PREPROCECSSING SUSPENSION MODEL USING ANSA(SOLID MESHING)

MESHING AND PROVIDING APPROPRIATE CONNECTIONS FOR THE REAR SUPENSION

AIM:

• To mesh the components and provide appropriate connections using ANSA for the Rear Suspension model.

OBJECTIVES:

• Isolating the different components and Locking the views.
• Assigning PID’s to the different components.
• Creating the shell mesh and Defining the volumes.
• Generating the solid mesh and Assigning the Solid PID’s.
• Providing appropriate connections.

PROCEDURE:

 The above said procedure is done for each component separately by locking the components separately so that we can be able to generate good shell mesh as well as solid mesh.

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 this model since we have different components which are very close in this model there is possibility of the cons getting pasted resulting in the triple con which is itself a geometrical error. For example, tyre and rim which were very close to each other therefore the cons in the tyre and rim will get pasted while importing the model. It is suggested to isolate the parts and to perform the geometry check. If any triple con found release the con and paste it again by hiding the third surface by which you can eliminate the triple con errors

2.TOPO CLEANUP:

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.

3.DEFINING MESH PARAMETERS AND QUALITY CRITERIA:

In this model we do have different mesh parameters and quality criteria. Enter the mesh parameters and the quality criteria and save them separately.

i. Tyre and Rim:                    

ii. Other solid components:

iii. Shell Components:

Enter these values in Mesh parameter and Quality criteria and save them as different files. To check for errors for the particular components isolate them and retrieve the corresponding quality criteria and the mesh parameters.

• 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= Distance between the node and its opposite face.

                A=Area of the face.

4.MID SURFACE GENERATION:

Mid surface is the exact middle layer or surface of the component having uniform thickness over the part.For instance if the component or sheet metal having the thickness of 2mm then the surface at the 1mm from any of the face considered to be a midsurface. In this model the components having thickness>5mm it should be treated as shell component.

In ANSA can generate Mid surface by two ways:

     I.By Faces->Offset option

     II.By Faces->Middle option

I.By Faces ->Offset option:

This is manual method to generate the mid surface. Midsurface for te entire component is genetated by is generating the faces one by one by Faces->Offset command where you have to give the offset distance manually(Usually Thickness/2). In this method we have an option the keep the original face or to delete it. Once the mid surface is generated for the entire model they don’t have the associated with each other. ANSA will indiacte this by using a Single con on the boundaries of all the surfaces. To stitch all the faces with each other  Face->Topo option will be used. Select Face->Topo->Select the faces to want to Stitch->Middle mouse click, Now the faces were attached with each other indiacting the double con where the two surfaces attached togather. In this method the child surface orientation may differ from the parent surface so it is advisable to perform Faces->Orient one the Topo performed after Mid   surface generation.

In step 1 the direction shown outside the face therefore the offset value is entered in negative value as we want to create the surface in the opposite direction. In step 3 you can see the mid surface generated in the model indicated by a single con as it is shared by only one surface.

II.By Faces->Middle option:

In this option ANSA will create a mid surface automatically when any two surface is selected.

5.SHELL MESH 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:

For Solid meshing we should use tria 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. For shell meshing we should use mixed elements(Quad and Tria).

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

Shell Meshing :

Since we’re generating shell mesh load the Mesh parameters saved for shell mesh.Select Mesh Generation->Spot Mesh from the mesh module. Select the face to mesh. On confirmation the 2D mesh will be generated. Improve the shell mesh by using the Reconstruct, Smooth, Split, Swap, Join options. Once the Shell mesh is done to join them with the volume the node pair should be perfect. We can’t able to join the shell Geometry to volumetric geometry. We cab be able to attached them with the FE only. Therefore project the nodes with the con on the volume.

Solid Meshing:

Step 1:To create a volumetric mesh first the surfaces must be meshed with Tria or ortho tria elements on appropriate surfaces.

6.DEFINING VOLUMES AND SETTING PID:

To define the volume go to volume mesh module Select Volume->Define->Manual. Under that select Auto.

Now select the volumes and press middle click.

The volumes were defined and we can find the on Volumes->List. On clicking the volume name it will highlight the volumes.

7.VOLUMETRIC MESHING:

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 surface, 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 volumetric mesh is generated is shown below..

8.IMPROVING QUALITY:

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

1. Improve->Fix Quality
2. 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. Select the  Improve->Fix Quality under volume mesh and select the off element which needs to be fixed and upon confirmation ANSA will adjust the nodes to clear the Quality criteria. One disadvantage of auto fixing option is if ANSA found any off elements on the surface of the geometry it will try to adjust the nodes on the surface resulting in kinks or dents. Therefore it is suggested to verify the surface once the auto fix option is used to clear the quality criteria.

ii.Grids->MV Free:

It is an manual fix option. In this method we should move the nodes based on Global coordinates or by using Local coordinates.

In this model we didn’t have any off elements on volume mesh.

9.PROVIDING CONNECTIONS:

Connections can be provided by using connection manager only on shell elements.  Since we have solid elements in this model connections can be given by using NASTRAN deck only.

In the image shown the many node RBE2 should be created and the master nodes should be joined by using 2 node RBE2 elements.

Step 1: In NASTRAN Deck select Elements->RBE2->Many Nodes. Select the nodes to form   RBE2 element.

Step 2: On clicking Middle click various options as shown below open. You can apply the DOF of the RBE2 Element and its part.

Step 3: On middle click RBE2 element will be formed as shown below

Step 4: RBE2 elements should be created as shown in the image below

Step 5: Master nodes of those RBE2 elements must be connected by using 2 node RBE2 element. Select Elements->RBE2->Two Nodes from NASTRAN deck. Select the node pairs.

Step 6: On middle click two node RBE2 elements were formed between the node pairs.

In this location the rotation about x axis should be allowed to feel the effect of the suspension. If they were connected by arresting all DOF we cannot be able to feel the effect of spring on the suspension as the wheel will be rigidised with the chassis.

Step 7: To release the DOF about the rotation on X axis. Click on Elements->Info. Now select any two node RBE2 element. ANSA will show the property card as shown below.

We can see that in CM databox all the DOF were arrested. We should release the 4^th DOF i.e. rotation about x axis as shown below.

MODEL AFTER MESHING AND PROVIDING CONNECTIONS

LEARNING OUTCOMES:

• Learned methodology of creating a Shell mesh along with Solid mesh with various components.
• Learned to join the Shell FE with Solid Elements.
• Learned to represent the Coil Spring with 1D element.
• Learned to provide connections in various locations with appropriate DOF.