CFD Meshing for BMW car

Aim - To surface mesh a BMW car model for wind-tunnel analysis.

Tool used - ANSA

Procedure -

1. Use fine option over the entire region to smoothen the curves. This tool basically uses a length criteria to smoothen the surfaces.

2. Clear the single Cons

Make necessary changes to the geometry to clear single cons. Triple cons should be avoided whereever possible. Make sure that the external geometry is not changed much, as we are intrested in the external flow simulation.

Geometry check

Errros should be resolved. But, warnings can be ignored, if unable to solve.

The below shown chassis geometry simply cannot be used for meshing as it has too many sharp edges and cons very close toe ach other. It also has interfering surfaces. All these must be resolved.

3. Assigning PID's

4. Surface meshing of the car

Make sure you solve all the off elemernts after mesh generation.

Usefull tools - Move, reshape, reconstruct.

Quality criteria

PID	Target length	Min length	Max length
Body	3 mm	1 mm	5 mm
Lights	4 mm	3 mm	5 mm
Grills	1 mm	0.4 mm	2 mm
Tyres	2 mm	1 mm	3 mm
Mirror	2 mm	1 mm	3 mm
Window	2 mm	1 mm	3 mm
Silencer	3 mm	2 mm	5 mm
Logo	1 mm	0.5 mm	2 mm

5. Model build up using Symmetry

6. wind tunnel creation and PID assignment

Relative option is used to create new nodes relative to the exisitng nodes. These new nodes are then joined using the curves option and faces are created using the curves.

An important point is that the wind tunnel should be large enough to capture the flow physics, but not so large that it rises the computational costs. A standard wind tunnel size can be obtained from research papers.

7. Wind tunnel surface meshing

Use spacing option to create variable element size mesh.

8. Delete unwanted volumes

Conclusion

• The geometry modification to remove single cons and off mesh elements should not be such that it completely distorts the geometry.
• Volumetric meshing of a geometry of this size is computationally very expensive.