Week- 5 - Solid meshing

Solid meshing of Rear view Mirror

1.Aim:

To perform tetra meshing for a Rear view mirror model as per the given quality criteria using ANSA.

2.Objectives:

1. To check for geometrical errors and mesh the Surfaces with the given 2D element Quality criteria:
   

   S.No	Quality Criteria	Value
   1	Target/Average length	1
   2	Minimum Length	0.5
   3	Maximum Length	2

2. To define volumes and to mesh them using tetra elements with the following quality criteria:
   

   S.No	Quality Criteria	Value
   1	Tet collapse	0.2

3.Theory:

• 3D elements are usually used to mesh volumes. They are derived from 2D elements that are used when the problem is unable to be simplified. 3D solid elements only account for translational displacements.
• These elements are suitable for three-dimensional analysis. They are mainly used to mesh solid parts and sheets of thickness more than 5mm, transmission castings. engine blocks etc.
• Tetra element, 4 noded with 4 triangle faces are formulated in three-dimensional space with three degrees of freedom per node. These are the translational degrees of freedom in the X, Y and Z directions respectively. In the figure Tetra element is shown for 4 nodes.
  

• 3D elements are obtained from 2D elements and are used when the model has more than 6mm thickness.

• Tetra Collapse: Ideal Value = 1.0
  Tetra collapse = min of (h/√A) ÷ 1.24

  where “h” represents the Height between the Node and the opposite face of the Triangle.

  “A” represents the Area of Opposite Face (area of the Triangle).

  
  

  Tet collapse values less than 0.15 or 0.2 are not acceptable.

4.Procedure:

Step 1: Importing the geometry

• First step is to import the geometry into ANSA software.We can do this by either of the two options:

1. File → open → select the file in supported format → open or
2. Simply drag and drop the file in required format into ANSA.

Figure:CAD Model

• There are four components in the imported model. We approach them separately with the same procedure.

Step -2: Geometry cleanup

• Cons resolution: We have to do the cons resolution to check the actual shape of the cons in the geometry. There are two tools for this. One is 'Fine' option in Auxilaries group under the Topo module. The other one is 'Length' option in Perimeters group under the Mesh module. The 'Length' tool helps to do this in a single step when compared to fine tool.
  Mesh module → Perimeters → Length → Perimeter/macros (do this for both) → select the whole surface → middle mouse click → give the value as 1 → apply.
  
  Figure: Selecting Length option from Perimeters group
  
  
  
  
  
  Figure: Assigning Perimeter and Macros values as 1
  
  
  
  
  
  
  Figure: Assigning Perimeter and Macros values as 1 for rest of the components
  
  
• Connectivity correction: There may be surfaces without connection in the imported model (Single cons). If present, we have to correct them using the 'Topo' option in Faces group under the TOPO module.
  Topo module → Faces → Topo → select the whole surface at once → Apply (Middle mouse click).
  
  
  Figure: Connectivity issue
  
  
  
  Figure:Correcting connectivity issues
  
  
  
  
  
  Figure: Correcting connectivity for remaining components
  
  
• Orientation correction: Sometimes, the given model is observed with yellow and grey color surfaces mixed together when viewed under ENT mode. This indicates mixed normal direction among the surfaces. We have to correct the visible surfaces to either gray(+ve) or yellow(-ve), preferably gray. The tool, 'Orient' is used for this purpose.
  TOPO module → Faces → Orient → Choose Select, Visible or Auto appropriately.
  If cone correction is done properly, choosing the visible option will change the colour of entire surfaces in one go.
  
  
  Figure: Orientation error
  
  Figure: Corrected Orientation
  
  
  
  
  
  Figure: Corrected Orientation for remaining components
  
• Geometry check: We need to check the geometry by using 'Geometry' option to find and correct any other errors in the imported geometry.
  Tool tab → Checks → Geometry → Tick the single cons as well → Execute → If any errors pop up in red color select all of them at once → right mouse click → Fix → If not fixed, try manually → Then execute the geometry check again, it will show No errors or warning exist.
  
  
  
  
  
  Figure: Geometry check
  
  
  
  
  
  
  Figure: Geometry check and fixing errors for remaining components
  

Step 3: Setting the Criteria and Parameter files

• We have to set the quality criteria as well as mesh parameter as specified.
  Utilities tab → Mesh Parameter → Set Element type: Tria , target element size, minimum element size → Ok
  
  Figure: Mesh Parameters file
  
  Utilities tab → Quality Criteria → From Shell tab set 2D element qualities and from Solid tab set 3D element quality(Tet collapse) → Ok
  
  
  
  Figure: Quality criteria file
  
  
• We also save them to be used later.

Step-4: Generating 2D mesh

• Cons resolution: We need to perform cons resolution again on the components with perimeter and macros values equal to 'Target element length'.
  Mesh module → Perimeters → Length → Perimeter/macros → select the whole surface → middle mouse click → gave the value as target element size(It is 1) → apply (Note: Do the same for macros option in Length tool as well).
  
  
  
  Figure:Cons resolution as Target Element Length
  
  
• Avoiding places of element failure: If there are near construction/ feature lines in the model. It may lead to element failure in quality. So, we need to suppress those line using 'cut' tool (Right click).
  Use the 'Measure' tool from utilities to find out the distance between two construction/feature line. If the distance found is less than the minimum element length (Here 0.5 units), suppress any one construction line.
  Topo module → Faces → Cut or
  Mesh module → Macros → Cut (Right click for suppressing)
  
  Figure: Measuring distance between feature lines
  
  Figure: Suppressing construction/ feature lines
  
  
• 2D meshing: We mesh with tria on flat regions and ortho tria on curved region. We mesh each part separately.
  Select 'Best' option from Mesh generation tab in the mesh module. First, select the flat areas. Choose the tria option from option list. Then apply (Middle mouse click).
  Mesh module → Mesh Generation → Best → Select surfaces, in option list use tria and ortho tria for flat and curved surfaces respectively → Apply (Middle mouse)
  
  
  Figure: Meshing Flat surfaces
  
  After meshing on flat surfaces, double click on the 'unmeshed' (or right click → show)- this will show the unmeshed curved surfaces. Then again select the 'Best' option from Mesh generation and select the 'ortho tria' from option list.
  
  
  
  Figure: Meshing curved surfaces
  
  Then click on the 'Neighbour All' to view the whole part.
  
  
• Quality Correction: Click on the 'Hidden' option which will show failing elements(off elements), unmeshed surfaces etc.
  We can use reconstruct, smooth, improve options on failing elements to correct them.
  Mesh module → Shell mesh → Reconstruct/ Improve/ Smooth
  
  Figure: Errors in 2D mesh
  
  Figure: Correcting errors by using Reconstruct option
  
  Figure: 2D mesh after correcting all errors

Step-5: Pre volume check

• Here we check for unmeshed areas. The 'Hidden' tool will show, whether there are any unmeshed surfaces or not. If there are, mesh those areas.
• We also check for single cons, double cons and triple cons in the model. But we have initially corrected them using Topo and Geometry tools. We can use Check → Geometry option for checking this as well.
• Penetration and Intersection Check: We check for intersection and penetration on the 2D elements. If any errors found, we have to correct them.
  Tools → Checks → Penetration → Intersection → Execute
  
  
  
  Figure: Penetrations and Intersections check
  

Step-6: Volume Definition

• Before converting 2D to 3D elements we need to define the volume for creating 3D elements in them.
  Volume mesh module → Volume → Define → Select Detect on: Visible → Ok
• The above step will create the volume for visible geometry.
  
  
  
  Figure: Defining Volume
  

Step-7: Generating the 3D mesh

• After volume definition, we generate the 3D tetra mesh in the volume using 'Tetra Rapid' tool.
  Volume mesh module → Unstructured Mesh → Tetra Rapid → Volumes → Click on the volume → Apply (Middle mouse click)
• After creating the 3D tetra mesh, we have to check for elements which are failing for tet collapse. The ' Hidden' tool will show the failing elements. If any elements fail the criteria, correct them.
  
  Figure: 3D Tetra Meshing
  
  Figure: Tetrameshed component
  
• Using 'i-Not' tool we can mask some elements to see how the elements have captured the volume.
  
  Figure: Using i-Not tool
  

5.Outputs:

5.1 Component 1

Figure: Final meshed component 2D

Figure: Final meshed component 3D

5.2 Component 2

Figure: Final meshed component 2D

Figure: Final meshed component 3D

5.3 Component 3

Figure: Final meshed component 2D

Figure: Final meshed component 3D

5.4 Component 4

Figure: Final meshed component 2D

Figure: Final meshed component 3D

5.5 Final Meshed Model

Figure: Final 2D Meshed Model


Figure: Final 3D Meshed Model

6.Results:

1. Corrected geometrical errors and meshed the surfaces with the given 2D element quality criteria
2. Defined volumes for each coponent and meshed them using tetra elements as per the given quality criteria.
3. Successfully carried out 3D Tetra meshing on the given Rearview mirror model.

7.Learning Outcomes:

1. Meshing 2D elements in the given model by satisfying criteria. (Use of tria and ortho tria).
2. Creation of 3D elements in defined volume by converting 2D elements to 3D.
3. Checking and correcting element quality failure, geometry etc.

8.Conclusions:

This challenge is very helpful to understand Solid meshing, how 2D tria elements are converted into 3D tetra elements, how the errors are rectified, different tools and its uses.