How to Solve Tet Collapse in Tetra (Unstructured) mesh?

There are two types of meshing done in Volume mesh. 

1. Hexa Mesh
2. Tetra mesh
   

Steps to Generate A Tetra Mesh:

Below are the steps to generate a tetra mesh:

• Perform Geometry cleanup & Geometry check.
• Give quality criteria and assign mesh length in the MESH deck. (Main quality parameters are: Min length, Max length)
• Generate surface mesh with TRIA elements. 
• Solve all OFF elements for the meshed surface. ( OFF elements in the case of surface mesh will be the elements that are failing for Quality criteria such as  Min length, Max length )
• Define a Volume in the V.Mesh deck.
• Mesh the volumes with either Tetra Rapid or Tetra FEM algorithm.
• Solve all OFF elements for Volume meshed components with given Quality Criteria. ( OFF elements in the case of Volume mesh will be the elements that are failing for Tet Collapse).
  

What is A Tet Collapse?

Tet Collapse is the most important Quality criterion which is always checked while Tetra mesh is Performed (Tet Collapse is applicable to Tetra Elements only).

• As we know the Tetra element is a four-noded element which has four sides and four nodes. (As shown in the image below)

 

• The height of the Tetra element (h) is measured from each of the four nodes to its opposite face and then divided by the Square root of the opposite face’s area (A). The minimum of four resulting values is then normalized by dividing it by 1.24.

 

Therefore, Tet Collapse can be calculated by,

 

• As the Tetra collapses, the value approaches 0.00. While a perfect Tetra element will have a value of 1.00  (Note: Tet collapse values less than 0.15 or 0.2 are not acceptable.)
  

So, If the height of the tetra element is less than the desired height then that element will fail for Tet Collapse.

How to Solve A Tet Collapse?

Method -1: Using ‘Fix Quality’ (Auto-Correction):

• For this, Isolate the failed elements and do Neighbour 1st level. (By doing Neighbour 1st level we can see the neighbour elements which are connected with isolated elements)
• Click on Fix Quality to solve the failed elements. (Refer to the below image to find out the Fix Quality tool)
  
  

• By doing this, ANSA will apply some small nodal movements which will eventually increase the height of the Tetra elements and failed elements will be solved automatically.
• But, this step may move the nodes outside the surface and create raised edges or kinks on the surface (i.e Node can move outside the surface or inside the surface) which will reduce the surface quality.
  
  

Method -2: Through MV Free (Manual Correction):

This is the method of solving Tet Collapse without disturbing the surface quality. 

• For this, isolate the failed elements and do Neighbour 1st level. (By doing Neighbour 1st level we can see the neighbour elements which are connected with isolated elements)
• Click on MV Free and select a Node which can be moved such that the height of the Tetra element can be increased. (Refer to the below image to find out the MV Free tool)
  

 

• Drag the node in the direction of height till it obtains the acceptable height of the element.
• Care should be taken while selecting the node for the movement. The node which is on the surface should not be moved in order to solve the failed element.
  

Hints:

While performing Volume mesh(Tetra mesh), a few things should be checked properly.

1. Geometry should be closed. (i.e. Geometry should not have any Single Cons)
2. Surface Mesh elements must satisfy quality criteria parameters. (i.e. Min length, Max length)
3. Volume mesh elements must satisfy quality criteria (i.e. Tet Collapse) without sacrificing the surface quality of the component.