Week 4 Challenge : CFD Meshing for BMW car

Aim: For a given model of Turbo Charger geometrical cleanup need to be done and perform 2D – 3D mesh with given mesh target length.

Procedure :

1. Given BMW geometry model is imported and geometry check is performed to see the geometry error like single cons, holes, etc. It has lots of single cons and other error. All of the single cons can be also viewed using wireframe and viewing options (turning ON single cons only)

2. TOPO option is then used to clean up some single cons.

3.To view all the curvatures accurately, under the mesh tab, parameters and macros are set to 1 mm.

4. Since the model is symmetric, half of the model has to be deleted by properly selecting half of the geometry and then delete it.

5. Since we require only the outside surface, we don’t have to retain any surface which is inside of the car, some inside surface is present in the headlight area, which has been deleted. Also, single cons present in the headlight outer surface have been deleted.

6. As shown in the figure there is some glitching present in the grill area which has to be deleted.

7. In the front grill area there are a lot of triple cons present which shows there are either a small surface area or 3 surfaces joining the single cons. By close examining, all the unwanted surface which is forming triple cons have to be deleted. Also since we want only the exterior portion of the grill, some surfaces have to be deleted which is lying inside which can be done by using project, coons, extend and delete options. The final surface which we require is shown on the left.

8. Now moving on to some other region as shown in the figure there are a lot of triple cons present forming separate volumes which need to be fixed by deleting the extra surface (figure on the right)

9. Now the vertical ribs (left in the figure) need to be open from the inside of the car which can be fixed by deleting the surface (right in the figure). With the help of cut faces, insert hot points, coons, delete, etc. option the closed surface can be created as shown in the bottom figure.

10. In the BMW logo region there are some triple cons present which represent there are some overlapping surfaces, which need to be fixed by deleting overlapped surfaces, then with the help of the topo and coon faces option all the error can be removed. The final corrected surface is shown in the figure.

11. In the windshield area there are some triple cons present, which need to be fixed. The surfaces which is inside of the car need to be deleted, so then it can form only one inside volume.

12. Inside surfaces of the side glasses and rear windshield have to be removed as well so that they can form a single volume.

13. Triple cons on the side mirror have to be removed by deleting the inside surface. Also, we should delete the surface connecting the mirror and car, so that it forms only one inside volume.

14 . Now in the front tire there is triple cons present, with the help of the I-not option, tire has to be isolated. Now the overlapped surface of the rim of the tire has to be deleted from inside so that it forms only one common volume. Similarly issue with the rear tire can be resolved.

15. Under the body, there are lot of overlapping surface present which have been deleted and recreated so that, there is no triple cons and all the created surface form only one volume

15. After finishing all the cleanup and checking for all the errors in the geometry check, PID needs to be assigned to different surfaces as shown in the figure.

16. After assigning PIDs, it's time to create tetra mesh before proceeding to create volume mesh. But before creating surface mesh we have to assign quality criteria, according to the specific solver. We are going to use different quality criteria for different PIDs. Open the quality criteria tab and select skewness, min length, max length, min angle quads, max angle quads, min angle trias, and max angle trias. Assign the value to each of these parameters as shown in the video for different PIDs.

17. After assigning the quality criteria, we should delete CONS forming length less than specified Target length by using the face cut option and clicking the right mouse button over the CONS. We have to do this for all of the PID’s. The target length for each of the PID’s is shown in the video which is body – 3mm, lights – 4mm, grills – 1mm, rims and tyres – 2mm, optical rearview mirror – 2 mm, window – 2mm, silencer – 3mm, logo – 1mm.

18. After deleting all the surfaces which are forming CONS less the target length, we can proceed with the surface meshing. Let’s start with the smallest part which is the logo. In the quality criteria window specify the minimum length of 0.4 mm and maximum length as 2 mm. In the mesh, the parameter window specifies the target length of 1 mm, change the element type to tria. After doing this, go to the mesh tab and specify perimeter and macros of 1 mm. Then with the help of spot mesh generate the surface mesh for the logo.

19. Now to check the element, whether it is failing our quality criteria or not, switch from shadow mode to hidden mode, and select view as an entity instead of PID’s. The elements which are failing are highlighted by different color as per quality criteria, we should try fixing all these issues by switching back to TOPO mode and edit the small surface by deleting and recreating that. After fixing the surface, re-mesh it. And if the fail elements are still there try using reconstruct option under shell mesh. The final surface is shown below.

20. With a similar approach grill can mesh as the target length of the logo and grill is the same.

21. Now we start with the window PIDs, target length of window PID is 2 mm, we can set 1mm as min length and 3 mm as max length. Define perimeter and macros to 2mm and generate the surface mesh using the spot mesh option. Since no elements are off, we can proceed to the next PID’s

22. Now we can mesh Light PIDs’s, target length of light PID is 4 mm, we can set 3mm as min length and 5 mm as max length. Define perimeter and macros to 3mm and generate the surface mesh using the spot mesh option. Some elements fail in maximum length, so with the help of the move option under the grid, try to adjust the node, so that off elements get fixed.

23. Now we can mesh Tyre and Rim PIDs, target length of Tyre PID is 2 mm, we can set 1mm as min length and 3 mm as max length. Define perimeter and macros to 2mm and generate the surface mesh using the spot mesh option. Around 1220 elements fail, so first try using reconstruct option to reduce some elements. After using this there are 18 off elements lift. Now we are try fixing these elements manually using the move grid option, cut macros, and reconstruct option, final correct surface will look like the figure shown below.

24. Now we start with silencer PID’s, the target length of silencer PID is 3 mm, we can set 1 mm as min length and 5 mm as max length. Define perimeter and macros to 3mm and generate the surface mesh using the spot mesh option. There are some elements that are failing, try using the reconstruction option which should fix the issue.

25. Now we can proceed with rearview mirror, target length of this PID is 2 mm, we can set 1 mm as min length and 3 mm as max length. Define perimeter and macros to 2 mm and generate the surface mesh using the spot mesh option. There are some elements that are failing, try using the reconstruction option which should fix the issue.

26. Now we can proceed with the body PIDs, target length of this PID is 3 mm, we can set 1 mm as min length and 5 mm as max length. Since the body part is connected to other parts we should not create a mesh in a usual manner as we created for other PID’s as it will delete the mesh in the other PID’s. Define perimeter of 3 mm for body PID’s but deselect all the edge which are connected to the other PIDs, after doing that use spot mesh to generate the mesh.

27. Generated mesh have so many off elements, so try using reconstruct option by making all PID’s visible to reduce off elements. And if you just visible body PIDs there are only 11 elements that are failing, which we can reduce manually.

28. After resolving all of the quality issues, it’s time to copy the half model to another side, so that the full model of the car can be obtained. Under the transform option, click on copy and select all the entities except the logo, then click the middle mouse button. In the popup window of copy entity, select mirror 3 points plane and select three points YZ plane, and click ok. It will reflect half of the model and after verifying everything, click on the finish.

29. In the wireframe mode check for single cons, which is present in the front windshield and logo area. Use the paste cons option to fix it.

30. Now to construct a wind tunnel around our car, we have to measure the length of the car (X) which is 1300 mm. so the front end of the wind tunnel is 4X distance from the front end of the car, the back end of the wind tunnel is 6X from the rear point of the car, Top end of the wind tunnel is 3X distance from the top point of the car and the side of the wind tunnel is X distance from the side of the car.

31. Now create a point from the bottom of the car at a distance of 75mm using the relative point option in the –Y direction. Now measure the distance between the last point (which we have used to create point below at 75 mm) and the front of the car which is 650 mm. so the front of the wind tunnel is created at a distance of 5850 mm. similarly create a point to the rear of the car at the distance of 8410 mm. Same way create a point from the roof of the car at a distance of 3900 mm. Now select the front and the endpoint of the wind tunnel and create a relative point in +X and –X at a distance of 1300 mm.

32. Now to create a surface we need to create the curves by using the create curve option. Then use the coons option to create the bottom surface.

 Now measure the distance from the bottom surface of the wind tunnel to the top surface of the car which is 375 mm. so the top surface of the wind tunnel will be at 3X + 375 mm which is 4275 mm.

33. Now use the offset option in the faces, to create the top surface with the offset value of 4275 mm. After creating the surface, delete points and curve as it is of no use.

34. Now with the help of coons face option create the remaining surfaces and assign the newly created surface to new PID’s for the tunnel.

35.Now switch to mesh mode and define perimeter and macros of 100 mm for the top surface and 20 mm for the bottom surface of the wind tunnel.

36. Now for the vertical cons, the mesh length is going to vary from 20 mm to 100 mm, which can be created using the spacing option. Set biasing to geometric, factor to 1.1, dmin to 20, and dmax to 100.

37.Now start meshing bottom surface, define target length of 20mm, quality criteria as min length 15 and max length of 25 mm, then generate the mesh using spot mesh

38. For top surface, define target length of 100mm, quality criteria as min length 80 and max length of 120 mm, then generate the mesh using spot mesh.

39. Now generated the mesh for other surfaces one by one using spot mesh directly as length is already defined.

40. After finishing, all the surface mesh for the tunnel, switch to the volume mesh tab and click define. With the whole domain selected click ok. Ansa will automatically define the closed volume, select the one between the tunnel and the car, delete the rest of the volume. Your model is ready to create the tetra mesh.

Learnings :

1. Different options are explored in-depth to remove single cons in the TOPO clean-up tab.
2. Learned how to do the half model cleanup instead of the full model which saves time tremendously.
3. For large assembly, it’s better to do component-wise cleanup and meshing one by one.
4. Learned to do meshing with quality criteria in mind.
5. Before doing meshing it’s better to remove small length cons not satisfying minimum length criteria.
6. For a component that is attached to some other component having a different target length, it is preferable to do the meshing at last for that component.
7. Learned how to see the fail element using the hidden mode and explored different options to fix the issue like move grid, smooth mesh, reconstruct, etc.
8. Explored the option called spacing in mesh mode which allows creating a grid of varying length.
9. Learned how to convert half model to full model using the symmetry option.
10. Learned how to generate the volume mesh from 2D mesh.

 

Result and conclusions:

1. Given BMW car model is cut to half to save time
2. The model has been cleaned by using a different TOPO clean-up option before doing surface mesh.
3. Different PIDs have been meshed with different quality criteria and mesh target length (given). PID’s which is not connected to each other adjacently should have meshed first.
4. After doing surface mesh, the Full model is created by copying the half model.
5. The standard wind tunnel is generated according to car dimensions.
6. The spacing tool is used to gradually grow mesh length from the road to the top of the wind tunnel.