Cleaning the topology and Meshing the BMW M6 CAD model using ANSA

Objective :

• To clean the given topology of BMW M6 CAD model using ANSA.
• To name its different parts and assign PID's.
• To Surface mesh the given model.

Topology clean up :

The given BMW M6 CAD model was first checked for geometrical errors, below is the figure showing the CAD model which was imported to ANSA.

The imported model had several errors including single, double and triple cons. To eliminate these errors we have used plenty of tools in ANSA which are listed down.

Tools used for clean up :

Below listed are the major tools that were used to clear to topology of the given model.

1. Planar - Defines a new planar faces, which is generated automatically.
2. Fitted - It is used on the selected surface, which are non-planar faces.
3. Coons - Creates curves which is used to patch a surface.
4. SET PID - Assigns selected shells, faces or group of entities to a new PID or Existing PID.
5. Extrude - Creates faces by extruding the selected cons.
6. Transform - Used to Copy, Move and Rotate using set of informations.
7. Tangent - Used to make a tangential extension to a selected curve or entity.
8. TOPO - Performs topology based on given values.
9. Intersect - Used to interpret a point on a surface or Cons.
10. Delete - Used to deleted all the cons, surface and any entities which are selected.
11. Zone cut - Used to offset selected cons and make a zonal cut.
12. Split - Used to shell and solid elements into smaller elements by selecting their nodes or edges.
13. Split X - Used to split quadilateral element into four triangular elements.
14. Swap - Used to swap two elements nodes or edges mutually.
15. Move surface (MV Surf) - Used to move meshed single node into a Macro Area's surface.
16. Move Free (MV Free) - Used to move a node or group of nodes freely.
17. Paste - Used to manually or automatically paste nodes.
18. Perimeter Length - Used to assign nodal distribution of mesh on node lengths.
19. Mesh Generation Free - Generates the mesh for the assigned perimeter length or size.
20. Reconstruct - Used to recreate the mesh on the already meshed surface to fine tune the mesh on basis of the given mesh parameters.
21. Spacing - Used to control nodal spacing on perimeter length.

Procedure followed for Topology clean up :

Below image shows the different errors which were in the given CAD model,

As the body of the given geometry had the more errors it was chosen first,

• Pimarily all the single cons were fixed using Cons and Fitted options.
• All the single cons were converted into double cons.
• As there were many unwanted surfaces creating volume for the geometry, they were all deleted.

Now moving onto the grills, vents and head light region, the errors found were shown below.

• There were plenty of single cons which were deleted and recreated and converted into dounle cons.
• All the unwanted volume surface were deleted.
• The grille was recreated to avoid Concave and Convex errors.
• Major missing parts were recreated using Coons and Fitted.

Moving onto Chasis and tail lamps, the errors found were.

• Chasis had many complex overlapped surfaces, which were first deleted and recreated using Fitted and Coons.
• Surface housing Exhaust was also recreated.
• Single cons were converted into double cons.
• The tail lamps had volume surface which was deleted.
• Triple cons were Released and converted into double cons by recreating the surface.

After the elimination of all the errors on the one side of the model, it was then Transformed by Copying using Symmetrically by Mirror 3 Points Plane.

Although, the front wheel had major errors it was not mirrored previously, now it was also Mirrored Symmetrically using the error free another front wheel.

Boundary conditons :

The imported model was seperately named and PID's were assigned but, it was all clumpsy and was put into a single folder. Therefore, all the parts were separately named and PID's were assigned separately.

Below figure shows the model after assigning different PID's,

Different colours on the various parts indicate the PID's they have been assigned to.

Meshing :

Before meshing the geometry was Geometry checked and Single cons, Triple cons, Concave and convex surfaces, Mixed surfaces and Needle edges were removed.

After eliminating all the erros, the geometry was meshed based on the below Perimeter Length assigned to different parts.

• Body = 3mm
• Lights = 4mm
• Grills = 1 mm
• Tires = 2mm
• Rims = 2mm
• ORVM = 2mm
• Windows = 2mm
• Silencer = 3mm
• Logo = 1mm

The elment used for messing was Tria element.

After assigning different Perimeter Lengths to various parts, the model was meshed using Mesh Generating by Free.

Checks Manager :

After completely meshing using different variable mesh Perimeter Lengths, the geometry is check for surface mesh checks in the checks manager.

Checks manager is used to check different parameters which might be the main cause for the mesh to fail, they are shown below.

Mesh Parameters and Quality Criteria :

Quality Criteria for the Mesh Parameters were checked for Skewness, Minimum length, Maximum length, Maximum and Minimum angle quads and Maximum and Minimum angle trias.

Skewness is the measure of the deviation from the ideal equilateral triangle to the distorted triangle. It can also be defined as the irregularness of the triangle elements.

Maximum Length is the parameter which has a specified value in the quality criteria which should not be exceeded.

Minimum Length is the parameter which has a specified value in the quality criteria which should have a minimum value specified and value cant go below it.

Above are the three mandatory quality criteria for the mesh parameters.

Below figure shows the Quality Criteria checked and cleared,

Wind Tunnel :

Wind Tunnels are large tubes having a test body inside and air passed inside. As the actual testing for aerodynamics for a car is expensive, a wind tunnel is built to simulate and get the aero efficiency of a car.

Automotive wind tunnels fall into two categories:

• External flow tunnels are used to study the external flow through the chassis
• Climatic tunnels are used to evaluate the performance of door systems, braking systems, etc. under various climatic conditions.

Most of the leading automobile manufacturers have their own climatic wind tunnels.

The Wind Tunnel we used had the following dimensions,

1. Front face - Four times the front dimension of the car.
2. Rear face - Six times the rear dimension of the car.
3. Top face - Three times the read dimension of the car.
4. Bottom face - In close proximity with the tire.
5. Right and Left face - One times the centre plane to left and right sides.

Below images shows the wind tunnel arrangement we made for the CFD analysis for our model,

A Spacing Perimeter of 20 to 100 with a factor 1.1 was assigned to the pillars of the Wind Tunnel for minimizing the computation time to create the mesh for the Wind Tunnel.

Conclusion :

• The given BMW M6 model was cleaned for different types of surface errors using set of tools available in ANSA.
• The model was Meshed using the given Perimeter Lengths for various parts.
• A Wind Tunnel was created to simulate the test for CFD.