Week 3 Challenge: Shell (2D) Meshing of an Automobile Hood using ANSA Preprocessor tool

Course: Preprocessor for Structural Analysis using ANSA

Submitted by: Amanpreet Singh Whan

Supervisor: David Nirmal Paul

October 13, 2020

Abstract

The hood of an automobile is a critical part and offers protection from dust and other foreign particles. The report presents a strategy that was adopted to mesh the given hood model using the ANSA Pre-Processor tool for the fulfillment of the Week 3 challenge. The shell meshing is a three-step process that includes geometry clean-up, mid surface extraction and finally performing meshing using setting quality and mesh parameters. The max quads percentage and max trais percentage were found to be 98.99% and 1.01 % respectively.

Table of Contents

Abstract.

1 Introduction.

2 Aims.

3 Methodology and Results.

3.1 Import CAD Model and do Geometry Clean-Up.

3.2 Mid Surface Extraction.

3.3 Meshing.

4 Discussion.

5 Conclusion.

6 Learning Outcomes.

Appendix.

1 Introduction

The hood or so-called bonnet is a crucial component of the front portion of an automobile. More than just merely acting as a hinged cover for the engine, the hood offers protection to the engine and other parts of the vehicle from dust, other foreign particles, and most importantly absorbs the noise and vibrations generated by the engine due to excitation on rough roads and at higher speeds. Nowadays, the hoods are essentially designed to be aerodynamic to reduce the impacts of air resistance and add a luxurious look to automobiles.

A hood normally consists of four parts namely:

• Outer Panel
• Inner Extract
• Latch Reinforcement
• Hinge Reinforcement

2 Aims

• Check and fix all the geometry errors of the given Model for Geometry Clean-up purposes.
• Extract/ Generate Middle Surface for all the components of the given model.
• Perform 2D or Shell Meshing of the components of the model as per given Mesh Quality Criterion to generate an overall good mesh flow for the entire model.
• Investigate and Improve the mesh quality wherever required.

3 Methodology and Results

The section describes the different stages that were performed using an engineering tool, ANSA Pre-Processor for the successful generation of an overall good mesh for the completion of the Week 3 Challenge Automobile Hood project. These stages can be further broadly sub-categorized into three key steps, starting from importing of cad model to geometry clean-up for mid surface generation and finally carry out meshing of the extracted mid surface as per quality requirements.

3.1 Import CAD Model and do Geometry Clean-Up

After opening the ANSA Pre-processor software, the first and the foremost step is to import the CAD geometry and perform topology using the topo command in the faces section of Topo Module Buttons for the entire model. In the next step, go to the Checks Manager and select geometry from the options list. The main function of the Checks geometry is to locate geometrical issues such as unmeshed macros, unchecked faces, needle faces, collapsed cons, triple cons, and cracks. Even though most of these errors can be easily eliminated using the Auto-Fix command but issues concerning triple cons must be fixed manually by releasing Cons or Hot-points (nodes) and joining cons/ nodes back again using the topo command. Before executing the Checks geometry operation, make sure to turn on the most essential above stated geometry issues.

What follows hereafter is the set of images showing different results obtained before and after the completion of geometry clean-up using ANSA Pre-Processor software.

3.2 Mid Surface Extraction

The mid surface can be generated both manually and automatically. A manual mid surface can be obtained using Faces->Offset command in the Faces section of Topo module buttons. Moreover, the mid-surface generated using manual operation must be assigned a new PID which can be easily done using the Set PID command in the faces section. However, ANSA directly assigns a new PID to a Mid Surface that is extracted automatically, and it is usually accomplished using Mid Surface->Skin command in the faces section.

The results of the mid surfaces obtained manually (using the command, faces->offset) for the different components of the given CAD model are presented in the following pictures.

3.3 Meshing

Meshing is a phase where the CAD geometry is discretized to represent the computational domain in which the solver will discover a solution. A refined mesh helps to achieve better results.

The meshing conducted for the given model is explained in the following steps:

1) After the extraction of mid surfaces, the Perimeters length was set to 5 mm in the perimeters section of the Mesh module buttons. This will help to generate a mesh of constant element size.

2) In the next step, the values of Quality Criteria and Mesh Parameters were entered as per challenge requirements to get a good flow of the mesh over the model. The values used for the quality criteria and mesh parameters are presented in the following tables.

Table 1-Quality criteria and its respective assigned values

Quality Criteria
Criteria	Value
Aspect Ratio	3
Skewness	45
Warping	15
Jacobian	0.7
Min length	2
Max length	7
Min angle quads	45
Max angle quads	135
Min angle trias	30
Max angle trias	120

Table 2- Mesh Parameters and its assigned values

Mesh Parameters
Parameters	Value
Target length	5
Minimum target length	2
Maximum target length	7
Distortion distance	20%
Distortion angle	0

3) Next, an even number was assigned to all the edges of the holes present inside the geometry using the ‘Number’ command in the Perimeters section. Also, the edges of the holes were aligned parallel to the next neighboring edges of the geometry.

4) The ‘Best’ Mesh Generation algorithm was used to produce mesh for the model.

5) Quality failing elements were eliminated using multiple approaches which include Reconstruction of mesh, smoothing of mesh, Moving & Pasting of Grids and Splitting, Swapping & Joining of elements.

The results of the meshed components of the model are presented in the following pictures.

4 Discussion

ANSA Pre-Processor was used to mesh the CAD model as per asked Quality Criteria and Mesh Parameters to generate a good overall mesh flow for the successful completion of the Week 3 Challenge. To fulfill the requirements of the challenge, Geometry clean-up along with Mid Surface extraction and Meshing was performed and the obtained results are presented in the figures shown above.

In figure 3, it can be clearly seen that the provided CAD model had multiple errors such as needle faces, cracks, unchecked faces, and triple cons which were fixed using both manual and automatic methods.

In PID mode, selecting the component’s generated mid surface and the parent component while keeping the parent component in transparency mode and it was found that the mid extracted falls exactly in the middle of the parent component of geometry, concluding mid extraction performed was accurate and precise.

The construction lines were manipulated by a certain amount in some critical areas of the geometry where quality could possibly fall to generate an overall good flow of the mesh for the entire model. However, this doing has increased the number of trias in these critical areas. In figure 19, the component (outer mid surface) has a total of 812 off or quality failing elements. All of them have Minimum Length errors and it has occurred because the maximum length of that specific portion (rounded region) is 1.948 mm while the minimum element size used for the mesh is 2 mm and that is the sole reason why quality has failed in that region.

The following are the figures that represent a few of the examples of manipulating construction lines and areas where minimum length errors have occurred.

5 Conclusion

In conclusion, the main aim of the challenge was to do a geometry clean-up of the given Hood model, generate mid surfaces of its components, and perform 2D meshing. The max quads percentage was found to be 98.99% while the max trias percentage was found to be 1.01%. Lastly, the meshed model was assigned a thickness by enabling the ‘Draw shell as a solid’ command in the Quality Criteria- Presentation Parameters tab.

6 Learning Outcomes

Throughout this challenge, I have gained knowledge about different commands that are used to fix geometry errors and are used to generate the mid surface of any CAD Model with both auto and manual options. I have also learned that the Mesh Quality can be fixed using several operations like ‘mesh reconstruction’, ‘mesh smoothing,’ ‘grids move or paste’, and ‘elements split, swap & join’.

Appendix