Generating a CFD mesh over the BMW M6 model to perform external aerodynamic simulations

Aim:

To check and solve all the geometrical errors on the given BMW M6 car geometry and assign appropriate Property IDs (PIDs). Then, deploy the surface mesh for different parts (PIDs) with the given target length and element quality criteria using ANSA. Then enclose the car model in a virtual wind tunnel, and deploy CFD mesh in it to perform external aerodynamic simulations on it. 

Problem statement:

1. To check and solve all the geometrical errors on the given BMW M6 car geometry and assign appropriate PIDs.

2. Deploy the mesh on the car surface (surface mesh). The target lengths for PIDs is as follows:

1. Car body - 3 mm
2. Head and tail lights - 4 mm
3. Grills - 1 mm
4. Rims and tyres - 2 mm
5. Optical rearview mirror - 2 mm
6. Window and windshield - 2 mm
7. Silencer - 3 mm
8. BMW car logo - 1 mm

3. Enclose the car in a virtual wind tunnel of appropriate dimensions (this is done to perform external aerodynamic simulations on the car body) and deploy the volume mesh.

Solution:

1. Import the given geometry into ANSA

2. Since the given car geometry is symmetric in nature the geometry is sliced into 2 parts about its symmetric plane. On this half model, all the geometric errors are resolved and then meshed, and finally, the resultant model is copied to the other side.

3. When the given car geometry is imported to ANSA, the small features of the car geometry were not captured accurately. This is because the mesh resolution is higher than some geometry features. To resolve this problem, Mesh> perimeters> length= 1mm

4. The topology is cleaned up by deleting unnecessary faces in the interior of the model so that ANSA does not create unnecessary volumes by doing so all the triple cons are converted to double cons, and the single cons only appear on the symmetric cut plane. since to perform external aerodynamic simulations on the given car model, the interior components do not matter, hence, some interior faces and features are deleted and only minor changes are done on the exterior of the car geometry. 

5. Appropriate boundary flagging is done on the obtained car model.

6. Once all the geometric errors were resolved, the surface mesh is deployed on different car components with their respective mesh target lengths. In this problem, only triad elements are used. The surface mesh is first deployed on the smaller components, which are not in contact with other parts/PIDs, then it is done on the remaining components. Once all the surfaces have meshed, we must check whether or not the generated mesh satisfies the mesh quality criteria or not. If not then the surface mesh must be reconstructed. The mesh quality criteria for the car components are as follows.

The mesh generated on the car body:

a. BMW car logo:

b. Car silencer:

c. Car windows and windshield:

d. Optical rearview mirror:

e. Tyres and rims:

f. Grills:

g. Head and tail lights: 

h. Car body:

The mesh generated for the entire assembly:

7. Once the mesh is generated on all the regions of the car model, we can now generate the complete car model. This is done by the use of symmetry option. Transform> copy>  entities> symmetry> mirror 3 point plane, then select any 3 points on the symmetric plane. Once this is done, all the entities will get copied to the other side of the symmetric plane. The geometry my show some single cons once this is done, this can be rectified by performing topological checks. Topo> faces> topo

8. Once this is done, the car geometry must be enclosed in a box (virtual wind tunnel) to perform external aerodynamic simulations. If the length of the car is 'L' meters then, the virtual wind tunnel dimensions are as follows:

a. Distance from the wind tunnel inlet to the front of the car= 4L

b. Distance from the aft of the car to the wind tunnel outlet= 6L

c. Distance from the sides of the car to its respective wind tunnel sides= L

d. Distance from the floor to the top surface of the wind tunnel= 3L

The virtual wind tunnel is created by first, identifying the origin of the global coordinate system, then, the location of wind tunnel floor corners is calculated and points created, these points are joined with curves. There must be a minimal gap in between the tyres and the wind tunnel floor to simulate the aerodynamic flow over the car accurately. A face can be created from these curves. To get the top surface of the wind tunnel, a copy of this face is offset at a distance of 3L. The side faces and the inlet and the outlet of the wind tunnel can be created by using the reference of the wind tunnel top surface and floor cons.

9. Now a surface mesh must be generated on the wind tunnel surface. Since we want finer volume mesh near the surface of the car, which is resting on the floor of the wind tunnel, we must provide smaller mesh size for the wind tunnel floor and larger mesh size on the windtunnel top surface. The mesh size on the side walls and the inlet and the outlet of the windtunnel must increase gradually from the floor to the top surface of the wind tunnel. To achieve this first the wind tunnel floor and top surfaces are meshed separately with triad mesh elements and the mesh target length of 20mm and 100mm respectively. Now, to mesh the other faces in an gradual increasing manner, go to mesh> perimeters> spacing, select the 4 edges, take biasing as geometric (1.1), and dmin= 20, dlimit= 100. Next, mesh the geometry with using spot mesh, and check for quality criteria errors if any. If there are any errors, it can be rectified mesh> shell mesh> reconstruct.

The mesh quality criteria for the wind tunnel is as follows:

Mesh generated on the wind tunnel surfaces:

a. Wind tunnel top surface:

b. Wind tunnel floor:

c. Wind tunnel inlet, outlet and side walls:

10. Once the surface mesh is generated on all the components, volumetric mesh can be created. In volume mesh> define> auto-detect> whole data base. Here, delete all the unwanted components, ie, the volumes enclosed by the solid surfaces (in this case 4 tyres and 2 silencers). Now right click on the fluid volume detected and click on re-mesh using tetra CFD mesh, and the required mesh will be generated. Generating the volumetric mesh requires a lot of computational power and storage, hence, it cannot be performed on a general purpose laptop. It can be performed easily on more powerful multi-processor systems.

Conclusions:

Hence, the surface mesh was generated on the car body and on the wind tunnel surface with the set mesh quality criteria and mesh target lengths.