PREPROCESSING OF A SOLID MODEL

PREPROCESSING OF A SOLID COMPONENT USING ANSA

AIM:

• To mesh the given Solid model using Preprocessing Software (ANSA).

OBJECTIVES:

• Clearing geometrical and topological issues in the model.
• Generating Shell Mesh on the Model.
• Defining the volumes in the model.
• Generating Solid Mesh on the volumes.

PROCEDURE:          

1.GEOMETRY CLEANUP:

Importing the model from other software to ANSA results in some geometrical deviations which needs to be cleared before proceeding to other steps. To perform geometry check Tools->Checks->Geometry. In General needle faces and Unchecked faces error occurred which is common while importing the model to ANSA from Modelling software. In this model we don't have any geometrical deviation.

As our geometry is clean without any translation issues or geometrical deviations so we can proceed to topological activity.

2.TOPOLOGICAL ACTIVITY:

Topology describes how the different faces and features of the geometry connected with each other.

Boundary Cons of the faces and Features were called as feature lines other Cons in the Model were called as Construction Lines which were generated during modelling of the component. Construction lines can be altered as we want to get the better mesh flow. Feature lines should not be disturbed as it will result in Change in geometry however if the cons Length is less than the minimum element size it will result in element failure under such circumstances the feature line can be altered up to some extent without giving up the design feature of the part .In the part shown below the distance between the two-feature line is less than 0.5 mm which does not meet our quality criteria results in off elements. To remove the fillet Select Faces->Dach option.

Fillet will be removed using following steps.

The resolution of the curve can be improved by using Auxilaries->Fine command. To do that Select the command and Click multiple times on the curve that needs to be refined.

3.DEFINING MESH PARAMETERS AND QUALITY CRITERIA:

Quality of the mesh is user defined one as it varies for different applications. Quality of mesh will be defined by Mesh parameters and Quality Criteria. Mesh parameters option is available in the Utilities Bar.In that window average element Size, Minimum Element Size, Maximum Element Size and order of Mesh will be defined as shown below

Mesh Quality criteria also need to be defined from the Utilities bar.

Our quality criteria inputs for the mesh were

1. Average Element Length - 1 mm .
2. Minimum Length    - 0.50 mm
3. Maximum Length    - 2 mm
4. Tet Collapse    - 0.2

• Maximum Length:The maximum length allowed for the element edge.
• Minimum Length: The minimum length allowed for the element edge.
• Tet collapse is represented by a formula shown below.

            Tet Collapse=Min Value of [ `(h/sqrt A)/1.24`]

            h= Distance between the node and its opposite face.

            A=Area of the face.

4.SHELL MESHING GENERATION:

Mesh generation is process of discreting the entire model into smaller elements.

In ANSA mesh generation is done by set of algorithms available for meshing.

Image shows options available for meshing.

2D mesh has two types of elements:

• Quadrilateral or Quad Elements
• Triangular or tria Elements

Quadrilateral Elements:

Quad elements were the elements having four nodes. Square is considered to be an ideal quad element and the deviations were calculated in comparison with the square element. Quad elements were structured elements. They have short processing time and also have highier accuracy. Therefore Quad elements were preferred over the tria elements.

Triangular Elements:

Tria elements consists of three nodes. Equilateral Triangle is Ideal Tria element and deviations were compared with the Equilateral Tria Element. Tria element is said to be unstructured mesh. Stiffness of the Tria element is high which results in highier processing time and also have low accurate results. Therefore we should try to avoid the maximum possible number of triangular elements in 2D Meshing . In 3D mesh we’ll be using the tria elements as they will capture the feature better than quad elements.

Rotating Quadrilateral Elements:

Rotating quad elements were formed when any one corner of the 5 quad elements meet at a point to form a Star like structure. Even thought they were quad elements they should be avoided in the model as they act like the tria elements.

Meshing:

In curved faces we need to use ortho tria elements and on the Flat surfaces we need to use Tria elements because the ortho tia elements were good at feature capturing we can also use ortho tria elements on the flat surfaces but it will increase the computational time during simulation. Therefore at possible places it is suggested to use tria elements than ortho tria elements.

In the options list we can switch between the element types.

Step 1: Select Mesh Generation->Spot Mesh from the mesh module. Select ortho tria element type from the options list. Select the curved faces.

Step 2: Mesh will be generated as shown below with off elements.

Step 3: Select the element Type as tria from the options list. Select the flat surfaces

Step 4:Mesh is generated  as shown below on middle click

In ANSA mesh can be generated easily but the mesh will have lot of elements will be generated such that they fail to fulfill the Mesh Quality. Mesh Flow and Mesh quality improvement is the most important steps in order to achieve good mesh quality and Good Mesh Flow. To improve the mesh Quality there are many options available in ANSA. But for solid meshing we will generally use Reconstruct option as we primarily focus on to get rid of off elements.

1.Reconstruct:

This option is similar to Mesh generation but key difference is that Mesh Generation option will generate the mesh only once but reconstuct option will try to give all possible arrangements of the mesh either by altering nodal numbers, altering the type of element or altering the element size within the allowable limit  and also eliminates maximum number of errors.

Below steps to perform reconstruct option.

Step 1: Select Shell Mesh->Reconstruct. Select the ortho tria element type from the options list and select all the faces. Select the Edges and the click OK.

Step 2: Now ANSA will show the preview of the reconstructed mesh model if the rearrangement .

Step 3: On middle click The mesh preview is applied on the component.

As we can see ortho tria elements were applied on the flat faces also. To change them into tria elements the same steps said above should be followed by selecting the    element type as tria and select only the flat surfaces to reconstruct.

On Middle click the preview of the mesh is applied on the Face.

Model after generating mesh for the entire model.

5.DEFINING VOLUMES AND SETTING PID:

ANSA does not consider the closed surfaces as volumes unless or until the volumes were defined. In order to do the volumetric meshing defining volume is important. We will see how to define the volume once the shell mesh is created for the entire model.

Step 1: Select Volumes->Define->Manual under Volume Mesh Module and Select Auto mode in the dialog box appearing. Now select the volumes which needs to be defined.

Step 2: On Middle click ANSA will show property card.

Step 3: Right Click->New->PSOLID.

Step 4: Solid property dialog box opens. Enter the properties and click Ok.

Now solid property is defined for the volume. But it is hidden as the volume is unmeshed.

Now the properties window opens up showing the new Solid property created with the PSOLID.

Volumes defined can be viewed from the Volumes->List from the Volumes module.

6.VOLUMETRIC MESH:

3D mesh has two types of elements:

• Tetra Elements(Unstructured mesh).
• Hexa Elements(Structured mesh).

Tetra Elements:

Tetra elements were the 4 noded 3D elements which are formed to fill the volume. A ideal tetra element is the Equilateral triangle Pyramid. These elements will be formed such the on face of the element matches with the shell mesh element. These elements were constructed based on the volume mesh algorithm availabe in ANSA. By default ANSA has the Growth ratio of 1.2.They fill the volume by arranging the elements in a random manner which results in unstructured mesh. Tetra elements were preferred for the low simulation time. Even though the accuracy of the simulation results were lower than Structured mesh they were on the accepable level.

(The elements size will increases by 20% as they goes deeper inside the volume is defined as Growth Ratio. For example if the element has 1mm on the shell element, the size of the next element inside the volume will be 1.2 mm)

Hexa Elements:

Hexa Elements were the 8 noded element. An ideal hexa element is a cubic element. The arrangement of the elements inside the volume will be similar across the cross section of the volume. Hence hexa mesh is called as structured mesh. The results were more accurate when compared to unstructured mesh. Since they have highier simulation time it is used only in the important parts such as Engine block and Transmission components.

Step 1: Select Unstructured Mesh->Tetra Rapid or Tetra FEM under volume mesh module. Select the volume to be meshed

Step 2: On Middle Click the Mesh will be generated inside the volume indicated by number of tetra elements formed and Volume left unmeshed. In the above image unmeshed volume shows 4 now one volume is mesh and the below image shows unmeshed volume as 3. The mesh formed also consists of 1 off element as shown below

The cut section below shows the tetra elements were filled inside the meshed volume in unstructured manner. The other volumes were hollow as they were unmeshed.

6.IMPROVING TETRA MESH QUALITY:

The off element shown is the tet collspsed element. It can be cleared using

• Improve->Fix Quality
• Grids->MV Free

i.Improve->Fix Quality:

It is an auto fix option used to clear the off elements. It can be done by following steps.

Step 1:Select the  Improve->Fix Quality under volume mesh and select the off element which needs to be fixed.

Step 2:On middle click ANSA will fix the issue by moving the nodes

ii.Grids->MV Free:

It is an manual fix option. This option can also be used to clear the off elements in Shell mesh also. By this option the quality is cleared by following steps.

Step 1: Select Grids->MV Free option from the mesh module

Step 2: Right Click on any edge to create a local coordinates so that you can decide on which direction we need to move the node. The direction can be enabled in the MV free dialog box. We can also do it without creating the local coordinates at that time ANSA will move the nodes based on the global coordinates.

Step 3: Select the Node you want to move and middle click.

Step 4: Now the selected node is free to move. Move the node until the Quality criteria is achieved, once the node is acceptable click OK

Now the model will be free of bad quality elements.

ENTIRE MESHED MODEL

CONCLUSION:

We have created the solid mesh using the above said steps. In this model we’ve used ortho tria elements on the curved faces. We can use tria mesh on the curved faces whose curvature is high.

LEARNING OUTCOMES:

• Learned methodology of creating a Solid mesh.
• Understood key differences between Shell mesh, Solid Mesh. Structured and Unstructured mesh in solid meshing.
• Learned how to define the volumes and creating a solid property for the same.
• Learned how to improve the quality of solid meshing.