Week 8-3D Tetra Meshing Challenge

CHALLENGE - 9 : 3D Tetra Meshing

AIM:

• To create a Teramesh for given three models with the following quality criteria.

OBJECTIVE:

• Understand about 3D tetra meshing generation.
• To study about different methods of tetra meshing and 3D mesh quality checking.
  
  
• To learn about 3D meshing from 2D elements.

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. uses for solid parts and sheets of thickness more than 5mm, Transmission castings. engine blocks etc.
Tetra element is the 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 and 10 nodes.

Tetra Meshing Techniques: There are two methods of tetra meshing

• Automatic mesh: This approach is limited to simple geometries and pre-requisite is error free CAD model. User select the volume and software automatically carries out meshing as per specified element length, quality criteria etc.
  Advantage: Very quick, no meshing efforts
  Disadvantage: Results in very high number of nodes and elements. There is no control on mesh flow lines and specific mesh pattern requirement (like bolted, welded joints or contact surface simulation).
• 2D (Tria) to 3D (Tetra): Most commonly used method. Quad or tria meshing is carried out on all the outer surface of the geometry, quads split into trias and then converted to tetras.
  
  

Tetra Collapse: Ideal Value = 1.0 (Acceptable > 0.1)
Tetra collapse = h*1.24/A  (Defined as distance of node from opposite face divided by area of the face multiplied by 1.24)

GIVEN CRITERIA:

• MODEL - 1: Housing Model
  

  TetraMesh Generation Method: 2D to 3D Conversion

  Elements sizes: Min- 2 Units, Target- 5 Units, Max- 7 Units 

  Tet collapse: 0.15
  
  

• MODEL - 2:
  

  TetraMesh Generation Method: Quick Tetra Mesh/ Volume Tetra Mesh

  Target Element size: 3 Units.

  Tet Collapse: 0.15
  
  

• MODEL -3:
  

  TetraMesh Generation Method: Quick Tetra Mesh/ Volume Tetra Mesh

  Target Element size: 4 Units.

  Tet Collapse: 0.15
  
  

PROCEDURE: Each model are meshed and saved separately.

Step 1: Importing model and cleanup:

• To import the geometry, go to file → Import → Import geometry → Import.
  Model-1: File name:housing.stp
  
  
  Model -2: File name:Week 8 Model2.iges
  
  
  
  Model -3: File name: m3Turbo Charger_Week_8.hm
  
• After importing the geometry, the geometry is checked for any missing surfaces or any other errors and then correct it. 
  
  

Step 2: Criteria setting and Topology cleanup

• The criteria and parameters changed according to the given criteria for each model.
  Preferences → criteria file settings.
• Remove the solid before midsurfacing, Delete → Solid → Select solid → delete entity. Then using 'By topo' geometry color mode, check the model for free lines and errors.
• Then we have to avoid places where there are chances of element failure. So use toggle tool on unwanted lines.
  

  Tools used in topology

  Tools>Edge Edit Panel 

  Equivalence (multiple edges at a time): Search surfaces for pairs of free edges and combine into shared edges

  Toggle (edge pair at a time): Select an edge; equivalences with other free edges found within a user specified tolerance

  Replace (1 edge pair at a time): Select 2 edges to equivalence together. Control which edge to retain and which to move.

  Geom>Surfaces>Ruled or Geom> Quick edit > filler surface or 2d> spline> surfaces only (to create missing surfaces).

  Defeaturing: Update the Criteria length as per given element Quality criteria which is Target/Average length for each model. Removing all unnecessary edges, fillet radiuses etc. also maintain distance between two edges or nodes not less than Minimum Length.

  Tools used in Defeaturing

  Toggle edge: Left-click a free edge (red) to make it shared (green), or a shared edge to make it suppressed (blue).

  Right-click a suppressed edge to make it shared, or a shared edge to make it free.

  In either case, the edge must meet the criteria for the type of edge you wish to change it to. For example, toggling a free edge to a shared edge requires the presence of a similar edge within the distance you specify in the tolerance field.

  Replace (1 edge pair at a time): Select 2 edges to equivalence together. Control which edge to retain and which to move.

  Geom> Quick edit > filler surface (to create missing surfaces).
  
  

Step 3: Meshing

• Model -1: Here two parts are there, Each part is meshed separately
  
  The model 1 should be meshed by converting 2D to 3D tetra elements.

• Tria shell meshing (2D meshing): Before meshing, assign given element Quality criteria in hypermesh by Preferences>criteria file setting.
  
  
• Start meshing by, 2D → automesh → select surface, element size =5, mesh type = trias → mesh.
  
  
  
  
• Check quality index for 2D tria elements created from Quality Index or Check elements tool. Correct the failing elements as per criteria by using smooth, element optimize, rebuild etc. Before using rebuild, the parameter should be corrected with mesh type as trias only.
  
  
  
  
• First set tetramesh criteria. 3D → Tetramesh → TetraMesh parameter
  
  
  
• Tetra meshing: Tetra meshing is the 3D meshing process where the tetra elements are used to fill the volume.
  3D → Tetramesh → Instead of comps. select elements and select displayed → mesh.
  
  
• By using mask tool, check how tetra elements are created in the model.
  
  
  
  
  
• Delete 2D shell elements (tria element).
  Delete → element → By config. → select config. as tria3 → select entities →  return →  delete entity
  
  

• Model 2: Model 2 is meshed with quick tetra mesh or volume tetra mesh.

• First set the criteria as well as parameter.
• Setting Tetramesh parameter, 3D → Tetramesh → TetraMesh parameter
  
  
  
  
• TetraMeshing: Two methods are there:
  Method 1: Quick TetraMesh:
  View → Browsers → HyperMesh → Utility → Geom/mesh → Quick tetramesh → select comp. → we can fix mini. tria angle, max. feature angle, max reverse angle, mesh size(4), minimum edge size, mini. element size → mesh type (trias only) → mesh
  
  
  
  
  
  Method 2: Volume Tetra Mesh
  First set tetramesh criteria. 3D → Tetramesh → TetraMesh parameter
  
  3D → Tetramesh → Volume tetra → select solids / surface → set element size → mesh
  
  
  
  
• By using mask tool, check how tetra elements are created in the model.
  
  
  

• Model 3: Model 3 is meshed with quick tetra mesh or volume tetra mesh. There are four parts in the given turbocharger model. Casing, Hub, Supporting plate and Holder housing. Each part is meshed separately.

• First set the criteria as well as parameter.
  Setting Tetramesh parameter, 3D → Tetramesh → TetraMesh parameter
  
  
• Volume TetraMesh: First set tetramesh criteria. 3D → Tetramesh → TetraMesh parameter
  
  3D → Tetramesh → Volume tetra → select solids / surface → set element size → mesh
  
  Casing:
  
  
  
  Hub:
  
  
  
  Supporting plate:
  
  
  
  Holder housing:
  
  
  
  
• By using mask tool, check how tetra elements are created in the model.
  
  
  
  
  
  
  
  
  
  
  

Step 4: Quality checking

• Each model is checked for mesh quality as per given criteria. Before quality check, correct criteria as well as parameter settings.
  Tools → Check elements → 3D
  The mesh is checked for tet collapse, mini. length and maximum length.
  If any error found, correct it manually by using replace node, distance tools etc... Also we can use tetra remesh tool.
  To see the failing element, click on Tools → Check elements → 3D → click on quality you want to check (say tet collapse)→ save failed → return
  Then, Tools → mask → element → retrieve → mask → reverse, This step will shows the element which are failing.
  
  Tet collapse correcting by using node replace tool.
  
  
  
  Editing maximum/minimum length error using distance tool
  
  
  
  

FINAL MODEL:

Quality Check:

• MODEL - 1:
  
  Minimum element length check
  
  
  Maximum element length check:
  
  
  
  Tet collapse check:
  
  
  
• MODEL - 2:
  
  Tet collapse check:
  
  
  
• MODEL - 3:
  
  Tet collapse:
  

Masked Model for Tetra Mesh checking:

• MODEL - 1:
  
  
  
  
  
• MODEL - 2:
  
  
  
  
• MODEL - 3:
  
  
  
  
  

Meshed Model:

• MODEL - 1:
  
  
  
  
• MODEL -2:
  
  
  
  
• MODEL - 3:
  
  
  
  
  

RESULT:

Tetra meshing is done successfully for all 3 model by eliminating geometrical errors and considering the given element quality criteria using Hypermesh.

CONCLUSION:

This challenge helps us to learn about 3D tetra meshing, how meshing and element cleanup is done. This also gave an idea about meshing 3D tetra element from 2D shell elements (tria element). This challenge gave knowledge about various tools in 3D tetramesh tab.