To mesh the TRUNK BIW component of an Automobile Using ANSA

OBJECTIVE: To mesh the trunk BIW model for structural analysis using ANSA.

2. PROCEDURE:

2.1 Geometry Cleanup:

• First the model is imported to the ANSA software and checked for topological errors.

• These geometrical errors have to be rectified by the user using some of the tools available in ANSA from the modules buttons namely:

  a) Faces -> New -> Coons (or) Fitted

  b) universal Delete Option

  c) Faces -> Topo

  d) Faces -> Cut & Project Cut

  e) Cons -> Paste & Release

• After rectifying all the geometrical errors, once again geometry check has to be executed to confirm that there are no errors.

2.2 Mid-surface:

Why Should we Extract Mid-Surface?

To perform Structural Anlaysis for the components having thickness less than 6mm, it is always recommended to extract mid-surface from the component and proceed for Meshing. Meshing the whole component which has very less thickness is computationally expensive hence mid-surface is extracted from the component for which thickness is assigned and then the model is meshed.

• The mid-surface for the model is been already given and hence e have to proceed for the meshing part.

2.3 Meshing:

Why Do We Carry Out Meshing?

The Basic idea of Finite Element Analysis (FEA) is to make calculations at only limited (finite) number of points and then interpolate the results for the entire domain (surface or volume). Any continuous object has infinite degrees of freedom and its just not possible to solve this problem in this format. Finite element method reduces the degree of freedom from infinite to finite with the help of discretization or meshing (consisting of nodes and elements).

• To mesh this component first the target element length is to be set. Here element length taken is 4mm.
• For this, shift from Topo mode to Mesh mode from the modules section and select Perimeters -> Length option and select the complete model by dragging over the model using left mouse button and click on middle mouse button and enter length as 4mm.
• Now inorder to mesh the component ANSA provides many algorithms for meshing such as Free, Spot, Batch, Best, Adv. Front etc. Which ever algorithm gives good mesh can be chosen. Here Batch mesh algorithm is chosen because when compared to all other algorithms batch mesh was giving good mesh.
• To get a good mesh one has to mesh the component surface by surface and final meshed component has the following mesh elements.

• Now after meshing te component, click on the Hidden elements option under drawing styles section and off elements can be seen. These elements are the one which are exceeding the given quality criteria.

• Most of the Off elements were rectified while meshing surface by surface. In some parts like the beads present in the component, the quality criteria is failing for minimum length.
• These off elements can be eliminated when the bead geometry is altered. But since geometry capturing is very important than the quality hence the off elements are neglected.

• As it can be seen in the above picture that the edge length of the bead is 1.53mm whereas the min. length criteria is 2mm and hence it is failing in those areas.
• Except the bead areas all other off elements are rectified as shown below.

• The off elements can be rectified using some of the tools such as:

  a) shell mesh -> Reconstruct , Smooth

  b) Elements -> Split , Swap , Join , New , Delete

  c) Grids -> MV Surf , Paste , Align

Why to have less no.of traingle elements?

In all the above models we see that the triangle percentage is maintained less than 5%. This is because triangle elements are stiffer than the quad elements i.e. Triangular Finite Elements are stiffer in bending problems because they have constant strain across the element. This simply makes it impossible for an FE solver to calculate the bending response properly.

(OR)

The triangular element is called as a constant strain triangle(CST) element. Value of the stress tensor in the whole element will be constant . This may not be true in the real situation. So this approximation may create a lot of error.

That is the reason it always advised to have less no.of tria elements in the component.

3. LEARNING OUTCOMES:

From the above Trunk BIW model, following were the learning outcomes:

i) clearing geometrical errors using different ANSA tools.

ii) improving the mesh quality and mesh flow using different ANSA tools.

iii) Reducing the trai percentage by using paste and split option.

4. CONCLUSION:

• The TRUNK BIW component is meshed completely by satifying the quality criteria.