3D Tetra Meshing

3D Tetra Meshing

Housing Model

Aim- To generate a 3D Tetra Mesh on the given component's by using different method's.

Objective-

Generate 3D tetra mesh by using 3 method's called

• Convertion of 2D element's into 3D element's.
• By doing Quick Tetra Mesh from user interface panel.
• By doing Volume Tetra Mesh from 3d panel.

 

Intro to 3D Meshing and Some Terminologie's -

3D Element Types-

Figure 1-3D Element Types

[Note:Penta and Pyramid element's are rarely used.But parabolic element's are very very rare element's only used for 1 or 2 specific types of condition's.]

 

Shell Element-

Figure 1.1-Shell Element's

 

Example of shell elements (CTRIA3, elements (CTRIA3, CQUAD4, CTRIA6, CTRIA6, CQUAD8)

• First Order 4 or 3 nodes.
• Second Order 6 or 8 nodes.
• DOFs 6 degrees of freedom per node.

Solid Element-

 

Figure 1.2-Solid Element's

 

Example of tetrahedron tetrahedron, pyramid, penta and hexa elements elements

• First Order 4, 5, 6, 8 nodes.
• Second Order 8, 12, 15, 20 nodes.
• DOFs 3 degrees of freedom per node.

 

Tetra Meshing Method's

There are two methods of tetra meshing:

1) Automatic mesh:

This approach is limited to simple geometries and the pre-requisite is an error free CAD model. The user just has to select the volume and the software automatically carries out the meshing as per the specified element length, quality criteria, etc.

Advantage: Very quick, no meshing efforts.

Disadvantage: Results in a very high number of nodes and elements. There is no control over the mesh flow and the specific mesh pattern requirement (like bolted, welded joints or contact surface simulation).

 

2) 2D (tria) to 3D (tetra):

This is the most commonly used method. Quad or tria meshing is carried out on all the outer surfaces of the geometry. During the tetra meshing the quads are automatically split into trias which then serve as the “basis” of the tetra elements.

 

Common Algorithms for Tria To Tetra Conversion -

• Advancing Front: This algorithm is very powerful and the most commonly used algorithm.
• Delaunay algorithm.
• Tria-quad mesh.

 

Procedure -

1. Housing Model

Phase 1- Importing

a. Hence we are importing a given CAD geometry into hypermesh.
b. There are file formates like IGES,STEP,Parasolid where we can import these file formats into any CAD,CAE Softwares.
c. But in hypermesh we can only import three file formates like

• IGES [Initial Graphics Exchange Specification].
• STEP [Standard for the Exchange of Product Model Data].
• Solidworks.

IGES,STEP,These two are standard file formats which are used most in industries.But now a days in industries,they are aslo using parasolid file format.

Figure 2-Importing Geometry

 

Figure 3-Imported Housing Model

 

Phase 2- Examining 

• Before working on the model.We have to check the geometry if there are any errors like

1. Damaged Geometry.
2. Free edges in unnecessary areas.
3. Unnecessary points on the lines.
4. Unnecessary Connections and connectivity error.
   

• By assessing the given three models.We can see the components in every model.

Housing Model

Two Component's

Figure 4-Housing Component's

 

Phase 3 - Creating 2D Shell Element's on the Geometry

• For this Model,I'm using 2D tria to 3D tetra method.

• First delete the solid's if it exsist's in the component.
• With the surface geometry,automesh the component from 2D panel.
• While meshing the component,Change element size to 5,In the mesh type drop down menu,switch to the trias and mesh the component.
• 2D Panel`⇒`automesh`⇒`surface`⇒`displayed`⇒`element size-5`⇒`mesh type-trias`⇒`mesh`.

Figure 5-Meshing Process

 

• After meshing the component,If it is in wireframe elements mode,Switch to the shaded elements mode.

Figure 6- Switching element mode

 

• Now I have made 2D Shell element's,Check for the element quality and eliminate the element's failing for min and max length.
• Here element's are not failing for min length.
• It is failing for max length.try to eleiminate them.

Figure 7-Element's are not failing for min length

Figure 8-Element's failing for max length

 

To check the element's failing for max length.

• Go to the Tool's⇒Check elems or (F10)⇒Max length⇒It will show elements failing.

To clear  the element's failing for max length.

• Go to the Tool's⇒Check elems⇒Max length⇒Save failed.
• Tool's⇒Mask or (F5)⇒Elements⇒Right click⇒Retrieve⇒Mask⇒Reverse⇒Mask adjacent.

 

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Figure 9-Failed Element's will be displayed like this

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Figure 10-Make it unmask adjacent

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Figure 11-For this particular region,Element's will be failing for max length,Remesh that particular region with element size as 4,Before it was 5.Change it.So the element size will be applied for that particular region only.

 

Figure 12-Now element's are not failing after remeshing with element size as 4.

 

Figure 13-Remeshed

 

3.1 Start Tetra Meshing

3D Panel⇒Tetramesh⇒Element's⇒Mesh.

Tetrameshing: Standard Tetrameshing

• Standard tetrameshing: 3D > tetramesh panel > Tetra Mesh sub-panel

Process:

• Generate a surface mesh of shell elements

• Check quality and connectivity of the plate elements.
• Generate the tetrahedral mesh.
• Delete the original surface mesh.
• Edit if necessary to obtain good quality.

Figure 13:1-Tetramesh Subpanel

 

• Requirements for the shell mesh:

• Enclose one, and only one, continuous volume..
• There can be no free edges. (Otherwise not a solid geometry)
• There can be no T-connected edges.
• There can be no duplicates in the mesh.
• Elements should not fold over and overlap each other.
• Avoid very low minimum tria angles.
• Avoid a large difference in size between adjacent elements.
• Avoid a large difference is size between two sides of a wall thickness.

• For quad elements in the shell mesh:

• Can split quads into 2 trias and create tetra elements under them.
  – OR –
• Can keep the quad element and create pyramids under them.

• Floatable Trias:

• Adjacent tria faces on the tetrahedral mesh may have their diagonal reversed
  from the shell mesh if tetras are better quality.

Figure 13:2-Flotable Trias

 

• Fixed Trias:

• Adjacent tria faces on the tetrahedral mesh always match the shell mesh.

Figure 13:3-Fixed Trias

Figure 14-Tetramesh

After Tetrameshing,Mask the elements in a particular region and see,whether the tetra element's are created as 3D(Solid).

3:2 Check whether the element's are failing for Tet Collapse

• In 2D Tetramesh,We will be checking whether element's are failing  for only min and max length.
• In 3D Tetramesh,We will be checking whether element's are failing for only tetcollapse.

3:3 Other Checks for Tetra Meshes

1. Quality checks for 2D tria elements: Before converting trias to tetras, all the quality checks as discussed for shell elements should be applied.
2. Free edges: Conversion from tria to tetra is possible only when there are no free edges.No free edges indicate the mesh is enclosing a volume.

What is Tet Collapse ?

Tet Collapse can be calculated by measuring the distance of a node from the opposite face, then each of the four values will be divided by the Square root of the opposite face's area. The minimum of four resulting values is then normalized by dividing it by 1.24.

Figure 15-Tet Collapse

 

Tetra elements whose collapse value falls below the value specified are highlighted when the tetra collapse function is selected.These elements remain highlighted until the Check Elems panel is exited.

Tetra collapse calculation: At each of the four nodes of the tetra, the distance from the node to the opposite side of the element is divided by the square root of the area of the opposite side. The minimum value found is normalized by dividing it by 1.24, and then reported. As the tetra collapses, this value approaches 0.0. For a perfect tetra, this value is 1.0.

Figure 16-Tet Collapse Formula

 

To check the 3D element's mask the element's.

Figure 17-Element's have been masked and reversed

 

3:4 Tetra Remeshing to clear the element's failing for tetcollapse

Before Tetra remeshing,Go to Tetramesh parameter's⇒Tick all Check Boxes Fill Voids⇒Fix midnodes⇒Smoothing to get better quality.

In Tetramesh panel,In boundry faces drop down menu change it to remeshable and mesh.

Figure 18-Tetramesh Parameter's

 

Why ticking the checkbox of Fill Voids ?

Mesh all volumes, if your geometry includes volumes inside of another volume. For example if you had a sphere inside of a larger sphere, checking this option would cause the volume of the inner sphere as well as the volume between the two spheres to be meshed.

Why ticking the checkbox of Fix Midnodes ?

In case of 2nd order tetras, this option fixes the mid edge node of surface mesh while volume meshing.

Why ticking the checkbox of Smoothening ?

Apply an extra stage of calculation to improve overall mesh quality. Additional smoothing and swapping steps will be performed and tetra elements will be split to achieve a smoother mesh transition. If tetra elements are used in the boundary layer those elements will be excluded from smoothing to maintain the original distribution.

 

Figure 19-Unmask Adjacent to Tetra Remesh

 

Figure 20-After Tetra Remeshing

 

3:5 Manually clearing the element's failing for tetcollapse by using

• Distance Tool
• Translate Tool

Using Translate Tool-

Figure 21-Element Failing for Tet Collapse

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Figure 22-Using Translate Tool

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Figure 23-Failed element have been cleared using translate tool

 

Using Distance Tool-

Figure 24-Element failing for tetcollapse

 

Figure 25-Failed element have been cleared using distance tool

 

After Tetramesh,Make sure to delete the 2D shell elements.

Phase 4

Similarly as i did for the above component,Do the same thing for this component also.

Figure 26-Creation of 2D Shell Elements

 

Figure 27-Element's filing for min and max length

 

Figure 28-Mask the Elements

 

Figure 29-Mask Adjacent and Remesh it

 

• After remeshing the 2D shell element's,Some element's which are failing for the min and max length will be cleared,Some won't be cleared.
• So you have to clear that element's manually using distance tool.

Figure 30-Clearing element's using distance tool

 

Now 2D shell element's are not failing for min and max length.

 

Figure 31-Element's are not failing for min and max length

 

4:1 Start Tetrameshing on the component 

• 3D Panel⇒Tetramesh⇒Element's⇒Mesh.

Figure 32-Tetrameshing Panel

 

• After tetrameshing,Check Whether the component has been meshed with 3D tetra element's.

 

Figure 33- 3D Tetramesh

• Check the element's whether they are failing for tetcollapse or not.

Figure 34-Element's failing for tetcollapse

 

• To clear the element's failing for tetcollapse Tetraremesh the element's.
• While tetraremeshing ,Make sure you are ticking the check boxes of fill voids,fix midnodes and smoothening in the tetramesh parameters panel and give the tetcollapse value.
• Unmask adjacent and tetraremesh the element's.

Figure 35-Unmask Adjacent

• Before tetraremeshing,Do this thing

Figure 35-Tetraremeshing Process

 

 

Figure 36-After tetraremeshing no element's are failing for tetcollapse.

 

•  In the above figure,After tetraremeshing no element's are failing for tetcollapse.If 2 or 3 element's are failing for tetecollapse in the sense,Use Translate and distance tool to clear them.

• You can clear the element's failing for tetcollapse by using Solid Mesh Optimization.

Figure 37-Solid Mesh Optimization

 

Figure 38-Solid Mesh Optimization Panel

 

• Go to the edit criteria and uncheck all the boxes except tetcollapse,Cause you are checking only for tetcollapse.

 

Figure 39-Edit Criteria Panel

• Here if you click the check,It willl show element's failing for tetcollapse,131 element's are failing.

Figure 40-Element's Failing

 

• To clear the element's failing for tetcollapse,Click the fix button to eliminate the elements failing for tetcollapse.

Figure 41-0 element's are failing

 

[Note:Make sure to delete the 2D Shell Element's after 3D Teterameshing]

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Figure 42-Deleting 2D Shell Element's

 

Model-2

Only One Component

 

Figure 43-Model 2

 

 For this component,Make quick tetramesh or Volume tetramesh.

Quick Tetramesh

• Quick tetra mesh tool is accessed by going to View>Browsers>Hypermesh>Utility>Geom/Mesh>Quick tetramesh.
• In the Quick tetra mesh tool, the component & surfaces are selected, mesh size is set to 3 unit's, and mesh type is set to Trias only. After clicking on mesh, a new component will be generated with the tetra mesh.

• Geom/Mesh page of the Utility menu.
• Provides a quick method of generating a tetramesh.
• Maintains specified minimum element quality criteria.

• Resulting tetramesh may deviate from the geometry to maintain good element quality.
• User can select “sacred elements” or “sacred surfaces” to force the tetramesh to closely follow the original elements/surfaces.

• Click Help on the Quick Tetramesh panel for a description of the inputs.

Figure 44-Geometry

Figure 45-Geometry/Meshl

 

Figure 46-Quick Tetramesh Panel

 

Figure 47-Switch to the elements and proceed

 

Figure 48-Select Crank Housing and Mesh

 

Figure 49-Meshed

 

• Now the component has been meshed with Quick Tetramesh Method.
• After meshing Quick Tet's component will be created individually.
• Now rename that Quick Tet's Component or Organize it to the Crank Casing Component.

Figure 50-Quick Tetramesh

 

• Make Sure to delete the 2D Shell Element's.
• Similarly what you did to the Housing Model.

 

• Check for the element's failing for the Tetcollapse.

Figure 51-Element's failing for the tetcollapse.

 

Figure 52-Tetraremsh Panel

• Try to clear them using Tetraremesh.
• We can also try to clear them using Solid Mesh Optimization.
• If one or two element's are failing,We can clear them manually using Translate and Distance Tool.
• Unmask Adjacent and tetraremesh the element's.
• Element's will be cleared.

Figure 53-Unmask Adjacent Element's

 

Figure 54-Tetraremeshed Component

                                                                                                                                    

                                          

Model-3

Four Component's-Two Casing,Central Hub,Hub

Figure 55-Model 3 Component's

 

• For this Model,I'm using Volume Tetramesh method.

• Volume tetrameshing: tetramesh panel > Volume tetra sub-panel Mesh > Create > Tetra Mesh.
• Provides a quick method of generating a tetramesh.

Has some specialized options:

• Use Proximity – Creates smaller elements next to small features to make a smooth transition from small to large.
• Use Curvature – Will place more elements along curved surfaces based on user specified settings.

Figure 56-Volume Tetramesh

 

• Use proximity and use surface curvature options.

Figure 57-Proximity and Curvature Option's

 

• Check whether the element's have been meshed as 3D tetra or not.

• Delete the solid's in the component.

Figure 58-Component meshed with 3D tetra's

 

Figure 59-Element's failing for tetcollapse

 

• Tetraremesh the failed element's.
• After Tetraremeshing the element's,Failed element's for tetcollapse will be cleared. 
• Zero element's failed for the tetcollapse.

Figure 60-Tetraremesh the element's

 

• Now no element's are failing for the tetcollapse.

[Note:Similarly do the same thing for the casing 2 component also.]

• Casing 2

Figure 62-Casing 2 Component

 

• Do Volume Tetramesh for the component,Which i done same thing for the previous component.

Figure 63-Element's failing for tetcollapse

 

• Do Tetraremesh for the element's failing for the tetcollapse and delete the solids.

Figure 64-No element's are failing for tetcollapse after tetreremeshing the element's which are failed.

 

• If one or two element's are failing for the tetcollapse,To clear them,Use Distance and Translate tool.

 

Hub Component

Figure 65-Hub Component

 

• For this,I'm using Volume Teramesh.In the enclosed volume drop down menu,I'm switching to the surface and meshing the component,Cause the component given to me has surface no solid.
• Volume tetrameshing: Tetramesh panel > Volume tetra sub-panel > Mesh > Create > Tetra Mesh.

Figure 66-Volume Tetrameshed Component

 

• In this component,While doing volume tetramesh,The element's are not failing in the component.

• Check the component,Whether it has been meshed with 3D tetra element's or not.Mask th element's and see.

 

Figure 67-Component has been meshed with 3D tetra element's

 

• Now Reflect the 3D tetra element's to the other side.
• Before reflecting the element's,Create nodes in the arc centre using Tool Nodes.
• Geometry > Nodes >Arc Centre >Select the lines around the arc >Create.

Figure 68-Selecting the lines to create a node at centre

 

Figure 69-Nodes panel

 

Figure 70-Node Created Using Interpolate Node

 

Figure 71-Nodes Panel

 

• Now Reflect the element's to the other side.
• Tool > Reflect > Element's Duplicate >Current Component > Y axis > Base > Reflect.

Figure 72- Reflect Tool

 

Figure 73-3D Tetra element's have been reflected to the other side.And no element's are failing for this component

 

Central Hub Component

• For this component,I will be using Volume Tetramesh to mesh this component.

Figure 74-Central Hub Component

 

• Volume tetrameshing: tetramesh panel > Volume tetra sub-panel > Mesh > Create > Tetra Mesh.
• Provides a quick method of generating a tetramesh.

Figure 75-Volume Tetramesh Panel

 

Figure 76-Meshed with Volume Tetramesh

 

• The above component has been meshed with the volume tetramesh.
• Check for the element's failing for the tetcollapse.

Figure 77-Element Check Panel

 

 

Figure 78-Element Check

 

•  For this component,After doing volume tetramesh,No element's re failing for the tetcollapse.

 

• Check whether the component has been meshed with the 3D Tetra Element's.

Figure 79-The component has been meshed 3D Tetra Element's

 

Figure 80-Meshed Component With Volume Tetramesh

 

• Here for this component with volume tetramesh,zero element's are failing,No need to tetraremesh. 

 

Final CAD Model Image -

Figure 81-Model 1

Figure 82-Model 2

 

 

Figure 83-Model 3

 

Figure 84-Model 1 3D Tetramesh

 

Figure 85-Model 3 3D Tetramesh

Result -

• Hence TetraMesh Generation Method-2D to 3D Conversion for Housing component with the following quality criteria has been done.
• Hence TetraMesh Generation Method-Quick Tetra Mesh/ Volume Tetra Mesh for the given model with the following quality criteria has been done.
• Hence TetraMesh Generation Method-Quick Tetra Mesh/ Volume Tetra Mesh for the given assembly model with the following quality criteria has been done.

Conclusion and Learning Outcome -

• In this Week 8 Challenge,I came to know how to 3D mesha component properly without dent's and kink's.
• Get to know how to 3D Mesh with different three methods and advantages of those methods.
• How to do Quick Tetramesh,Volume Tetramesh.
• How to do Solid Mesh Optimization.