3D Tetrahedral Meshing of RearView Mirror

AIM

We need to create a 3D Tetra mesh model of the Rear View Mirror component given

OBJECTIVE

The component having more than 6 mm thickness are meshed in a special way.Those componenets cannot be meshed using the 2D meshing technique, because the range of error will happen to the simulation results will be much more than acceptable when we try to mesh the components over higher thickness with the 2D meshing technique.So the method we are using is called 3 D meshing technique.Here in ANSA we need to create the component into a closed volume before move on to the 3D meshing.There are two different types of 3D meshing i) Tetrahedral meshing ii) Hexa modelling. The differance between these two types of meshing techniques are not small.Tetrahedral meshing is also called Non- Structured mesh whereas the Hexa mesh is called structured mesh.This differance is because the tetramesh is actually the 3D representation of normal tria 2D mesh.When the 2D tria meshed surface is converted to 3D,it will form tetrahedron elements.But the tetrahedral elements formed in tetra modelling will be non structured.It means the mesh will follow a pattern only upto some layer and thererafter the mesh will be in derailed fashion and this is because of the algorithm inbuilt and its growth factor of the tetra elements.This technique is quite easy.But this is not the case with the hexameshing.Hexameshing is the 3d meshing of the normal 2D Quad elements.This mesh will be highly structured and patterned.So this meshing technique cannot be used in case the componenet we want to mesh is highly complex in nature.At the same time ,for the sake of getting results we can rely on tetrameshing for complex 3D componenets .This is because the time taken for meshing with tetra elements comparitavely less and results are even acceptable.But when it comes to accuracy the HEXA modelling can give the best possible results.But the problem is the computational time ,because the computational time required for the hexa meshing is higher than the tetra hedral elements.And the hexameshing requires special strategies to mesh the model.It is a tedious process.But the tetrameshing is simple and easy.

PROCEDURE

Here we are given a component in automobiles called rearview mirror.This is attached in the front windshield of the car for providing the rear view to the driver.The model we are having have 4 differant properties assignnd to its 4 parts namely the rearview glass,Plastic backcover,Swiveling support and a double ball joint. THe glass is fixed on to the mirror plastic case and the double ball joint which allows for all the rotational degree of freedom and the swivelling support helps us to position the mirror according to the proper view for the driver.The double ball joints when positioned in a specific way there may be possibility of the formation of straight line connection.When a collison or sudden stop occurs the passengers are thrown infront and there is a possibilty that the driver or codriver may hit into the mirror and gets injured especially when the straightline connection forms,because at that tim the mirror will be firm and doesn't change its position even if the hard force of bombardment with the passenger happens.So that the modern vehicles have the technology of snapping back the mirror while the collison or suidden stops occurs.So that the occupant injury can be reduced to a great extent.

Import of the model

At first the model needs to be import into the ANSA.THe model is in IGS file.First open ANSA > New> Open > browse for the file > Open.This will import the model into the ANSA.

Fig 1 RearView Mirror Model after Import

Topological Cleanup

When the model is entered into ANSA there will be so many CAD translational issues so we need to first do the topological clean up.Here we should first apply the TOPO on the entire model. Here in this model the CAD issues got cleared with mere topo application.This can be verified using CHECKS > geometry > select the netire model > Middle mouse click.This ensure that the CAD is not having any translational issues and so we can move on with the Tria meshing.

Fig 2 Model after the Topo Clean Up

Setting the Length as 1 in perimeters and Macros

This is done because we needs to mesh the model with the minimum element size as 1 mm. And the refining the hotpoints at 1 mm distance from on another will helps us to see the model more accurately.This can be done by Go to Mesh Module >Length > select the enitre model > Set perimeter as 1 > OK.Similarly put the radio button in Macros and select the geometry and set the macro length to 1 mm.

Fig 3. Model after setting the lenth as 1 ( In TOPO View)

Geometrical constructions

This is done to ensure that our tria elements we are going to deploy will not be failing at the areas in the model.Inorder to ensure that we need to make some geometrical constructions in such a way that the tria elements will not fail for minimum or maximum lengths.CUT option in Topo module can be used to deploy and remove the geometric lines in such a way that the tria elements will not fail at any areas.

2D Tria Meshing

Here the model is having 4 parts and 4 parts haves the plane of symmetry.So we can just cut the 4 parts of the model in half using PLANE CUT option.This is done by Topo >Plane cut > select three nodes in the plane of symmetry > Middle click. This will cut the entire model into half.Then using DELETE option we can cut out the geometry into half.We can mesh this half geometry completely and later we can use the trasform copy option to make it as the full model.

Fig4 Plane Cut executing on the Swivelling Support Part

Fig 5 Deleting the half geometry using Delete option

Similar to the Quad meshing ,the process is similar.But we have two types of Tria elements in ANSA or any other preprocessor.They are Ortho Tria and Tria elements.Ortho is nothing but right angles tria elements and the other one is normal trai elements.The significance of the Ortho Tria is that it can be deployed in curved and complex areas to capture the feautrs properly.Capturing feautres properly is very essential.this can be very welll undergone by using the ortho tria elements.There surface capturing ability is awesome while the normal tria elements will not be capable of capturing the fillets and complex areas perfevctly.So at the curved and fillet aress we will be using Ortho Tria elements.

To deploy Ortho tria elements, First go to mesh parameters and set the average length as 1 mm and set element type as ortho tria and press OK.Then set the quality parametrs in Shell Tab as minimum length as .5 mm and maximum length as 2 mm.In th solid tab we need to set the Tet Collpase as .2.In ANSA it is givn as Collpase only.The 3 D mesh having the tet collpase more than .4 will be the best mesh.Tet Collapse is nothing but the height of the elements.

Fig 6 Ortho Trias used in curved areas and fillet areas for better feature capturing

Now we are set to proceed with the meshing process.Like what we done in shell meshing, Here we can use the SPOTMESH algorithm to deploy the TRIA elements.After the mesh deployement we can obviously use Reconstuct algorithm to reconstruct the mesh deployed.One thing we need to taken care is that the curved and fillet areas are needed to mesh with orthotria elements.But when it comes to flat areas we need to use the normal trias.This is done because the Ortho tria is computationally expensive while the normal trias are comparitavely less expensive.Once the genral 2 D meshing is done we can move on to 3D meshing.

Fig 7 Glass after 2D tria meshing

Fig 8 Plastic cover after 2D tria meshing

Fig 9 Ball in the double ball joint

Fig 10 Swivelling support triameshed

3D TetraMeshing

3D meshing is need to be done in a volume.So first what we need to done is that create a volume .We already have the model as a volume with us, but we need to define that as a volume then only the ANSA will be able to figure out it as a volume.For that in VOLUME MESH MODULE we have the define option.Select Define > Choose Manual > Select the entire model >Middle click > Right Click > New > PSOLID > GIve name and Corresponding Property Id > Ok. This will create a new volume that is recognisable by ANSA.So Then we can move onto 3 D meshing process Here in ANSA there is TETRA RAPID And TETRA FEM algorithms are avilable to create the 3D Tetramesh for structural analysis.Here we can use TETRA FEM to deploy the Tetra mesh in the volume.For that TETRA FEM >select the voliume >Middle click.This will deploys tetra mesh between the 2 D meshed surfaces.ANSA dont bother about the pattern or structure of the mesh, instead it will try to connect every tria element in one surface to every tria element in other surface.So that the mesh will be unstructured and this is predominantly due to the growth factor of the mesh.

Fig 11 4 Volumes defined for 4 parts

RESULTS

Fig 12 The entire meshed model in Property ID View 1

Fig 13 The entire meshed model in Property ID View 2

Fig 14 After masking some portion of tetramesh to get clear visibilty of tetrahedral elements

CONCLUSION

Since the geometry is complex and so deploying Hexa mesh on this model is very tedious job even if the hexamesh is the ideal mesh.As far as results are concerned we can obtain a good result using the tetrahedral mesh itself and at the same time the meshing time is very less compared to hexameshing.And feature capturing is to be done in best possisble way we have to use the orthotria elements at the curved and fillet areas.So the meshing of rearview mirror is done using tetrahedral elements.

Link for the files

https://drive.google.com/file/d/1dU5qcJeKy4vEycqr_AvKabFzur0b5UlP/view?usp=sharing