ANSA Week 3 Challenge - Meshing of Car Hood

OBJECTIVE

To mesh the given hood component after proper geometry cleanup and midsurface generation using the ANSA preprocessing software. Also, to ensure the generated mesh follows the given quality criteria and the corresponding thickness of each surface is assigned to the mesh.

QUALITY CRITERIA

 
THE COMPONENT

PROCEDURE

1. The part has 4 main components and they have different thicknesses (which are measured using the measure tool). They are as follows: Inner Panel - 0.75mm, Outer Panel - 0.75mm, Hinge Reinforcement - 1.2mm and Latch Reinforcement - 1.2mm.

2. Initially, the part will not have proper resolution so to ensure proper detailing and viewing, we can use the length option which can be accessed through the Mesh module. This tool works in creating a finer representation of the model based on how finely the length of the feature lines is broken up into. All the surfaces need to be selected to use this tool.

3. Before anything else is carried out, a geometry check and cleanup need to be carried out and ANSA has a tool that takes care of that. The checks tool can be accessed from the upper toolbar. Checks > Geometry.  With all components visible, the checks can be 'executed'. The geometric discrepancies will be highlighted by the tool and they can all be selected, right clicked and fixed by clicking 'fix' as shown in the screenshot.

4. After cleaning up (and reorienting the faces of the part - using the 'orient' tool), we can assign properties and property IDs to accommodate the midsurfaces that will be created. Doing this will also make it easier for viewing components separately. To create a property, go to properties from the upper toolbar and right-click the list. Then go to New > PSHELL > PSHELL to create a new property ID.

This opens a new window where we have the option to edit many of the particular property's attributes. In this case, we are to assign a thickness (T). We can go ahead and assign one of the two thicknesses. The process will be repeated for each of the components' midsurfaces. We can do the same and assign properties for the main components. The intention is to view the midsurfaces (and their mesh) and parent surfaces separately.

After assigning thickness and clicking 'OK', we can attach the property to the particular surface/component. This is done by right-clicking the particular property and clicking 'apply'. We can then select the surfaces that are to have that particular property and middle-mouse click/press enter to apply the property. Now the property has been assigned. Now that particular surfaces can be hidden by clicking the bulb icon beside it, making it easier to focus on certain surfaces while working on them.

5. Now, we shall generate the midsurface of each surface. This can be done using the 'midsurface' tool in the faces menu or using the 'offset' tool from the Topo module. The midsurface tool would be ideal here as the components have a constant thickness. The offset tool would be more effective in cases of varying thicknesses. For practice purposes, I went with the offset tool.

After selecting the offset tool, it will ask us to select the surfaces to offset. We can select the surfaces on one side of the component. After that, we can assign the offset value (half the thickness).


The offset tool will show the direction of the offset in the form of an arrow. If it is in the opposite direction of the required offset, the offset value needs to be in negative, as shown in the screenshot. If we proceed, the surface will be offset and we will have the option of keeping the parent surfaces.

The midsurface tool also requires surface selection but you would need to select all the surfaces of the particular parent component. Furthermore, there will be an option to not delete the parent surfaces on the bottom right panel.

The offset can then be assigned their respective property, through the 'apply' option discussed in the previous step.

6. After midsurface is generated for each of the components, we can go ahead and prepare to mesh the midsurfaces. To start off, we can assign the quality parameters as per the challenge. We need to access the 'mesh parameters' and 'quality criteria' options from the top toolbar. And then we can assign the values as per the table. The mesh type will be 'mixed' and minimum and maximum lengths would be 2mm and 7mm respectively.

The quality criteria can be entered as follows:

7. After that, we can go ahead and assign the target length. This is done by accessing the 'length' tool again from the Perimeters section under the Mesh module. We need to select the entire midsurface and assign the target length (which would be 5mm).

8. To facilitate better mesh flow and ensure good mesh quality, the midsurface needs further preparation with regards to proper surface cuts. This can help prevent tria formation.

Before we do that, we observe that each of these components can be separated into symmetric halves (mostly). To save on time, it's good practice to mesh half the component and reflect the mesh to generate the other half. So, we can go ahead and delete a half for each of the inner extract and outer extract components. Whereas for the hinge reinforcement, we can delete one of the two since they are mirror images of each other. The delete tool can be accessed from the top toolbar section.

Cuts can be made using the 'Cut' tool from the Mesh module (under macros). We can go ahead and try our best to split up the surfaces to rectangular regions to ensure the least number of trias. Existing feature lines that may hinder this can also be suppressed. This is done by right-clicking the lines using the same 'cut' tool.



In other cases, this needs to be carried out to prevent minimum length failure. The most famous example of this in the hood component would be that of the hemming region of the outer extract panel. The feature lines on either side of the hemming peak need to be suppressed to prevent element failure.

9. After preparing the surfaces, we can go ahead and mesh them. We need to use the 'best' mesh option, which can be accessed from the 'Mesh Generation' section. It's good practice to start from an inner region of the component with free edges (single cons) and mesh the surrounding regions in an outward direction. For example:

10. After meshing a region, analyze the mesh and see if it can be improved. If it can be improved, we can make use of the 'Reconstruct' tool from the Shell Mesh section in the mesh module. It basically remeshes the selected mesh keeping the quality criteria in mind and tends to produce a better mesh. The tool gives the option to rerun the tool so we can ensure the produced mesh is up to the standards we are looking for.



This process is repeated until we mesh the entire component.

Mesh quality can be checked by switching to hidden mode. The option is available on the bottom toolbar. The failing elements will be highlighted in the colours as per the legend on the side of the screen (of the quality criteria).

11. Sometimes, the reconstruct tool doesn't work in getting rid of some trias. Trias are okay but certain types of trias need to be avoided - like opposite trias or trias touching feature lines. Trias can be worked on using the swap, split, join and smooth tools. The first three are available in the 'Elements' section and the smooth tool is available in the 'Shell Mesh' section.

In the following example, there are trias touching each other and in close proximity:

In (2), the splits are created [as can be seen in (3)]. The adjoining trias are joined in (4) and all the surfaces are smoothed in (5).

The opposite trias are taken care of by splitting the quads between them using the split tool, then joining the two adjoining trias formed (involving the opposite trias) using the join tool. The resulting mesh can then be smoothed using the smooth tool to improve the mesh flow.

14. After all the regions to be meshed have been meshed, we can go ahead and work on transforming the component to produce the symmetrical region to complete the mesh for the entire component. This can be done by accessing the 'transform' tool from the top toolbar and selecting 'copy'. We can then select the entities that need to be symmetrically replicated. After that, we can move on to the next window and go to the symmetry tab and proceed without changing any of the options.


The half replicates and completes the component as shown:

Some geometry checkups and fixing may be required based on the complexity of the component (through Checks > Geometry).

15. Finally, as required, draw shell as solid needs to be activated. This can be done by going to Quality Parameters and accessing the Presentation Parameters tab. In that section, the 'Draw Shell as Solid' box needs to be checked.

MESHED COMPONENT IMAGES

OUTER EXTRACT PANEL


INNER EXTRACT PANEL

HINGE REINFORCEMENT

LATCH REINFORCEMENT

As seen, none of the components has OFF elements.

ALL MIDSURFACES ASSEMBLED:

There are 2624 trias out of the total 158727, meaning the tria percentage is 1.65%.

LEARNING OUTCOMES

1. Application of several versatile tools that are part of the ANSA ensemble.

2. Understanding of the more automated geometry cleanup process and other geometric tools used to facilitate meshing process in the TOPO module.

3. Understanding of midsurface generation options available in ANSA.

4. Understanding of meshing techniques, property creation and application, quality criteria and mesh cleanup procedures.

CONCLUSION

The given hood part was meshed as per required quality criteria, after having carried out topology cleanup and generated midsurfaces for each of its components and thickness was assigned to each of them.