Week 4 Challenge : CFD Meshing for BMW car

Aim:

For the given model, check and solve all geometrical errors on half portion and Assign appropriate PIDs. Perform meshing with the given Target length and element Quality criteria. After meshing the half model, Do symmetry to the other side.

Objective:

- Target lengths for the different parts of a model are as follow:

1. Body - 3 mm
2. Lights - 4 mm
3. Grills - 1 mm
4. Rims and Tyres - 2 mm
5. Optical Rear view mirror - 2 mm
6. Window - 2 mm
7. Silencer - 3 mm
8. Logo - 1 mm

Model: BMW M6

            

• Procedure:

1. Topo/geometry clean-up:

a. Upload the ANSA file onto the screen to perform topo/clean and meshing in the later stages.

b. Go to mesh and choose the length and enter 1 mm under perimeters to smoothen the sharp curves of the car and give it a fine look.

                   

                                                              fig 2: meshing for smooth edges

c. Since the car is symmetric from the middle to an end, we can delete one side of the car and perform the necessary operations on the car and use the symmetry option to produce the complete car.

 

                                          fig 3: deleting half-car body

 

                               fig 4: performing topo clean up

 d. We can now start with the topo clean-up by removing the single and unnecessary triple cons. First click on topo and box select the entire to identify or joins any unjoined single cons or triple cons so that we can identify the required clean-up accordingly.

- Front grill area:

a. Begin with the front grill area and the headlights, and delete all the extra parts to have a single volume throughout the car.

b. Now to remove the single and triple cons and to maintain a single volume, to create a single volume we need to have double cons everywhere, to do this we need to perform a few cuts and re-surfacing of parts to maintain a single volume.

 

                                       fig 5: Front grill region

 

                              fig 6: Deleting and creating new surfaces

 

                                            fig 7: geometry cleaned grill

 c. After performing the cuts and removing the extra triple cons surfaces by using the delete option and other tools, we can now re-meshing create surfaces and maintain a single volume.

 

                                                    fig 8: creating a new surface 

 

                                              fig 9: Wireframe mode

 - Windows, rear lights, roof region, and logo:

a. Delete the triple cons or extra surfaces to remove the triple cons, these double surfaces are not necessary since we need to have a single volume.

b. Do the same for windows, rear lights, and roof areas where extra internal parts are present which are of no use to create surface or volumetric mesh.

c. The logo of the car is under a single surface, therefore using project cons, create a separate surface and delete the extra surfaces to maintain a single volume.

                                     fig 10 a,b: geometry cleaned windows and rear lights

  

                                                          fig 11 a,b: Logo geometry clean-up and wireframe mode

 - Design feature and mirror:

a. Perform topo and identify the triple and single cons, and delete the extra unwanted surface to maintain a common volume.

                                           fig 12: Design feature of the car

 b. Use the intersect option to identify the triple cons at the intersection area and can delete the smaller surfaces individually.

c.  Extended the single cons on the adjacent surface to create triple cons and delete the unwanted surfaces by doing that.

d. Delete and re-create surfaces accordingly to have a smooth and perfect shape at tight corners. Also uncut the lines which are necessary which might cause trouble during meshing. 

                            fig 13 a,b: Design feature wireframe mode

 e. Isolate the mirror and perform topo clean-up, hide the surfaces blocking the triple cons view.

                              fig 14: Optical rear view mirror

 f. Delete the extra surfaces and create a single volume, also at the car and mirror connection region perform extend on the car surface and create a single volume by deleting the extra surfaces.

 - Wheel and Base region:

a. Isolate the wheels to perform geometry clean-up, rectify the triple cons, hide the surface blocking the view, and delete the overlapping surfaces causing triple cons.

b. Create larger surfaces after deleting smaller surfaces that are parallel to each other.

 

                                   fig 15: Tyre PID and wireframe model

 c. The base of the car is intersecting with the tire, therefore we need to delete the excess surface and also delete the extra design surfaces which cause smaller edges, this causes problems while creating mesh.

                            fig 16: Using curve feature while topo clean-up

 d. Therefore we need to clean the entire base and create new surfaces accordingly having smooth edges.

 

                                 fig 17: Base geometry clean-up

 e. After making the surface changes, we need to check if the base surface has no intersection with the tire region.

                      fig 18: checking interference between tire and body

 - After checking and performing topo-clean-up for the entire half-car model. We need to perform geometry checks to find small cracks are unchecked faces extra which is not good for the geometry, therefore we need to perform checks and clean the geometry accordingly.

 

                                                   fig 19 a,b: Checking for geometry checks

 - Once the geometry is checked and has no errors, we can now assign names to different components using PID.

                                            fig 20: Assigned PID regions 

                  fig 21: PID table

 - After assigning the names, we now choose the quality criteria method. Choose OpenFOAM solver next to volumetric mesh and choose the given quality criteria below. 

          

                             fig 22: Quality criteria table

 - Using the measure tool we can calculate the smallest distance between to cons or hot points and depending on that, we can assign our minimum and maximum length accordingly in the mesh parameters, if we do not do this we can get the failed mesh.

                

                                                                fig 23: Using the measure tool

2. Surface meshing:

• We need to mesh the components individually since the given mesh targets are different for each PID region.
• Assign the mesh parameters and quality parameters for each PID accordingly, assigning the correct min and max length will reduce mesh failure and off elements. 

- Quality criteria for logo and grills region:

- Logo and grill surface mesh:

                                                fig 24: Logo surface mesh

                                               fig 25: Grills surface mesh

                                          fig 26: grill surface mesh close view

- Rims and tires, window and rearview mirror surface mesh:

- Quality criteria and mesh parameters:

 

- Surface mesh:

                                         fig 27: A surface mesh of windows

                                                fig 28: surface mesh of tyre

                                             fig 29: surface mesh on Rear view mirror

- Silencer and lights:

- Quality criteria and mesh parameters for silencer:

 

- Surface mesh:

                                           fig 30: surface mesh on a silencer

                                          fig 31: Surface mesh on lights

- Body surface mesh:

                                              fig 32: Car body surface mesh

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

                                  fig 33: Checking for off elements in entity mode of car body

After meshing, we can choose re-construct under shell mesh to reduce the off elements. And later the remaining off-elements can be edited manually.

                                        fig 34: Using the re-construct option to reduce off elements

- Final surface mesh of car model:

                                    fig 35: Complete car model with 0 off elements

- Performing symmetry on the car model:

• Perform symmetry under the transform option to create the exact same opposite body of the car to create a full-body car.
• Use the copy option and select the half-body car and deselect the logo since we don't need a copy of the logo.
• After choosing the model, middle click mouse, we get a window, choose symmetry and choose mirror 3 points plane and choose 3 points on the edge to be performed symmetry.
• After choosing 3 points, the middle clicks twice, and the symmetry is performed, click the middle click again to confirm the geometry and complete the process.
• There will be a few single cons present near the middle section, use paste or other tools to solve the problem.

                         

                                                             fig 36: Symmetry conditions window 

     

                                        fig 37: Surface mesh on full car body after performing symmetry

    

                                                                     fig 38: Final car model

- Creating wind tunnel:

1. First, we need to create reference points to create wind tunnel surfaces.
2. Select relative under points in topo mode, create a point below the car take one reference point and enter the correct values.
3. And calculating the wind tunnel dimensions appropriately, we can create the wind tunnel accurately.
4. The wind tunnel dimensions are:                            ( standard distance of the tunnel wrt to car model)

   1. car model: 1306mm                                                      .................. (x)

   2. Front surface distance(from reference points): 5875mm   .................. (4x)

   3. Back surface distance(from reference points): 8480mm    .................. (6x)

   4. Top surface distance(from reference points): 4323mm      .................. (3x)

5. After creating the bottom surface use the curves for having the reference of cons, we can delete all the hot points and curves.
6. Create all the other wind tunnel surfaces using the new surface option.

                                               fig 39: Using the relative points tool

                                             fig 40: Wind tunnel

- Surface meshing of wind tunnel:

a. We need to apply surface meshing depending on the mesh length near the car and regions far away from it.

b. Therefore we need to apply different mesh lengths at the bottom and top wind tunnel surfaces.

c. We need to use the spacing option to have a mesh length varying from a small mesh length size to a larger mesh length.

d. Surface meshing can be achieved by doing this accordingly.

                                                                          fig 41: Surface mesh of wind tunnel

- Volumetric mesh of wind tunnel-car volume:

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

b. Since performing the volumetric mesh takes 2-3 days, the volumetric mesh is not performed as given by the challenge question.

- Volume regions:

                               fig 42: Volume between wind tunnel and car model

                                     fig 43: Car body single volume

                                                   fig 44: Tyres volume

                                        fig 45: Silencer volume

 Conclusions:

1. BMW car model is topo-cleaned and surface meshed appropriately.
2. The wind tunnel is created and the surface meshed with reference to the car model size.
3. Different volumes of the car model are created since the silencer and tires are not connected to the car body.
4. A single volume is created of the car body so that it can be avoided at one go when creating a volumetric mesh.
5. Required physics tests can be performed on the wind tunnel volume model after volumetric meshing successfully.