets with a radius less than given by the user. 
  • Connect weldings: After the Casting function is executed the meshed parts will be extended and connected. 
  • Intersection angle: This parameter tells ANSA where to do modifications in geometry based on intersection angle. 
  • Stop at geometry errors: A geometry check will be performed before the middle skin extraction. If any errors exist, then the procedure will stop.
  • Handle as single solid: In cases of two closed volumes the one inside the other, the middle skin will be calculated in the space between the two. 

The image below shows the parameters configured for midmeshing using Casting,

The image below shows the midmesh generated,

As can be seen in the above image, the mesh is not of good quality and has a lot of off elements. Also, the feature capturing is not good in a lot of regions. To repair the mesh, manual intervention is required. So, the complete mesh is reconstructed using the reconstruct tool and smooth tool. There are also regions with back to back trias touching each other, opposite trias, trias touching on feature or touching the free edge. They were also repaired as done in the manual meshing method.

In the following images, some regions with bad mesh and corrected mesh are shown,

The final mesh generated is shown below.

After manually reconstructing the mesh, all the off elements were eliminated. Trias are reduced from 139 to 37 only. Though the features are still not perfectly captured in some regions, it is still much better than what ANSA gave initially. The reason for that is that there was no geometry associated with the mesh and therefore, altering the mesh around free edges becomes very difficult.

3. Batch meshing (Automatic method)

Batch meshing is another method available in ANSA to generate mesh automatically. For this method first midsurface is extracted either manually using offset tool or automatically using skin tool. And then Batch meshing is done for the extracted midsurface. Batch mesh tool automatically performs geometry cleanup and generates mesh based on the meshing parameters and quality criteria. Unlike the casting method, this method provides detailed options to control how ANSA would generate the midmesh. The batch mesh tool allows to create customizable batch meshing sessions to perform automatic mesh generation on Geometry and FE data. In addition, it provides the ability to manage flanges, holes, fillets, tubes, and perform de-featuring.

The first step is to extract the midsurface for the component. This is done using the skin tool available at Topo>Faces>Mid surfaces>skin and complete component is selected. ANSA automatically extracts the midsurface and assigns it to a new PID with 1mm thickness. The extracted midsurface is then checked for any geometrical errors and fixed. The image below shows the extracted midsurface,

From here on, ANSA will handle the geometry cleanup and meshing itself. To perform the batch meshing, go to Tools>Batch mesh and click on New>Meshing Scenario. Next, add the midsurface PID to be meshed in contents as shown below.

The image below shows the Batch mesh manager window after adding the PID and configuring the mesh parameters and quality criteria.

Mesh parameters and quality criteria were kept the same as used for the manual meshing previously. Then click on Run. And ANSA automatically generates the mesh. The generated mesh is shown below,

In the generated mesh there was only one element failing. Feature capturing was not good. A lot of features were defeatured.  To repair the mesh, manual intervention is required. So, the mesh is reconstructed wherever required using the reconstruct tool and smooth tool. Tria management was also not good, i.e., back to back trias touching each other, opposite trias, trias touching on feature or touching the free edge were present. These issues were also rectified as done in the manual meshing method. Geometrical changes made by ANSA were not reversed, only mesh quality was improved manually.

The image below shows some issues in the automatically generated mesh, and the next image shows the reconstructed mesh.

Most of the fillet features were also defeatured by ANSA which was not necessary, as shown in the image below, where the orange edge represents the toggled/joined double CON,

These geometry modifications are not reversed and kept as same, during reconstruction.

The final mesh generated is shown in the image below,

The final mesh has zero off elements. Trias are also reduced from 185 to just 37. And an overall better mesh is achieved.

Conclusion:

A quick comparison of all three methods is given below,

Therefore, manual meshing was found to be the best method based on mesh quality and feature capturing as other methods also require manual reconstruction at last. But, if quick meshing is required then casting and batch meshing can be preferred.
Also, if a component is geometrically simple than, the automatic methods would give good result in less time and should be preferred. But if the component is very complex, then the automatic methods might take same time or even more to generate a good quality mesh.

Though it is hard to say which of the automatic method would give better results as that depends on the component but still, batch meshing gives more control options and would work fine on most of the components. And, if time is not a constraint, then manual meshing should be preferred.