PRE-PROCESSING OF CAR HOOD ASSEMBLY USING ANSA -CAE

PRE-PROCESSING OF CAR HOOD ASSEMBLY USING ANSA

OBJECTIVE:

• Introduction to shell meshing i.e. 2D meshing
• Geometry clean up with no topological error
• Mid surface generation
• Meshing the geometry with the target length of 5mm

The quality criterion to be followed:

ABSTRACT: A car hood or a car bonnet is the part that covers the engine of the automobile. It is derived from the old English word 'hod' which means head. Its function is to protect and access the main engine, battery, and other crucial parts for repair and maintenance. As it is in the front portion and class A areas, therefore, it is adding a luxury look to any car model. so it very important to optimize the aerodynamic design as well as stiffness to minimize the air drag and flow over the bonnet. So in this challenge, we are going to perform the 1st step in CAE i.e Pre-processing of car hood geometry on ANSA.

METHODOLOGY:

1. Import geometry
2. Topological error cleanup
3. Mid-surface extraction
4. Quality criterion setup
5. Meshing and Mesh improvement

Introduction:

Hood

the car bonnet consists of usually two major parts i.e. an inner bonnet and outer bonnet. The inner bonnet is critical for providing the structural strength, while the outer bonnet is for providing a homogeneous style and maintain aerodynamic of the vehicle body. The outer body is not that important for strength as the inner body but it has a vital function in the case of pedestrian safety, being the first surface of contact with pedestrians. while designing a car bonnet various requirements should be fulfilled safety aspects, crash behavior, etc.

Geometry parts

The imported geometry consists of 5 parts:

1. Outer panel
2. Inner panel/extract
3. Latch reinforcement
4. Right hinge
5. Left hinge

PROCEDURE:

Geometry cleanup

After importing geometry into the ANSA environment we observe many topological errors while doing the geometry checkup as shown below.

keeping the feature as a single con, triple con, crack, needle face checked, etc. click the execute button, you will see many errors. For proceeding further we need to get rid of these errors. There two options you can follow i.e. 1) manual fixing of these errors one by one or by just telling ANSA to fix by itself. If you want to fix the errors by the manual method you need one by one approach the error area and fix it by either deleting the small redundant surface, releasing the triple cons, deleting hot points, repairing surfaces, etc.

otherwise, you can use the hybrid option to minimize your time consumption by auto fixing most of the areas and rest manual fixing. The reason since mesh needs to be generated it not enough that CAD model just looks correct but instead involves cleanup aspects as well as topological aspects.

After fixing the topological errors store the lock view for all the part such as 1) Outer panel 2) Latch & hinges 3) Inner extract (as shown below), this will help in extracting the mid surface for each of the sub-parts of the hood geometry

Next, we have to measure the thickness throughout the part & assure uniform thickness across the entire part. If thickness changes are observed then all alike thickness has to be kept under a separated property.

So by measuring the and storing data for thickness for sub-parts it can be concluded that the outer panel and inner extract are of approximately 0.75 mm and latch and hinges reinforcement are 1.2 mm thickness.

Orientation

Orientation must be checked thoroughly by the user every time working on a model. All models should have the correct orientation because the orientation issue will cause the final time issue when you merge the model with a complete vehicle. It is always preferred to use the grey orientation (entity model) on the top side (as grey being +ve).

Mid surface extraction

Before we discuss about mid surface extraction we should know about 2D elements and why should we carry out on mid-surface?

2D Element

when two of the dimensions are very large in comparison to the third one. 2 dimensions out of 3 are known by creating a 2D element and 3 dimension data is provided by the user as additional input data

why should we carry out on mid-surface?

Element thickness specified by the user is assigned half on the top and half on the bottom side. Hence in order to represent the geometry, it is necessary to extract the mid surface and then mesh.

All sheet metal parts, plastic components like instruments panel, etc. In general 2-d mesh is used for a part having width/thickness ratio>20.

There are two way to extract mid surface

1) The manual mid surface method

2) The Auto mid surface method

Manual mid surface generation

This can be done by manually selecting the surface and giving an offset option in the direction opposite to the grey surface

The offset is -0.75/2( negative direction ) because the generation of the mid surface has to be done in the direction opposite to the arrow(grey surface +ve)

similarly, it can be done for other subparts there for inner extract the thickness measured approximately 0.75mm and for the hinges and latch thickness is 1.2mm

Note:

1) While giving the offset make sure you do not press YES or else you can delete the parent geometry

2) use Fine option under the auxiliary section in TOPO mode to get a better resolution. (select fine and click on the curved and circular areas)

Auto mid surface generation

If the geometry is closed then we can go for auto mid surface generation option but it will auto-delete the parent geometry.

steps 1) press the mid surface tab under face 2) select the full geometry 3) confirm the mid location

Note: your parent geometry will be deleted but can be recovered back by using an undelete option. (you have to select the geometry again and do topo )

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once the mid surface is generated from either of the methods now we have to set the New property ID different for all 3 subparts with a respective thickness (as shown in the figure below).

Note: Also check for geometry after the completion of mid surface generation by keeping the single con option unchecked

Quality Criterion Setup

We can say that result quality is directly proportional to the element quality. The parameters like skew, included angle, Jacobian, warpage, etc. are the measure of how far a given element deviates from the ideal shape. If we talk about a square which means 90 degrees and equal side similarly equilateral triangle 60 degrees and each side equal. so we will discuss the quality criterion and try to understand them before assigning them into ANSA environment.

1) Warp Angle: it is defined as the angle between the normal of the two planes by splitting the quad element along the diagonal. so the maximum angle allowed between the two possibilities is called wrap angle. wrap angle is not applicable in the triangular element. the ideal value is 0 degree but less than 10 degree is acceptable as good practices.

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

3) Skewness: If the quad element is not square and has deformation then the skewness defined for as the 90 minus the minimum angle between the two lines joining the opposite mid sides of the element and similarly for the triangular element it is 90 minus the between the line from each node to the opposing mid-side. The ideal value is 0 but less than 45 degree is acceptable.

4) Jacobian: it is the scale factor used in the transformation of the co-ordinated system.(global to a local coordinate system). This is used for a faster analysis point of view. The ideal value is 1 but greater than 0.6 is acceptable.

5)Minimum element length: It is very important to check for crash analysis(time step calculation). it is also done to avoid the presence of any zero length element.

6) Target element length: It is the size of the element by which we can get the most nearer approximate value of the result compared to the experimental result.

To set up the quality criteria press the F11 key and check the boxes of the parameters of the required quality with required values and apply

Note: At least once again check the orient and do geometry check. if all good proceed further for meshing.

Meshing and Mesh improvement

Change the mode from 'Topo' to 'Mesh' mode

Before starting the mesh we need to set the element length. To do so we have to click the 'length' button under the 'parameters section'. After clicking the select in the pop-up window parameter and select the whole geometry and confirm enter the value 5 and confirm do the same for macros option. This alters the element length in the geometry by altering the no of hot points at the periphery as well as at your macro areas.

Also, set the meshing parameters before proceeding to mesh

so now we can start meshing the component individually but before we start meshing we will know about the 2D meshing and its good practices to follow to get a good mesh flow.

2D Meshing/shell meshing

The type of element allowed in 2D meshing are

1. Quad Elements
2. Tria Elements
3. parabolic Tria
4. parabolic Quad

As we know that the quads elements are better than triangular elements and parabolic elements are better than linear elements

But if the quads are better than tria then what is the need for using tria elements?

1. For better mesh transition
2. Avoid poor quality element in a complex geometry
3. better mesh flow
4. Tetra meshing (trias to tetra length)
5. Mold flow analysis

Best practices to follow to get better mesh flow.

1) Back to back tria element are avoided 2) on a planer surface tria should be avoided by merging them using split

3) no mesh transition on radius fillets/Curvatures (carried to planer surface ) 4) Hole feature should be even-numbered equally spaced

5) Avoid Trias at edges and on the features and construction lines

6) Avoid zig -zag distribution preferred smooth mesh(straight aligned nodes) 7) Rotating Quads are not preferred

8)constant mesh size (by using tries) Is preferred due to min. element length and time step criteria

9) Remove the redundant and excess construction line to get uniform mesh flow.

10) Divide the surface near the feature into to get better mesh flow pattern

11) Defeature the areas which are going to fail the quality criteria.

12) Mesh density near the critical areas ( features) is preferred.

so as we are now aware of what are the good practices for getting a better mesh flow. Now we can start our meshing process on ANSA

As we can see the inner extract is the symmetrical part so we can compare them and remove other symmetrical features. This can be achieved with the help of Rm. Bdl block under Faces in Topo mode. similarly, can be done for the outer panel

With the help of Rm. Bdl it is confirmed that we can remove the symmetrical half and perform meshing on it, later after the completion of the meshing process it can transform and mirror copy about the centerline the rest of the symmetrical part.

We should remove all redundant and unnecessary construction lines such as shown below in order to get better mesh flow.

Similar action to be performed for the outer panel and latch and hinges

Now we can finally start meshing the component and can get the better mesh flow by keeping in mind the best practices followed in meshing

We have multiple options to mesh (shell mesh)

1. Best mesh
2. Spot mesh
3. Free mesh

All the three works on the different algorithms but here in this model we will use the best mesh algorithm. Also after creating the best mesh we can reconstruct and smooth the mesh to get better mesh flow. ( shown below)

Note:

1. It is always advised to select the elements for reconstructing in a square pattern rather than an irregular pattern (algorithm works best in square selection )
2. It is also advised to macro area select the patch for smoothening the mesh.

Meshed Parts Results

Outer Panel

The hemming bend region which will surely fail for min length criteria so it kept as it is but make sure no trias at those regions.

Inner Extract

Latch and Hinges

As this area was failing the minimum length parameter it is advised to defeature the part. so we will compromise any one feature to save another feature (Don't remove any material from CAD geometry)

All the parts have meshed as per quality requirement and had good mesh flow (Smooth)

Learning outcome

1. Able to fix topological error
2. Able to generate mid surface for any complex geometry
3. Aware of the 2D elements and shelling meshing( Quads and Tria importance)
4. Aware of the quality parameters and there meaning during mesh generation
5. Able to understand and implement good practices of meshing any complex geometry
6. Competent in 2D meshing/ shell meshing even for complex geometry by achieving better mesh flow.

conclusion

From the above challenge, it can be concluded that methodology for any geometry is moreover very similar in case of shell/ 2D meshing. One has to follow the sequence stated above to get a better geometry cleanup followed by mid surface and finally meshing keeping in mind the good practices that strickly need to be followed in order to get better/smooth mesh flow. Few areas we need to compromise due to design constraints by the help of defeaturing and construction line adjustments but it should be only done until till and unless it damages your real geometry. The majority of elements must be kept nearer to the target length so as to get the nearest approximation to correct value (experimental reference data)

Hood model meshed ANSA file link: https://drive.google.com/file/d/1TjzXKrD6doZbtDdzAm7wpWYub96mj9QF/view?usp=sharing