Week 4 Challenge : CFD Meshing for BMW car

Aim: CFD Meshing on BMW M6

Procedure:

Improving the curves of the geometry

1. Go the surface mesh menu.
2. Click on length option in Perimeters menu of Surface mesh module.
3. Click on Macros in the open dialog box and select the entire model.
4. Click the middle mouse button and enter the length as I mm in the dialog box. Click on Ok.

Removing half geometry

1. Press on the “Delete” button in the top toolbar.
2. Click on the left mouse button and drag the mouse until the right half of the model is selected.
3. Deselect the unsymmetric parts of the model, i.e., the logo.
4. Click the middle mouse button to delete half of the model.

Deleting overlapped surfaces and unnecessary geometries

1. Using the “Delete” option present at top toolbar, delete the overlapped surfaces present in logo, headlight, vents, window, side vent, optical rearview mirror, etc.
2. As the volumetric mesh is to be deployed on the outer surface of the car, delete any interior geometry present in the car. This includes, the back side of vents, the internal geometry of rearview mirror, the internal geometries of lights, windows, windshield, car bottom etc.

Cleaning up the single cons and triple cons present in the geometry

1. After deleting unnecessary surfaces, a lot of single cons open up.
2. Clos these single cons by using the following options:
   1. Do TOPO cleanup using “Topo” option in Faces module of TOPO module.
   2. Paste two adjacent Single Cons.
   3. Extend Single Cons to nearby surfaces. If this creates triple cons then delete surfaces accordingly.
   4. Create new surface by using either the “Fill Hole” option in Cons menu or “New” option in Faces menu of TOPO module.
3. Eliminate the triple cons by deleting surfaces at various intersections.

Creating intersection and removing unwanted surfaces

1. The bottom of the car has several intersecting surfaces which are not detected by ANSA. Observe these positions and delete the relevant surfaces.
2. While deleting the surfaces, use the curve option to create a temporary Con for drawing new surfaces.
3. Clean up the bottom surface geometry.

Assigning PIDs

1. Assign PIDs to various parts of the geometry as defined below:
   1. Body
   2. Lights (Headlights, Tail light and Indicators)
   3. Grills (All the vents in the front of the car)
   4. Rims & Tyres (Contains all the tyre geometry)
   5. Optical Rear View Mirror
   6. Windows (Windshield, side windows and rear window)
   7. Silencer
   8. Logo

This will make our work easier while deploying surface mesh.

Deploying Surface Mesh on half geometry

1. On the above toolbar open the dropdown menu by clicking on the arrow on the right side of NAS.
2. Select OPENFOAM from the dropdown menu.
3. Open the mesh quality dialog box.
4. Click on the edit criteria visibility button and select the following criteria:
   1. Skewness
   2. Min length
   3. Max length
   4. Angle Shells
5. Unselect the other criteria and click on OK.
6. Set the Calculation to FLUENT for the relevant criteria seen in the mesh quality dialog box.
7. Set the color for all the parameters to easily detect which criteria is failing in the surface mesh. Click on Apply>OK.
8. Select entity mode from the lower toolbar and enable hidden view mode.
9. Deploy surface mesh on the various PIDs defined above with the given specifications:
   1. Body: 3 mm
   2. Lights: 4 mm
   3. Grills: 1 mm
   4. Rims & Tyres: 2 mm
   5. Optical Rear View Mirror: 2 mm
   6. Windows: 2 mm
   7. Silencer: 3 mm
   8. Logo: 1 mm
10. Deploy the surface mesh on the PIDs by isolating them in the following sequence: Logo > Grills > Lights > Windows > Optical Rearview mirror > Rims and Tyres > Silencer > Body
11. Before deploying mesh measure the minimum length observed in the particular PID and set the min length accordingly. After deploying the surface mesh to PID check the number of OFF elements mentioned on the right side of the screen. If any off elements are available select shell>Reconstruct>Visisble option and decrease the number of off elements as close to 0 as possible. When the off elements can’t be reduced any further go to individual error locations and use wither move, paste or origin tool in grids menu or join, or cut (right click) tools in macros menu.
12. It is necessary to deploy mesh on all other PIDS so that the body of the car can be meshed in one go.
13. After deploying mesh on all the surfaces, isolate the body PID>Set the quality criteria and mesh parameters accordingly>Open the Legth dialog box from perimeters menu in surface mesh module and select perimeters. Select all the edges which are not shared with other PIDs. To accomplish this, select the entire body by clicking the left mouse button and dragging the cursor over the entire geometry. Now enable other PIDs and deselect the perimeters shared with other PIDs, namely, Light, Silencer, Grills, Logo, Optical Rear View Mirror and Windows. Click on middle mouse button and set length as 3 mm. Set macros length and deploy the mesh using spot mesh>visible. A surface mesh is deployed on the entire body with the mesh length varying along the perimeters shared with other PIDs.
14. Fix any other off elements observed in the geometry.

Applying symmetry on the surface meshed model to obtain the complete car

1. After successfully deploying the surface mesh, Click on Tranform > Copy on the top toolbar.
2. Select the entire part of the model that is to be mirrored. Deselect the logo as it is neither selected nor any of its part was deleted earlier.
3. A dialog box will open. Go to symmetry tab. Set Use to Mirror 3-point plane. Select any 3 random points on the open surface/single cons of the car. The locations of these points are specified in the dialog box. Click on Apply.
4. A new dialog box opens. Set PID: No Offset; Part: Same Part; Set: None. Click on OK and then click on Finish after checking the geometry. The entire geometry is now mirrored.

Preparing wind tunnel and deploying surface mesh on the wind tunnel

1. To prepare the wind tunnel enable TOPO mode.
2. Measure the length of the car which comes to around 1307.88.
3. Select “relative” option in Points menu. Select the middle point located at the bottom of the model.
4. Set the relative co-ordinates for new point with dy = -1 and Distance = 65.
5. Using the points created in step 5 create new points at dz = +5885.46 and dz = -8501.22.
6. Using the points created in step 5 create 4 more points x = 1300 and x = -1300.
7. Join the points, create in step 6, using Create option in the Curves menu.
8. Create a surface using New option in Faces menu and delete the curves and points created earlier.
9. Create a top surface using Offset option in Faces menu. Set the offset distance as 4275. The top surface is created.
10. Create the lateral surfaces using edges of the available surfaces and new option from Faces menu.
11. Assign PIDs to all the surfaces.
12. Mesh the top surface with mesh length 100, max length 120 and min length 80.
13. Mesh the bottom surface with mesh length 20, max length 30 and min length 10,
14. Select the spacing option in perimeters menu of Surface Mesh module.
15. Set biasing as geometric, Factor as 1.1, Dstart as 20 and Dlimit as 100.
16. Select the 4 edges of the lateral surfaces.
17. Deploy the spacing factor.
18. Deploy surface mesh using spot mesh>Select and selecting the 4 latera surfaces.
19. Proceed to deploy volumetric mesh.

Learning Outcomes: Gained necessary expertise for surface meshing of a BMW M6 car. Learned about new functions, such as symmetry, use of curves to mark location of Cons, and preparing basic geometry such as cube in ANSA.

Result: A surface mesh, with specified mesh length, was successfully deployed on the given BMW M6 model.

Conclusion: The given model of BMW M6 was successfully meshed with given mesh length specifications.