Week 3

AIM :- Car Hood geometrical clean up , its mid surfacing and mesh generation using ANSA

ABSTRACT :- The hood is an opening body panel that covers the engine in a front-engine vehicle. The hood is also considered as another type of door in a vehicle as it consists of an outer panel and inner panel. The inner panel provides strength while the outer panel functions as the metal cover.Car hoods are often manufactured with built-in scoops made of steel, fiber glass or carbon fiber.

A car hood is a hinged cover that are sometimes held by struts to keep the engine bay from external elements that may ruin components like the engine and radiator. Apart from its function, the engine hood also contributes to the car’s aerodynamics by allowing air to flow unhindered along its smooth surface and onto the windshield.

Fig : exploded view of the hood

OBJECTIVE :- To ensures the safety of the people behind the wheel and also maintaining proper aerodynamic it is important to properly design the hood of the car . Here we are going to perform the preprocessing design on the hood using ANSA following the systematic methods and defined criteria .

METHODOLOGY :-

• Importing the model into ANSA software and seperating the model into 2 parts as inner and outer panel
• Assinging both of them separate PID or property identity
• Checking for geometricak error.
• Resolving those geometrical error.
• Extracting the mid surface of both parts.
• Assigning them separate PID .
• Perform topology clean up if needed.
• Assigning the stated quality criteria and mesh parameters as mentioned below in the tab

S.No	Quality Criteria	Value
1	Target/Average length	5
2	Minimum Length	2
3	Maximum Length	8
4	Aspect	3
5	Warpage	15
6	Skewness	45
7	Jacobian	0.7
8	Minimum Quad Angle	45
9	Maximum Quad Angle	135
10	Minimum Tria Angle	30
11	Maximum Tria Angle	120
10	Tria Percentage	15

PROCEDURE

1.Importing the model into ANSA software and seperating the model into 2 parts as inner and outer panel:

• First we will import the CAD hood model into the ANSA GUI.As we in the above picture we can see the hood cad based model .as we can see model is shown in 2 colors while we are in entity mode. Entity mode is in which there are no properties applied on the model ex. Thickness ,poision ratio, etc. so part is shown just according to its fits in the assembly.

Here the

Yellow = negative side

Grey = positive side

Now if we switch to PID mode , the part is shown according to the property color assigned in the PROPERTY card. Below shown fig

Make sure to activate the material as the pasrt in MATERIAL card.

Use the measure tool at different surface to measure the thickness of the part to ensure that there is uniform thickness.

• Segregating the hood into 2 parts as inner and outer panel :

For our convenience, we will divide the whole model into simpler model. Using the !not and feature angle option we will separate the hood into inner and outer panel .Simultaneously store this into 2 different locks and name them accordingly. Also then give them different PID so that we can see them visually in 2 different colors .

fig : selected upperhood in inot option

fig : innerhood seperated

fig : assigning pid to innerpanel

fig : seperated inner and outer panel

After that we can delete the rest of the property only keeping inner and outer hood

2.Checking for geometrical error :

When the CAD model is imported into ANSA we need to rectify for certain errors to make it performable into this platform and more like uniform geometry without any cracks.

To check for the errors in ANSA there is a tool called Geometry Check under CHECKS too. Errors we are encountered here are

Note :

Cons are lines around the faces denoted by different colours.

Red line = single cons(there is only one surface i.e no other surface is attached to it)

Yellow line = double cons( two surface attached to each other i.e to single cons join together to form double cons )

Cyan line = triple cons( 3 surface joining generate triple cons). Triple cons error can be eliminated by releasing the cons into single cons and eliminating the additional cons.

• Undefined faces : undefined faces are those where faces are damaged maybe by gaps or there maybe additional surface or presence of multiple cons.

Solution : isolate the undefined region and release the cons where its showing errors and try to eliminate additional cons or see additional surface created. Other way u can delete the face see for additional cons , release them and again create a fitted surface.

• Needle faces : neddle faces needs to be eliminated because those face can create problem in

Mesh generation as it may fail the mesh elements

Solution: delete those face , join them with adjacent faces or recreate the face .

2.Extracting the mid surface of both parts.

Midsurface meshing is opted for components where the geometry thickness is small when compared with the span length of the components, so if you see most of the automotive structures are of sheet metal and plastics, where the length of the component is large when compared to the thickness, hence for this reason we opt for mid surface extraction and assign the thickness of the component to the elements through properties on components method.To study the components specially thin walled under the load i.e its bending , shearing , tension and endurance virtually in software will take more time if go for volumetric simulations. Inorder to save time those 3d element is converted 2d element by extracting its midsurface .

Since the amount of elements is less for the shell approach, because the wall thickness is captured as a mathematical value instead of actually modelling the thickness, there will be less equations to solve. This results in simulations that run in minutes or an hour instead of days to weeks. That also means that a shell mesh approach can be easily used to perform iterative simulations. Another benefit is that changing the wall thickness in your simulations with the shell approach are easy, and you will see the results of those changes in a very short time.

Approach for extracting the midsurface :

• Midsurface by skin : these method is quick but the problem is that we lose the parent model though it can be recovered .
• Offset : these approach is used when the components is complicated you can extract the midsurface region by region also parent components is safe .

Select the region for the offset. While giving the offset value the arrow shows the direction of the offset or midsurface. the arrow goes from negative to postive side but we the region here is grey i.e positive so the arrow goes to that side .

The value here of for offset -0.75/2. These value is half of the thickness of the components. Minus sign is used because we want midsurace in middle of the component and oppposite to the direction of arrow . arrow is going for positive side so we are using minus as negative direction

Now make sure not delete the original faces.

red line denotes the midsurface created .

• Pid :

Using the INOT option separate the midsurface of the of both the parts.

Assigned them PID using SET PID option. Make sure to add the thickness value as 0.75. if thickness is defined the part is considered as invalid.

3.Perform topology clean up:

The extracted may have certain extra cuts and additional cons and hotpionts over its sufrace , we will try to eliminate those things. It necessary to eliminate those so that while performing the meshing the mesh will be more uniform and the element pass through the criteria

4.Assigning the stated quality criteria and mesh parameters before performing meshing :

There are certain quality criteria that are assigned to the elements of the mesh. before that we need to under stand the shell or elements. Mesh are divided into two types

1. Shell mesh /2-D mesh:

The general characteristics in shell model are listed as follows.

• When the thickness of the part is very small comparing with its flow direction (thickness/ dimension ratio is less than 0.1), the flow in gap-wise direction can be ignored.
• The combination of geometry approaching and thickness definition is called Thin Shell assumption, which constructs the shell model.

The Shell Element Type option list includes two element types you can use to shell-mesh models—triangles and quadrilaterals.

• Triangles— Meshes the surfaces with triangles.
• Quads— Meshes the surfaces with quadrilaterals. In this case, triangles appear only in areas where the mesh generator could not use quadrilaterals.

2.Solid mesh/3-D mesh :

From the point of view of the geometry of the molded parts, sometimes, ambiguity exists in identify the mid-plane and it is very difficult to be fully automatic. In addition, some 3D flow phenomena (gap-wise) will be lost by using 2D model to simulate, such as flow passes expansion and contraction area, flow passes spheres, flow passes a small dimension-to-thickness ratio area, and flowing material contains fibers. In these cases, the results obtained from the 2D-based CAE are less accurate or even misleading. Therefore, 3D-based (solid models) CAE analysis is in great demand.

common types of elements for solid model, such as TETRA, HEXA, PYRAMID, PRISM, and so on

The quality cirteria used here are

1. Average length =5

Average length is the distance between two points that created on the surface of the part. These is to ensure that average size of the element is 5.

2. Minimum length=2
3. Maximum length =8

These two value ensures that element defined size. Element outside these value are not created or if created needs to rectify

4. Aspect ratio = 3
5. Skewness = 45
6. Warpage = 15
7. Minimum Quad Angle =45

8. Maximum Quad Angle =135
9. Minimum Tria Angle =30
10. Maximum Tria Angle =120
11. Tria Percentage =15
12. Jacobian = 0.7

Same put the value in the mesh parameters mode .

5.Perform the meshing :

Perform the meshing keeping in mind that no rotating quads and no oppposite tria element present near each other also with proper mesh flow:-

Before performing the mesh flow make sure to defined the length of the perimeter and macros as 5. These is to ensure that adjacent nodes have a distance of 5 between them.

Switch to HIDDEN MODE then starting with the mesh generation . here we are using two mostly method of mesh generation as

1. Best
2. Spot mesh

Rectifying the errors of mesh :

• Warpage errors: these errors are shown in sky blue color here occurs in quad elements . to rectify these kind of error use the split option and try to split the quad into tria
• Min and max length error : these error arrises when element does set into defined value .to rectify these use split option or add number of cons .
• Rotating quads : to rectify the rotating quads use the split option , split the quad into trai .
• Opposite tria : these can be eliminated by using the split option and also using the paste option to join then as one

Mostly use tool here to remove the errors are :

1. Reconstruct
2. Smooth
3. Split
4. Paste

RESULTS :

• Seperated the hood into inner and outer panel
• Performed the geometrical clean up of the both pannel
• Assigned the property to both the pannel , eliminating extra property
• Successfully created the midsurface of both the pannel and assigned then the PID
• Performed the topology clean up on the mid surface and aligned the features properly
• Assigned the stated quality criteria and mesh parameters
• Performed the mesh generation with proper mesh flow

CONCLUSIONS:

As the hood consisted of two pannel , it has been successfully seperated into two pannel as inner and outer pannel for our ease . Then its futher preprocess were carried out as clean up , midsurfacing and lastly mesh generation with uniform flow successfully in ANSA.

LEARNING OUTCOMES:

1. Seperate the complicated parts into simpler one.

2. Divide the surface into smaller parts using cut to have the uniform flow of mesh

3. Certain rotating quads are necessary in the region where geometery or pattern is changing.

https://drive.google.com/drive/folders/1xg5vB57fA9UALj0In8tPMRwIb1oKoYZC?usp=sharing