BMW M6 Exterior Surface meshing using ANSA

BMW M6 MESHING - ANSA

The objective of this project is to perform meshing for a given BMW M6 exterior model using ANSA. The model is cleaned and checked for errors, then using the given element size, is meshed accordingly.

For any meshing process, the following steps are necessary.

1. Geometry checks
2. Surface cleaning (if any)
3. Boundary flagging
4. Meshing

Model

Geometry checks

The geometry check is done using the checks dropbox and selecting the geoemtry option. There are many errors in the geometry, so surface cleaning is needed.

Surface cleaning

The first step is to patch all the holes in the geometry and extra surfaces, represented by single cones open in red lines all across the surface.

When there are open edges, we can patch them using some tools such as:

1. Coons - Surface patch
2. Fill hole - Cons
3. New - Faces

The Coons tool is used to patch holes that are separate entities in the same surface. The rectangular boxes with it\'s whole surface missing, can be patched using this tool. Select all the edges in the geometry patch and click the middle mouse button to preview a surface creation patch.

In this section, the outline of the geometry is deleted at first after hiding all the other elements. Then, for computational efficiency, we shall delete one half of the geometry because they are symmetric in nature. Using this, we can work on topological errors in one side of the model and then replicate the other side using copy symmetry option. It reduces the computational work of surface cleaning as errors are same in both halves of the model.

All the triple cons are eliminated by deleting extra surfaces and cons in the geometry, either open or hiding within.

Extra surfaces are present in the windows, grills and headlights too. As we need the surface exterior geometry alone for simulation purposes, we can delete any entity with a volume and keep only the external surface skeleton for working. We only desire the double cons on the surface, indicating that there are no extra surfaces connected unnecessarily.

Then, holes in the geometry, and extrusions are delicately deleted so as to not affect the aerodynamic design of the car significantly. We use cut option most often and create hot points so that we can connect and delete surfaces as we wish. We maintain the curvature and major design curves as it is while modifing the geometry only to delete unnecessary protrusions.

For example, the intake grills contains a lot of volume elements and protrusions, which can be deleted by projecting certain curves in the geometry over the grill surfaces and deleting it as a whole. Then the open surfaces are patched using new faces option and made in such a manner that too many sharp edges might not arise.

After separating entities of the grills and volume elements, the open faces are patched to create a closed surface.

While creating and editing such surfaces intricately, each time we would have to also allot them into predefined PIDs which were already assigned in an unorderly fashion. We can clean up the boundary PIDs later, but first to assign the newly created geometries to respective PIDs, we can select the nearby entity attached to it, and with that the PID automatically switches to the predefined boundary and we can just click ok for that.

In case we see errors in the model but can\'t locate the source properly, we can add entities using the add option in the bottom taskbar and click near the vicinity to bring items in front.

Intersect is also a tool often used in the operation of deleting extra surfaces and cons that intersect. Added with that, the rear side of the vehicle and the tire sections contain multiple intersection errors which need to be fixed by adding needed surfaces too. During this process, the exhaust is hidden and as it comes back, the new surfaces intersect with the exhaust.

This is then solved using the extrude tool, that comes in handy when certain curves cannot be just created near a complex geometry intersection. The zone cut is first used to resize the cut in the geometry so that when extruded, we have sufficient clearance between surfaces and exhaust. This tool is used to create a cavity surrounding the exhuast so that it is separated from the body of the model. Project, New faces, Intersect, Extrude and many other tools are used as a whole to solve majority of the errors.

The tires also contain triple cons which are taken off and a hidden additional surface is seen. This is also deleted.

With that, the errors in one side of the model are solved respectively and checked using geometry check manager. Our aim is to also create the mesh for this one side itself and then create a copy symmetry so that meshing is also done overall. After that we also have to build our own wind tunnel with appropriate specifications. Now, let us first assign or rename boundaries and set new PIDs created.

Boundary flagging

Each boundary is set by:

1. Selecting Set PID from faces option in the module tab at the right of the screen.

2. Enable cross hatches option from the faces draw taskbar.

3. Select the respective boundary face and click the middle mouse button.

4. Right click in the dialog box and create a new boundary flag with the respective name.

5. Enter the name and double click created flag to assign boundary to that surface.

6. Click OK.

We can rename existing PIDs by clicking on them, which also highlights which entities have been selected in the model. We also have to allot each set of entities into specific groups and parts so that we can identify them easily as different sets of the car. This is done using the model browser. New part or group is clicked after selecting the necessary parts or entities for each module. Using this we can easily work on different aspects of the car by just highlighting its visibility in the model browser window.

The boundaries set are:

• Body
• Headlights
• Tail lights
• Tires
• Rims
• Front windshield
• Rear windshield
• Side windows
• Side mirror
• Grills: Intake, side, bottom
• Exhaust

After organizing each part into its own group, we can move on to surface meshing of the geometry.

Meshing

The required element sizes are given separately for each PID as:

1. Body: 3 mm
2. Headlights: 4 mm
3. Tail lights: 4 mm
4. Tires: 2 mm
5. Rims: 2 mm
6. Front windshield: 2 mm
7. Rear windshield: 2 mm
8. Side windows: 2 mm
9. Side mirror: 2 mm
10. Grills: Intake, side, bottom: 1 mm
11. Exhaust: 3 mm
12. Logo: 1 mm (within body)

All elements are tria mesh.

Now, the meshing process can be started by clicking the mesh icon in the right top corner to switch to mesh module.

Appropriate lengths for each PID are given using the length option.

We have to set the mesh parameters for appropriate mesh quality. We can change these values if we need to create allowances for smaller meshes in tight spaces. This can be reviewed during the mesh quality check.

The parameters entered for now are:

1. Min length: 0.9 mm

2. Max length: 5 mm

3. Distortion distance: 15%

After the meshing is done, we have to look at the mesh quality parameters that need to be there to check how good the mesh is. We look at parameters such as:

1. Min and max length

2. Skewness

3. Min and max angle for trias

All these are tested using the FLUENT calculation method for easier adaptation of the mesh into softwares like ANSYS FLUENT.

The shown parameters are color coded and these are highlighted on the model itself at various parts. Now we can solve all of these errors using these tools:

The cut macros can cut across grids of mesh using the length specific points in each edge. The join macros would join two adjacent mesh points together. MV surface is used to move the mesh along the existing surface by holding one of its vertices. Reconstruct mesh just reconstructs available mesh in the area selected with respect to limit parameters given.

In this case, some of the errors for sharp edges and corners have a very small minimum length mesh size. But if we delete these points, we cannot get a finer mesh. Hence, we change the mesh parameters into:

1. Min length: 0.2 mm

2. Max length: 5 mm

3. Distortion distance: 15%

This setup accomadates a wide range of smaller mesh sizes than we had intended to have. The computational time shall increase as an effect but we would get accurate results, without damaging or altering the curvature of the model provided.

All the errors are fixed and now as we check for geometry errors, including single cons,

We should get only this dialog box, including single cons that are made because of only deleting one half of the geometry and nothing else. Now, we can perform the symmetry copy option.

Three points are selected in different parts of the body to define the central plane. After that the symmetry option is performed and all the mesh is done on the other side too.

After this is done, we have to make the wind tunnel needed to enclose the car in an environment where we can use the mesh and acquire CFD simulation data or aerodynamic properties across the exterior of the car. For this, the given parameters are:

1. 4X for front entrance of wind tunnel

2. 6X for rear wall of wind tunnel

3. Bottom near the wheels itself

4. 3X for top wall of wind tunnel

5. X for side walls of wind tunnel

The \'X\' factor is the length of the vehicle, horizontally. We can measure that and it approximately comes upto 1300 mm. Hence, the factor of 1300 is multiplied with, say 4, to get the distance at which front entrance of the wind tunnel must be placed.

To start with, we can define separate points on each side of the car using relative points option and providing appropriate lengths. Then click the created point and define two other points in the same plane excluding the point in the current axis. By this, we create three points which can be used to then create a specific plane in that region. Plane option from surfaces tab is used to create the planes.

The planes are extended and copied into other places too using the predefined points on same axis. Same process is used to other axis too. Then the extended planes are made into a matrix, intersecting each other. Extra surfaces are deleted using intersect option and outlining the centre box matrix alone.

After this is done, the lengths and PIDs of the wind tunnel walls are given. The bottom wall has 20 mm, top wall has 100 mm and the middle portion is divided using spacing option for mesh length sizes evenly increasing from 20 to 100. The geometric option is used in spacing tab, which can be found near the length option itself.

Final win tunnel model will look like,

We can see the inner side of the model too, to check the orientation of the vehicle in the correct inlet direction.

The volume mesh of this wind tunnel is supposed to be done after this, but due to insufficient computing power and the operation being beyond the scope of this project, we defer from doing it.

Hence, a complete BMW M6 exterior body, surface cleaning and meshing has been performed successfully using ANSA.