Week 4 Challenge : CFD Meshing for BMW car

                        ADVANCED CFD MESHING OF THE BMW M6 MODEL USING ANSA

l. OBJECTIVE

1. Detailed geometry clean-up of the BMW M6 Model.

2. Generation of a surface mesh on the model.

3. Analyze and correct the quality of the mesh.

4. Setting up a wind tunnel around the car.

ll. METHODOLOGY

A. Part 1 - Cleaning All the Errors and Assigning the parts and PIDs

1. Initial Geometry.

The initial geometry is as shown in the figure. We need to clean the given geometry and generate a surface mesh.

Figure 1- The Initial Geometry Of the BMW M6 Model

2. Resolution of The Car

The resolution of the model is low because of the coarse mesh length. To fix it, we take the mesh length of the entire model as 1mm

Figure 2 - Initial Mesh Length Of the model

Figure 3 - Model Having A mesh Length of 1 mm

3. Cleaning All The Topo Errors Present in the Model

Now, we need to clear all the Topo errors manually. For this, we have to isolate the different parts from the rest of the model one by one and resolve the issues with geometry.

3.1 Symmetric Half Of the Model.

The body of the car is symmetric. Hence, we can correct all the topo errors on one side of the car, after which we can mirror it to the other side. Hence, we delete half of the car body 

3.2 Topo Errors present on the inner side of the car body near the wheel

In the first region, there are a lot of single cons present due to improper geometry. These can be fixed using several tools like "Extrude", "Project Cons", " Intersect Faces", "Releases Cons", "Paste Cons" and "New Faces".

Figure 6 - Topo errors present on the inner side of the car body near the wheel

Figure 7 - Fixed Surface

3.3 Window Region

There are several triple cons present at the windows due to the presence of the volume elements instead of surface elements in these regions. To fix this, all the extra faces are deleted. The procedure is repeated for all the windows.

Figure 8 - Volume Elements Present at the window

Figure 9 - Converting the volume elements into surface elements at the windows

 

3.4  Grills Region

In the grills region, overlapping surfaces are deleted. Also, since we are performing a simulation on the outer geometry and not volumetric simulations, hence the inner geometry of the forward grill need not have unnecessary volumes.

To remove then we delete some unwanted surfaces, smoothen the curve in different regions by using the "CUT" tool and then delete the previous curves. These smoothened curves will help us to create a new face to result in a closed volume.

Figure 10 - Errors present in the grill region

 Figure 11 - Errors corrected in the Grills region

 

3.5. Lights Region

The inner volume of the Headlight is not required as it will waste the computational power. To fix this, we have to delete the unwanted surfaces.

In backlights, triple cons are present on the light because of the presence of a volume on the inside of the lights. To fix this, we need to delete the volume elements present at the lights.

Figure 12 - Errors in the Lights region

Figure 13 - Errors corrected in the Lights Region

3.6. Mirror Region

Interference occurs between the mirror connector and the window. Also, triple cons are present in the mirror because of the presence of a volume on the inside of the mirror.

To fix this, we need to delete the interference surfaces to make them hollow and also delete the volume elements inside the mirror.

Figure 14 - Errors in the Mirror Region 

Figure 15 - Errors corrected in the Mirror Region

3.7. Logo Region

Triple cons are present on the logo because of the presence of a volume on the inside of the logo.

To fix this, we need to delete the interference surfaces to make them hollow and also delete the volume elements inside the logo and recreate the logo surface.

Figure 16 - Errors and correction in Logo Region.

3.8. Tyer Region

 

Tyer region has triple cons and intersection issues. We should remove unnecessary surfaces to eliminate triple cons and intersections.

Figure 17 - Errors in Tyre Region

 

Figure 18 - Error Correction in Tyre Region

3.9. Base Of Car

The baser of the car is intersecting with the tyre, therefore we need to delete the excess surface and also delete the extra design surface which causes smaller edges, this causes problems while creating a mesh.

Figure 19 - Errors in car base

Figure 20 - Errors corrections in-car base

 

4. Generating PIDs

The components of the model are segregated under different PIDs by naming them accordingly using  the Properties Tab

Figure 21 - Assigned PID Regions

Figure 22 - PID Table

B. Part 2 - Generating A Surface Mesh on the car body

The mesh parameters and quality criteria are set according to different PID regions and meshed using the "Spot Mesh" tool

Figure 23 - Mesh Parameters and Quality Criteria

1. Length of the Elements

The following element size is assigned to the model

1. Body = 3 mm

2. Light = 4 mm

3. Grills = 1 mm

4. Rear View Mirror = 2 mm

5. Windows = 2 mm

6. Silencer = 3 mm

7. Logo = 1 mm

8. Tyers = 2 mm

 

2. Surface Meshing

Each component has meshed individually as the given target length are different for each component.

Mesh parameters and quality criteria are assigned for each PID and correct minimum and maximum lengths are set to reduce mesh failure and off elements.

Figure 24 - Surface mesh for Logo

  

Figure 25 - Surface mesh for Grills

 

Figure 26 - Surface mesh for Windows

 

Figure 27 - Surface mesh for Tyers

Figure 28 - Surface mesh for Rear View Mirror

Figure 29 - Surface mesh for Silencer

Figure 30 - Surface mesh for Body

Since the body PID region is in contact with various other PIDs, the mesh length variation at the boundary region of 2 different PIDs causes the mesh to erase or cause mesh failure. Therefore, while entering the perimeters under a length in mesh dropdown. Select the regions which are not in contact with other meshed PID regions. This helps in avoiding the mesh being erased and can later be edited to erase the off elements.

C. Part 3 - Generating The Remaining Half of the model using the symmetry plane.

The other side of the geometry is created by using the "Copy" command. Using the "Copy" command 3 points located on the edge of the symmetry plane are selected and the command is executed.

Figure 31 - Symmetry Condition Windows

Figure 32 - Final Car Model

 

D. Part 4 - Generating A wind Tunnel

1. Dimensions of the wind tunnel

Let X - Length of the Car

A. Front of the Car - 4X

B. Rear of the car - 6X

C. Top of the Car - 3X

D. Bottom of the car - Proximity with the tyers

E. From the center plane (Distance from the Left and Right sides) - 1x

 

In this case, X `approx` 1300 mm

A. Front of the Car - 5975 mm

B. Rear of the car - 8680 mm

C. Top of the Car - 4323 mm

D. Bottom of the car - 75 mm

E. From the center plane (Distance from the Left and Right sides) - 1300 mm

 

Figure 32 - Wind Tunnel

Figure 33 - Car inside the wind tunnel

 

2. Meshing The Wind Tunnel

A geometric spacing is assigned with a factor of 1.2, with the smaller mesh elements increasing from the bottom to the top. This is because the top of the wind tunnel is less important compared to the bottom. Finally, the wind tunnel has meshed.

Figure 34 - Meshed Wind Tunnel

 

E. Volumetric Mesh of the Wind Tunnel - Car Volume

Since we are interested only in the volume between the car model and wind tunnel, we can delete other volumes of the car model which is not necessary.

Since performing the volumetric mesh takes 2-3 days, the volumetric mesh is not performed.

 

lll. CONCLUSIONS

The BMW M6 Model is cleaned thoroughly and the wind tunnel is set up. The surface mesh is also generated and the quality of the mesh is improved.