Solid Meshing of an automotive Rear view Mirror

AIM:

To do geometrical clean up on the given component and mesh with the given element quality criteria. Use tria elements to create surface mesh and generate tetra volume mesh (unstructured volume mesh).

 

OBJECTIVE:

The main objective of the project is to learn tetra or unstructured meshing with tetra elements.

 

PROCESS:

1. Perform geometry clean up and resolve geometry errors.
2. Assign quality criteria.
3. Use tria elements to do surface meshing (Shell meshing).
4. Generate volume and tetra volume mesh (Solid meshing).
5. Resolve tetcolapse issues.

 

SOFTWARE USED:

ANSA (Automatic Net generation for Structural Analysis) v19.0.1 64-bit

 

NEED FOR SOLID MESHING:

In the previous projects, we have measured the thickness of each of the components of the model, generated the mid-surface and deployed the shell mesh on the mid-surface. In this case, the cross-sectional thickness of the model is greater than 5mm. In some cases the cross-sectional thickness of the component cannot be measured. In such circumstances, we deploy the Shell mesh on the surface of the model and generate the Solid mesh.

 

PROCEDURE:

The given model is a Rear view mirror in a car which is designed to allow the driver to see the rearward through vehicle’s rear windshield.

                                                                                                                                       Fig. Imported CAD geometry

 

GEOMETRY CLEANUP:

Before the geometry clean-up, the shell element length needs to be set as 1. In the MESH module, PERIMETERS -> LENGTH the shell element length can be changed. This is done to get more hot points and better curve features. The perimeters & macros are set as 1 for the whole model.

                                                                                                                          Fig. Assigning shell element length on perimeters

The same way, it is also done for macros.

                                                                                                              Fig. Assigning shell element length on macros

Next, we have to stitch the surfaces of the model. Even though the surfaces are together they are not stitched together. To stitch them, we need to do a TOPO. It is under the FACES -> TOPO in the TOPO module.

                                                                                                                       Fig. Imported CAD Geometry

After the TOPO operation, the surfaces in the model are stitched as below.

                                                                                                             Fig. CAD Geometry after a TOPO operation

Now the geometry checks need to be done. The CHECKS command is under the TOOLS tab. Under CHECKS, GEOMETRY option is chosen.

                                                                                                                                         Fig. Geometry checks (1)

                                                                                                                                         Fig. Geometry checks (2)

 

                                                                                                                                         Fig. Geometry checks (3)

 The Geometry checks are run and geometrical errors in the model, if any, will appear.

                                                                                                                         Fig. Geometrical erros in CAD model

Now these 11 errors are selected and fixed. They can be fixed manually or can be fixed by ANSA automatically.

                                                                                                                  Fig. Fixing geometrical erros in CAD model

Once the errors are fixed, the Geometry check is done again once again to ensure there are no more errors left.

                                                                                                                                     Fig. Geometrical errors fixed

ASSIGNING QUALITY CRITERIA:

 

Before quality criteria is assigned, the MESH PARAMETERS are to be set. It is found under the UTILITIES tab.

                                                                                                                                Fig. Mesh parameters

                                                                                                                    Fig. Assigning Mesh Parameters

The Target element length, Minimum element length & Maximum element length are set as 1, 0.5 & 2 respectively. The type of elements is set as ORTHO TRIA. Next the QUALITY CRITERIA is set. It is also under the UTILITIES tab.

                                                                                                                   Fig. Quality criteria

                                                                                                          Fig. Assigning Quality criteria for Shell meshing

Now, since we will be doing shell mesh as well as the solid mesh, the QUALITY CRITERIA is to be assigned under SHELLS tab & SOLIDS tab. For SHELLS the MINIMUM ELEMENT LENGTH & MAXIMUM ELEMENT LENGTH are applicable, for SOLIDS, the MINIMUM ELEMENT LENGTH, MAXIMUM ELEMENT LENGTH & TETCOLAPSE are applicable.

                                                                                                                       Fig. Assigning Quality criteria for Solid meshing

Then under the PRESENTATION PARAMETERS tab, the ELEMENTS% box is checked and the slider for DETAIL ON DEMAND EFFECT is kept towards the ALL DETAILS. This is to enable the % of elements during meshing.

                                                                                                                       Fig. Assigning Presentation Parameters

SHELL MESHING:

Shell meshing is done with ortho tria elements and tria elements. Ortho tria elements help to capture the features such as fillets, curves, etc. better than the tria elements. So these are used in curved and fillet areas in the model. Other flat areas can be meshed using tria elements itself.It is advisable to use the ortho tria elements in the curved & fillet regions alone as the computation time for ortho tria is considerably more than that of tria elements. As a result, meshing the whole model with ortho tria elements would make the computational time for the whole model very high. This may sometimes lead to the crash of the ANSA application. In order to prevent this we use ortho trias for meshing only the fillet & curved surfaces.

During meshing, the type of element used can be changed between ortho tria & tria in the OPTIONS LIST tab in the right bottom of the window.

                                                                                                                       Fig. Shell meshing using Ortho trias

The method of meshing used is SPOT MESHING. It is in the MESH module under MESH GENERATION -> SPOT MESHING.

                                                                                                                       Fig. Mesh deployed with ortho trias

After meshing, the mesh is deployed in the selected areas. Similarly, meshing is done in other fillet & curved surfaces.

                                                                                                                       Fig. Shell meshing using Ortho trias on filelt & curved surfaces

It can be seen that the mesh flow is not good and is distorted in some areas. This can be sorted out by reconstructing the mesh in the particular surfaces. Reconstruction of the mesh is done using the RECONSTRUCT command. It is under the SHELL MESH module button.

                                                                                                                       Fig. Reconstruct mesh to obtain better mesh flow

After the reconstruction, the mesh flow is better than the previous mesh.

                                                                                                                      Fig. Reconstructed mesh

Next, to mesh flat surfaces (surfaces where there are no fillets or curves) we use tria elements. SPOT MESHING method is used and the type of element to be used for meshing can be changed in the OPTIONS LIST.

                                                                                                                       Fig. Shell meshing using normal trias on flat surfaces

After meshing, the mesh can always be reconstructed to obtain a better mesh flow. Note that the type of elements used in meshing can also be changed while using the RECONSTRUCT command.

                                                                                                                   Fig. Reconstruct mesh to obtain better mesh flow

                                                                                                          Fig. Reconstructed mesh

Similarly, other flat surfaces are meshed using the tria elements. Then the mesh is reconstructed using the RECONSTRUCT command.

                                                                                  Fig. Shell meshing using normal trias on flat surfaces

                                                                               Fig. Reconstruct mesh to obtain better mesh flow

                                                                                                               Fig. Reconstructed mesh

To mesh curved surfaces, we change to ortho trias for better feature capturing.

 

                                                                                              Fig. Shell meshing using Ortho trias on filelt & curved surfaces

                                                                                                     Fig. Shell meshing using normal trias on flat surfaces

Now, in some surfaces it can be seen that the construction lines are very small and are lesser than the minimum element length given in the quality criteria. The mesh in these surfaces will fail for the minimum element length.

                                                                                                  Fig. Measuring construction lines lesser than min. element length

In order to prevent this, we can re-arrange the construction lines in these surfaces.

The re-arranging of the construction lines should be done without changing the CAD model. Also the feature capturing should not be affected. The construction lines can be disabled using the FACES -> CUT in the TOPO module.

                                                                                                         Fig. Re-arranging construction lines at critical surfaces

Some of the construction lines can be disabled as the curvature is captured by another construction line. This does not change the geometry and does not affect the feature capturing.

                                                                                                          Fig. Construction lines re-arranged

After disabling, it can be seen the construction line changes colour to orange.

At another place, we can find fillets which are length lesser than the minimum element length.

                                                                                                     Fig. Measuring fillets less than min. element length

In this situation, we can use CURVES -> MIDDLE to create a middle curve between the two curves. As a result, the fillet is captured and the minimum element length issue is also resolved.

                                                                                                            Fig. Creating middle curve for fillets

First, one curve is selected and then the middle mouse button is clicked. Then the other curve is selected. Upon accepting, a middle curve (violet colour) is created.

                                                                                                             Fig. Middle curve created for fillet

Similarly, the same procedure is followed for the other fillet.

                                                                                                  Fig. Creating middle curve for fillets

                                                                                                             Fig. Creating middle curve for fillets

After the middle curves are created for the fillet curves, the curves are projected on the surface. The PROJECT command is under the CONS -> PROJECT in the TOPO module.

                                                                                                            Fig. Projecting middle curves onto the surfaces

Then the fillet curves are disabled using the FACES -> CUT.

                                                                                                                      Fig. De-featuring fillets

This way all the other surfaces are meshed with the relevant element type.

                                                                                                                 Fig. Shell meshing using Normal trias

                                                                                                                  Fig. Shell meshing using Ortho trias

                                                                                                                  Fig. Shell meshing using Ortho trias

                                                                                                                           Fig. Shell meshed CAD model

SOLID MESHING

Once the meshing is completed, the volume needs to be defined. It is done in the VOLUME MESH module. The volume is defined using the DEFINE command which is under the VOLUMES -> DEFINE module button.

                                                                          

The selection is done for the each part of the model one by one. This is done separately as the different parts of the component are made from different materials.

                                                                                                                 Fig. Defining volume for rotating ball

                                                                                                                   Fig. Defining volume for rotating ball

This way, the volume is defined for all components of the model separately.

                                                                                                                   Fig. Defining volume for mirror fixture

                                                                                                                   Fig. Defining volume for mirror

                                                                                                                       Fig. Defining volume for mirror holder

After the volumes are defined, the list of volumes can be seen with the help of LIST command. It is under the VOLUMES -> LIST module button.

                                                                                                                     Fig. Volumes defined for all components

Now, the volume mesh needs to be generated in these volumes. So we need to mesh them one by one.

                                                                                                                        Fig. Volume meshing with tetra elements

Upon right clicking the volume in the list, we can use the REMESH command to do the meshing.

                                                                                                                     Fig. Volume meshing with tetra elements

Now there are 4 meshing algorithms available. We will use the Tetra Rapid and Tetra FEM for meshing.

After the algorithm is selected, the solid meshing is done and the volume mesh is generated.

                                                                                                                   Fig. Volume meshed with tetra elements for rotating ball

Once the meshing is done, it can be seen that the Type and Status for the first volume in the list has changed from Undefined to Tetra Rapid and Unmeshed to Meshed.

Similarly the volume mesh is generated for all the volumes.

                                                                                                            Fig. Volume meshed with tetra elements for other components

RESOLVING TETCOLAPSE ISSUES:

Now to view the volume mesh, we need to switch off the Geometry & FE model from the Visibility tab in the left bottom of the window. Changing to the Hidden mode in the AUXILLIARIES tab from the right bottom will enable the quality criteria and show the failing elements.

                                                                                                                  Fig. Volume meshed component

If there are elements failing for quality, they can be fixed using the FIX QUALITY command under IMPROVE -> FIX QUALITY in the VOLUME MESH module.  This command is used to fix the failing elements automatically by ANSA.

                                                                                                                     Fig. Fixing quality issues with FIX QUALITY 

NOTE: Using FIX QUALITY command will fix the quality but may result in kinks or raised edges in the volume mesh model in some cases. The failing elements need to be fixed without resulting in kinks or raised edges in the volume mesh model.

Also the MV FREE command under GRIDS -> MV. FREE in the MESH module can be used. This command is used to fix the failing elements manually by moving their edges and nodes about the X, Y & Z axis.

                                                                                                                 Fig. Fixing quality issues with MV. FREE

One of the main quality criteria to check in volume mesh is Tetcolapse. This criteria is applicable for tetra elements only. Tetcolapse is defined as the Min. of the quotient obtained by dividing the height of tetra element with the square root of area of the opposite face.

                                                                                                                               Tetcolapse = (Min. of h/√A) ÷ 1.24

where,

            h = Height of the tetra element from the node to the opposite face

            A = Area of the opposite face

 

The h/√A value is taken for all the 4 nodes and the minimum of that value is divided by 1.24 to obtain the tetcolapse value for that tetra element.

                                                                                                             

                                                                                                                       Fig. Tetra element

After the failing elements are fixed, the volume mesh model should be visually inspected for kinks and raised edges. Kinks are the dips in one or many nodes of the tetra elements from the surface of the mesh. Raised edges as the word means are the raising of one or many nodes of the tetra elements from the surface of the mesh.

                                                                                                                      Fig. Cross sectional view of Tetra mesh

It can be seen that the tetra elements on the surface of the CAD model are structured but in the inside of the model the structure of the mesh is not followed. The algorithm has just filled the volume of the component with tetra elements. That is why this method is known as the Unstructured Volume meshing.

LEARNING OUTCOME:

In the above project, we have learnt the need for solid meshing, the process of solid meshing and the method to solve quality issues in the failing elements.

CONCLUSION:

Thus, the Geometry cleanup, Shell meshing and Volume meshing (Unstructured meshing) is done on the given Rear view mirror model and all the elements pass for the quality criteria.