Week 3

                                                                                      CAR HOOD ASSEMBLY PRE-PROCESSING USING ANSA

                                                                                                                     (WEEK-3 PROJECT)

OBJECTIVE:

• Import the CAD geometry of car hood assembly
• Clean up the geometry until no topological error is found
• Extract Mid-Surface for the geometry
• Mesh geometry with target/average length of 5mm

ABSTRACT

      A Car hood or bonnet is a part which is located at the front end and it covers the engine of the automobile. Its function is to protect the engine. We can easily open and close the hood during repairs or maintenance. Hoods are typically made of the same material as the rest of the body work. This may include steel, aluminium, carbon fiber . However, some aftermarket companies produce replacements for steel hoods in fiberglass or carbon fiber to make the vehicle lighter. In this project we are going to perform pre-processing of car hood geometry using ANSA which is the starting step of CAE.

METHODOLOGY

1. Import the CAD Geometry and do Geometry clean-up with no topological error.
2. Assign the property ID (PID) for each part
3. Generate mid surface
4. Setup Quality criteria
5. Meshing and improvement of the mesh

INTRODUCTION:

     HOOD

          Car hood mainly consists of two parts, inner bonnet and outer bonnet. Inner Bonnet usually provides strength for the engine, battery and other important parts in the front-end of the car. Outer Bonnet provides styling and gives aerodynamic shape to the car body. Various requirements are needed while designing such as safety aspects and the quality criteria.

                                                                                                                                                 Inner Hood of a Car

                                                                                                                                                      Outer Hood of a Car

PROCEDURE:

1. Geometry Clean-up

           After importing the geometry into ANSA there will be many topological errors. In order to prevent this, we do a geometry clean-up which is in the check’s manager.

        The errors in the above figure can be fixed automatically using FIX option or if the errors are not rectified, we must clear them manually.

    After all the error are rectified,if you check the geometry the exceution will be 100% then you can move forward.

2.ASSIGN PROPERTY ID (PID)

       Every part in the geometry has it’s own property. We can assign PID using ANSA. We have to select the part and click Apply, then we have to select the whole part using window selection to apply the PID.

     After assigning the PID we can always view that in the PID mode.

  LOCK VIEW

      This option is used store different part views of the geometry.One can always easily switch between the views. For eaxmple, you can switch between the parent geometry and mid surfaces of the parts.

MEASURE  

           We have to measure the thickness throughout the part. If there is any change in thickness, then we have to keep them under separate property.Now we have to store the measurements. It is observed that the outer hood and inner hood are approximately of 0.75mm thickness.

ORIENTATION

         We have to check the orientation and make it to visible in order to see the model clearly. We always prefer grey orientation over yellow in the entity mode.Enable cross hatches for better selection of the faces.

3.GENERATE MID SURFACE

         We have to know why we are generating the mid surface. If you take any geometry, half of the thickness will be above the mid surface and other half will be below. So in order to represent the geometry we have to create mid surface. There are two ways to extract mid surface: 1)Using off-set 2)Using mid surface option

1)Using off-set we can either create mid surface individually for each part or we can select the whole geometry using ‘Feature Selection’.Then give half the thickness i.e  -0.75/2 in the negative direction of the arrow.

2) Auto mid surface generation

           If the geometry is a closed one we can use this option. But it will delete the parent geometry after mid surface generation. This option is available under the midsurface tab. We have to select the whole geometry and confirm the mid location. A red line indicates the generation of mid surface which is shown in the figure below.

4.QUALITY CRITERIA

 1)Aspect Ratio: It is defined as the ratio of maximum element edge length to the minimum element edge length. The ideal value is 1 but less than 5 is acceptable.

2)Minimum/Maximum element length: It is very important to check this error as it is used for crash analysis check.

3)Jacobian: It is the scale factor used in the transformation of coordinated system. The ideal value is 1 but greater than 0.6 is acceptable.

4)Skewness: If the quad element is not a square and has deformation, skewness is defined as 90 minus the minimum angle between the two lines joining the apposite mid sides of the element and similarly for the triangular element it is 90 minus the line between the each node of the opposing mid side. The ideal value is less than zero but 45 degree is acceptable.

5)Warp Angle: It is defined as the angle between normals of two triangular planes formed by splitting the quad element along diagonals.

6)Target element length: It is the size of the element which we get should be nearer to the value compared to experimental value.

• Quality criteria can be accessed using F11 key. Now check the parameters which are required, give the values and then click apply.

• We must give meshing parameters before starting with mesh.

5.MESHING AND IMPORVEMENT OF THE MESH

   Now after asigining all the quality criterias and mesh parameters we can start 2D shell meshing. The type of elemets allowed in 2D meshing are of 4 categories.

   1)Quad elements 2)Tria elements 3)Parabolic quad 4)Prabolic tria.

                                                                                                                                                 2D Elements Types

                                                                                                       *L-Linear element                 *P-Parabolic element            *()-Number of nodes

• The quad elements are usually better than triangular elements.
• Similarly, parabolic elements are better than linear elements.
• For better merge transition we use tria elements at some places. It also provides good mesh flow.
• Trias at the edges and construction lines should be avoided for better mesh flow.
• We can recreate better mesh using reconstruction option which is in the shell mesh.

• We can also use smooth option to rectify some off-elements.

Options used for removing Trias

• Swap, Split and Paste options are used to remove trias originated on the geometry.
• Swap is available in mesh mode under elements tab. Swap is used to move the nodes as it rotates the common edge of two shells.
• Split is available in the mesh mode under elements tab. This option is used to split the trias into quad elements easily.
• Paste is available in the mesh mode under grids tab. It is used to paste the nodes manually/automatically.
• Holes feature should be even numbered and equally spaced.
• If there is any issue with the mesh extra hot points needs to be deleted on the geometry.
• In order to get better mesh flow, we need to cut faces using cut option which is in the Topo mode under Face grid. We can also create new faces.
• Rotating quads are not preferred.
• If there is any issue with a particular surface, one can always use universal delete option which is located under the search bar to delete that particular surface and create it using new-->fitted option.

                                                                                                                        Select the area                                          Delete the selected area

                                                                                                                                                              Create new surface

• After the completion of mesh generation for upper hood, 1)Quad elements: 50172 which is 99.53% 2)tria elements 236 which Is 0.47%. There are no off elements. 

• After the completion of mesh generation for lower hood, 1)Quad elements: 36347 which is 96.65% 2)tria elements 1258 which Is 3.35%. There are no off elements.

Mesh generations at some regions:

Learning Outcomes:

• Able to do topological clean-up with no errors.
• Able to generate mid surface for complex geometry.
• Aware of all the quality criterias and quality parameters during mesh generation.
• Understood about mesh generation and implemented mesh generation for complex geometry.
• Learned how to improve mesh flow and remove trias to achieve better mesh flow.

Conclusion:     

        From this project I can conclude, method of generating mesh for any geometry is very similar in case of shell meshing. I followed all the procedure mentioned in the grading policy. Practiced all the command in ANSA. Generated mid surfaces,able to defeature and do line adujstments in some areas. Able to mesh them successfully without any off elements. The majority of elements must be kept near to target length.

Hood Model of ANSA file link : https://drive.google.com/file/d/198m7mPNCqdhxAvBAjfl-D2ddnbTVHHAC/view?usp=sharing