3D Finite element Meshing on rear view mirror (Tetra Mesh)

Aim: -  Identify the components geometry and perform solid mesh (Tetra) on the component using a 3D mesh Deck followed by the given element Quality criteria. 

 

Objectives:-

-Is to understand the Tera mesh/ 3D mesh deployment over the model.

-Understand the tetra elements.

- Understand the difference between Manual mesh and the Batch meshing.

- Understand the Tetra Rapid and Tetra Rapid algorithms.

 

Methodology:- 

1) Import model and Go for Length option from the mesh module (Deck) to find out the actual geometry of a model

2)perform topo on it. As the model is too distorted after importing from different software. Topo helps to merge the adjacent surfaces and clear most of the errors in the process.

3) After visualizing the model, we get the idea about which category model will fall into. Like sheet metal/plastic/casting part. So, we can choose the meshing option accordingly.

4) For the sheet metal part which has a thickness of less than 6 mm or measurable thickness, we can go with the Shell Mesh option.

5) For casting / plastic part if the part having thickness more than 6 mm, solid meshing is preferred.

6)Check for geometrical errors and clean them.

7) After doing Geometry clean up, now we are ready to do meshing by preferring any of the above methods (Shell mesh / Solid mesh) as per the given model. (Here we are preferring solid mesh as the given model have a thickness more than 6 mm in some areas but we are going with 3d mesh all over the model)

8) Did solid meshing (tetra) by using a mesh deck. The next thing is defined volume and apply the volumetric mesh. For that, we can go to 'V. mesh' deck.

The Rear-view Mirror model

Here we are focusing on the given rearview model.

I had imported the given component as known as The Rear-view Mirror model.  This was a plastic component and as we all know the plastic component has different thicknesses through its whole geometry.

Also, as we discuss in basic steps for the component which have thickness more than 6 mm or complex geometry or both, the solid meshing is a preferred option. 

Because its less time consuming and give better result than shell mesh.

 

Quality Criteria

 

There are two categories in Solid meshing

• Tetra Meshing: - The tetra elements have 4 nodes as per the below image.
• 

We are using tria mesh here and for better flow pattern ortho tria on the outer surface of the part. So basically, on the surface, we do a normal Shell mesh but using Tria / ortho tria elements only. The basic idea is to create a close volume using shell mesh and then fill that volume using Tetra elements which are nothing but 3d / solid elements.

For example, take a cube. it has 6 sides. So, we do shell mesh on each side/surface of the cube and then fill it's inside with tetra elements.

These tetra elements only follow the surface mesh for the first layer of mesh then it just fills the entire volume without following any pattern that’s why it’s also called Unstructured Mesh. The inside tetra element doesn’t follow the surface mesh pattern through the geometry.

• Hexa meshing:- For its description we can say that it has 8 nodes as you can see in the image.
• 

 

Unlike tetra elements, the Hexa elements follow the mesh pattern through its volume. You can witness the same mesh pattern on either side of the body.

Also, the quad/ mixed elements are used in the meshing.

Let's take an example of a cube again, But this time we do shell mesh on the surface by using Quad / mixed elements and follow that pattern to the other side of the face.  We have to do this for each side of the cube. After shell meshing, we just have to patten that mesh so it can fill the inside empty volume.

There are options like Extrude, Solid builder, etc. in V.Mesh Deck. They extrude the pattern and create 3d elements inside the volume which have a uniform layer of mesh through that’s why it’s also called Structured Mesh.

 

Let's move on to our model again

Here After importing we get the usual result of distorted faces. As you can see that the geometry is in bad shape. Uneven orientation.

I did topo to cleaned it also there was no even orientation on the model so I set orientation too. By performing a length operation that is in Mesh deck in Perimeter block and set it to 1 we can manage the distance between an adjacent node to 1mm to get a clearer picture of the model itself.

As you can see in below image After performing the length option and Topo I get a result like this

The next thing I did was performed Topo operation from the topo Module. What Topo did was connects all adjacent surfaces/ singe cons and completes the geometry.

After that, I performed a Geometry check on it.

I get several triple cons and Needle face errors as you can see above. I used the Auto fix option this time and I get the same result as a manual one. (Crossed verified this)

 

The model is distributed in 4 different properties. I thought it's better to go on each part one by one. Named each part as mirror face, mirror body, round hinge, and handle respectively.

After Geometry cleans up, I have two options one is manually Defeaturing and do meshing, the other method was to use the Batch mesh option.

With the second method, I can defeature and mesh the model at the same time and the meshing pattern is auto-generated by ANSA itself. I just have to select which part I have to do batch meshing.

So, I decided to go with both of the methods to find out the difference between auto and manual one.

Batch mesh

                                                Fig.1

On the top bar, you can get the option of Batch meshing.

After clicking on 'New' you can get the option called 'meshing Scenario' where you can make a new meshing scenario for different parts. From the meshing scenario window, you can select the PID on which you want to work also the meshing criteria for it. In the above image, I create the four Meshing scenarios and named it accordingly for understanding.

 

                                          Fig 2

In the above image, you can see the meshing scenario window. As you can see I take one PID from the list of PID and moved it to a meshing block so ANSA can identify for which part it has to Batch mesh.

I repeat the procedure for all four PID’s (see fig 1) .  and run it by clicking on the RUN button. As soon as click on RUN Ansa start defeaturing and building mesh on its own and gave me the result.

Now you think why do I create separate scenarios for each PID’s /Parts?

Why not single-handedly select the entire model?

And the answer is I tried to do that but that I had face some issue as ANSA get confused the adjacent surfaces with overlap one and give me the errors that’s why I choose to go for individual parts.

Note: [ I will include errors I get during the procedure in the latter segment of the report with the appropriate screen shot]

The Final result of the Batch mesh is as follows.

These are the results of Auto Defeaturing done by Ansa.

[note: - Batch only did Auto defeaturing and surface meshing which was nothing but Shell meshing. It doesn’t introduce the tetra element in between.

As the tetra element generation procedure is the same for both Batch and Manual meshing, I will cover that part after explaining Manual meshing]  

Manual Mesh

IN Manual mesh I had called out each part which was set in separate PID one by one and did shell meshing using Tria / ortho tria elements depending on the nature of the surface.

[Tria:- When the surface was flat plane one.]

[Ortho tria:- When the surface curved in nature. This will give us better flow and pattern than the tria option]

Here are some screenshots of manual defeaturing.

You can see in the below image there were chances of the element failure as the measured distance was less than the minimum length set for an element (which was 0.5 mm.)

So, I had created midline using the 'Datch' option from the topo deck. Which solved the min length issue as well as captured my feature.

Here another example of such type of defeaturing.

Tetra Meshing

The tetra element generation procedure is the same for both Batch and Manual meshing.

1)First, we have to go to 'V. Mesh' deck and define the volume for given meshed geometry.

The best way to define geometry it's by Manual method rather than auto one. For that, we just have to long-press Define than it will show two options manual and Auto, Go for a manual one. It will generate the volume and you can check that with the 'List' option which is next to the 'define'. Also, the volume data window will be appearing just below the shell data window [left side one]. As you can see it was showing volume unmeshed: 1.  Actually, that means our volume creation is successful and we can move forward with tetra meshing.

There is two option available for tetra meshing and each of them has their individual algorithms around which they generate the mesh.

• Tetra Rapid
• Tetra FEM

For this model, I went with Tetra Rapid because as per name indicates it did deploy the volume mesh rapidly on the other hand Tetra FEM takes a longer time than tetra Rapid.

With the more complicated model, it may become a reason for the Ansa software crash.

But on the other hand, the mesh quality we getting from the tetra FEM is much better than the Tetra Rapid one. Actually, while volume meshing the Tetra FEM algorithms followed the surface patterns over the at least two layers of the mesh but the tetra rapid only followed a single layer after that the entire mesh is unstructured.

 Here you can easily see the difference between two mashing patterns. The FEM mesh is more organized and sharpens than rapid. Also, the number of elements generates during meshing is high in the Tetra FEM case.

Tetra rapid elements: - 187808

Tetra FEM elements: - 346022

The difference between the numbers of elements generated by both options is too high. That’s one of the reasons that tetra FEM meshing is more fine than Rapid and takes much time to do so. Also, these higher numbers of elements may cause trouble in further analysis process as analysis will take much more time in the case of tetra FEM.

So, to save the time I went with tetra rapid option because I thought it will not impact my end result that much.

Errors

• Errors Comparison of Batch and Manual Mesh

The main difference I noticed was of defeaturing. The auto defeaturing during Batch mesh eliminates almost all construction lines that ti was considered as not useful. 

On the other side with manual defeaturing, it’s on the user hands which construction line should be deleted and which shouldn’t. user first check the geometry takes measurements and then decide which are not appropriate construction lines or may become the reason for element failure.

Here are some screenshots which will give you an idea about what exactly happens in defeaturing in both cases.

If you take close looks towards the direction where the arrow pointing you get the idea about the result.

In some cases, Ansa deletes the construction lines even though it is not needed.

 

In this third image as you can see the batch option fails to do proper defeaturing even after the feature is failing for the minimum length quality criteria.

The error I get here is as follows,

This is a classic example of what will happen when we do wrong defeaturing. To solve this issue I performed manual defeaturing in topo deck and mesh it with Ortho tria elements.

• Random errors

At this point, I Faced minimum and maximum length failure issues to overcome that I did reconstruct. Here you can see the image before and after the reconstruction

 

 

This is another sort of error I came across the kink edges. A dent type pattern generates on the surface which eventually portrait on the mesh volume surface also and gives us the distorted result, It's not a good practice to leave this sort of kink edges. I get rid of them by using reconstruct and Origin command you can see the result yourself.

Errors while going for Batch mesh a complete component as one entity

These sorts of errors starting to pop up when I did batch mesh on the entire part as one solid entity.

Actually what happens here was when I went for complete model batch meshing the Batch mesh algorithm considers one end of a completely separate body as a part of its geometry and did meshing.

The result I get overlapping shell mesh on some surfaces which leads me to these errors.

To overcome these errors, I deleted the surface which gets included as one body even though it was part of a separate body. After this I get my error cleared as you can see in the below image.

But unfortunately, ANSA unable to consider it as a one-hole single closed body/volume that’s why my efforts to solid meshing on it were gone into vain. Because it’s practically not possible. After discussing with my mentor, I came to the conclusion that taking a complete model as one during Batch meshing is not a good practice at all. Therefore each assembly comes with some PIDs and we should approach them the way they are. Therefore I went with the separate PIDs method which I discussed earlier.

Final Result

As you can see there is a huge difference between elements generated by Batch mesh and Manual mesh.

As you can see in the above image the elements generated by the Batch mesh method is 478003 while the mesh generated by manual one carrying elements 512208 in numbers.

But as you can understand the errors, I mentioned earlier in the report even though Batch mesh elements are lesser in number the defeaturing /feature capturing is not that good, at least for this part.

Apart from this, I had also mesh the Intake Manifold using the same method.  [NOTE:- Quality Criteria is Different]

No Tetcollapse quality failure found out in the model.

[You can find a detailed report about it the following link:- https://skill-lync.com/projects/3d-finite-element-meshing-on-intake-manifold-tetra-mesh]

 

 Learning Outcomes: -

1) I learned about the 3D mesh. I get a chance to explore more of that volume mesh deck. Importance of 3D mesh (Tetra / Hexa).

2) Understood the right approach while doing meshing on the model which thickness is more than 6 mm / have indistinct shape, cast parts.

3)Learned new keys in the volume deck like,

Define – which will help to create a new volume.

List – which inherits the list of newly generated volumes.

Tetra Rapid / Tetra FEM – to generated Unstructured mesh inside the close volumes.

Then there are Reconstruct to reconstruct the 3d elements and Fix Quality to Auto fixed quality-related issue and many more.

 

Conclusion

• 3D meshing/volume meshing / Solid meshing only done on the part which has a thickness of more than 6mm and the casting parts.
• In Batch meshing, we can either go for Complete model meshing in one shot or we can approach the model PID by PID but out of both of them, the second one is always a good practice.
• Tetra Rapid and Tetra FEM work on different algorithms which eventually help them to generate the required solid mesh. While volume meshing the Tetra FEM algorithms followed the surface patterns over the at least two layers of the mesh but the tetra rapid only followed a single layer after that the entire mesh is unstructured as per the algorithms.
• Even though batch meshing gives us a lesser number of elements as the End result manual meshing is more accurate, in terms of defeaturing because in some cases it (Batch mesh) won’t do defeaturing correctly and that eventually lead to the bad meshing or quality failure.
• length and Max. length criteria wouldn’t get applicable to the solid mesh as the elements are 3D elements in nature and we can’t define them on the basis of 2d perimeters. Though we did check Tet collapse for 3d elements.