Week 3

PRE-PROCESSING OF A CAR HOOD BY ANSA

OBJECTIVE:

1. Checking Geometrical errors
2. Geometry clean up
3. Generation of Midsurface
4. Meshing the element with Target lenght of 5 mm
5. Assigning the Thickness for Geometry

ABSTRACT:

A Car Hood is used to cover engine and main parts of the car.These are attached to frontline of car through Hinges.This car hood provides engine,battery,Radiator etc and other components in all the conditions like environmental,accidents etc.It is important to maintain in a good condition.So this should be able to satisfy all the requirements and Quality issues that are designed from Enginner.

Today, we are going to do the 1st step of CAE process i.e. Pre-Processing of Car hood by using ANSA

METHODOLOGY:

• Geometry import
• Checking Topological errors
• Clean up
• Mid-surface extraction
• Quality criteria setup
• Meshing
• Mesh improvement
• Assigning thickness

INTRODUCTION:

A Car Hood which is also called as Bonnet is used to provide access to the Engine for repair and maintainance.Car Hoods are constructed from Steel or Aluminium.Hood of car is connected to the body of vehicle by HINGE.

The term CAR HOOD is an North American term,where as the the term CAR BONNET is a English term.

Car hood consists of inner and outer pannel.The Inner pannel provides strength and outer pannel is just of metal cover. The underside of the hood is often covered with a sound-absorbing material. Some high performance cars have ‘hood scoops’ which channel outside air directly to the air filter, which gives improved performance and efficiency. On passenger cars, a hood may be held down by a concealed latch.On race cars, the hood may be held down by hood pins.

GEOMETRY PARTS:

• Inner component
• Outer component

PROCEDURE:

After entering into ANSA we will open the geometry by using file command in ANSA.Open any one component by using or not commad and by neighbour all.Both the components are seperated.

GEOMETRY ERRORS

To find any geometrical errors we need to check the geometry for topological errors.

If the geometry is clean there will be no errors, if there are some errors,then they will be shown below.As there are no errors then we can go for geometry cleanup.

Assign PID for the above component.

Checking errors for 2 component

Fix the errors automatically,or by manually which cannot be done automatically

ASSIGNING PID

After errors are rectified Assign Pid for each component with different colour.

Steps to Assign PID:

• Go to properties.
• Right click the mouse button.
• Go to new-Pshell-Pshell
• Assign pid.
• Again right click on the assigned PID.Click on Apply and select the entire component from ANSA window.
• PID is Assigined.

EXTRACT MIDSURFACE

Midsurface meshing is opted for components where the geometry thickness is small when compared with the span length of the components. It can be extracted either manually or automatically using appropriate methods or a combination of both can be explained. It is based on the thickness variation and time constraints. In manual method, the faces are selected, and the offset value is given. In automatic method, the mid-surface is extracted in a form FE model straight. For this, the whole part is selected, maximum thickness value, target element size are given in prior.

For extracting mid surface take offset value half of the thickness.In this case the offset value is 0.75/2=0.375.

After extracting mid surface use or not command and neighbour all to seperate it from main part.Assign PID for both surfaces and lock the views.

CHECKING GEOMETRY

After extracting mid surface to the component check for geomentry errors and remove all unwanted lines curves etc.This will fail for Quality criteria issue.So before going into Mesh always it is a good practice to check for geomentry correction.The materaial or fillet do not fail for Quality criteria.

Below images show the geometry before and after clean up.

ORIENTATION

After extracting mid surface do orient.Orientation should be checked thoroughly by the user everytime on working 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 prefered to use grey orientation in entity mode because grey being +ve .

QUALITY CRITERIA:

Quality Criteria that should be followed.

This is the QC we need to follow for the Geometry that should be adressed in Quality Criteria propery.

We need to give target lenght in Mesh parameters of 5.

MESHING:

Before meshing make sure that the node to node distance is meeting the average lenght.For that we need to go for lenght and give length parameter lenght of 5 by selecting the whole model in ANSA window.

IMPORTANT POINTS IN MESHING:

• Do not mesh the entire component at once, do mesh for individual component,which gives better result.
• Reconstruct every time after doing best mesh.
• Make sure that no trias are opposite to each other.
• No trias should be at the edge.
• There should not be rotating Quads.
• If the component is very lage,try to make project cut and do the mesh.

After doing project cut,the model has to be meshed for each and every part.If there are any Quality criteria issues then try to Reconstruct the mesh and smooth which gives best results.

The above picture shows the mesh of the componennt,and most of the parts fail for Quality criteria.So try to RE-construct the mesh.

If we compare two images,the off elements are reduced.So doing reconstruct and smoothing the mesh gives the best practice.The elements which are failing for Quality crieteria can be done now manually done.

• RED colour issue is for angle,minimun lenght and maximum length
• Yellow colour is for ASPECT ratio,which is ratio of maximum length to minimum length.
• Blue colour is for warping angle,It is defined as angle between normals of two (triangular) planes formed by splitting the quad element along diagonals. Maximum Angle out of the two possibilities is reported as "Warp Angle". In the case of solid elements, an element's face deviates from being planar.
• GREEN colour is for SKEWNESS,which is defined as,skew angle is basically the angle between a normal/perpendicular to the alignment/centerline of the bridge and the centerline of the pier .

Best ways for mesh flow

As i mentioned above,no two trias should be in opposite direction.

This can be solved by making either one tria to move in forward direction or another one in backward direction.

• If tria should move in FORWARD direction then use SPLIT option.
• If tria should move in BACKWARD direction then use PASTE option.
• If tria should move in RIGHT or LEFT direction then use the SWAP option.
• The rotating Quad can be converted to TWO TRIAS by using SPLIT-NODE option.

In above case the forward trias are rectified by using SPLIT option.

Here tria is at the edge which is not acceptable,this can be solved by PASTE-COG opyion by picking up two nodes.Make sure that you are picking the correct and sequence order.

For construction line there should no be any trias connected.

Here green colour lines represent construnction lines,for that any trias should not be connected i.e,trias facing the edge or trias sharing the base with green lines.This can be done using PASTE and SPLIT options to make then as QUADS.

After performing there should no be any unmeshed and off elements in the model.

AS we can see,that there are no off elements or unmesh part in the model.This results our geometry accepts all the Quality criteria.

GOOD MESH FLOW

All parts have been meshed for Quality criteria and obtained a good(smooth)mesh flow.

ASSIGNING THICKNESS:

• Go to properties in list,double click on already assigned PID of outer and inner component.
• We can find the thickness table.
• Assign a thickness value of 0.75(for this model).Assigning thickness will be depended on model.
• Click ok.

After assigning thickness make sure that turn on DRAW SHELL AS SOLID in Quality criteria parameters.

CHECKING UP THE MODEL:

After giving the thickness make sure your mid-surface is extracted correctly and there are no orientation issues in the model.

Here we can see that mis surface is extracted correctly and there are no orientation issues i.e top side is of grey colour and bottom side is of gold colour.

LEARNING OUTCOMES:

• Able to fix topological errors.
• Able to extract mid-surface generation automatically and manually.
• Able to do geometry checkup before going to mesh.
• Got to know about Quality parameters and importance in mesh generation.
• Able to mesh the model.
• Able to remove trias and rotating Quads at the edges.
• Can do 2d meshing for complex geometry.

CONCLUSION:

From the above challenge,it can be seen that methodology for any geometry is similar in case of shell meshing.The procedure is followed to obtain better geometry,midsurface and better mesh flow that which accept the Quality criteria.In some cases we need to compramise for some parts so that the element should not fail for Quality criteria keeping in mind that we should not change the geometry that which effects design.The majority of the elements should be near to the target length and should accept Quality criteria.Keeping in mind all these cases the pre-processing of hood component is done by using ANSA.

Link of the project: https://drive.google.com/file/d/1HfOD1BSbn2PDuc7TNVKtBFVDHq6hqkxE/view?usp=sharing