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. Import the ANSA file using file>>Open onto screen to perform topo/clean .

b. Since the model is not having good quality of surfaces, so go to mesh and choose length and enter 1 mm under perimeters to smoothen the sharp curves of the car and give it a fine look.

c. Since the car is symmetric from the middle to any end, we can delete half portion of the car and perform the geometry cleanup for the same and later, we will use symmetry option to have the complete car geometry.

Note : We do not delete the BMW logo since it is not symmetric.

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:

1. Begin with the front grill area and the headlights, delete all the extra parts to have a single volume throughout the car.
2. Now to remove the single and triple cons and to maintain single volume, to create a single volume we need to have double cons everywhere, to do this we need to performs few cuts and re-surfacing of parts to maintain a single volume.

Note: Single cons will be there along the section of symmetry. These cons will remove after symetry to get complete car model.

Windows, rear lights, roof region and logo:

• 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.
• Do the same for windows, rear lights and roof area where extra internal parts are present which is of no use to create surface or volumetric mesh.
• The logo of the car is under a sinlge surface, therefore using project cons, create a separate surface and delete the extra surfaces to maintain a single volume.

Design feature and mirror:

• Perform topo and identify the triple and single cons, and delete the extra unwanted surface to maintain a common volume.
• Use the intersect option to identify the triple cons at the intersection area and can delete the smaller surfaces individually.
• Extended the single cons on the adjacent surface to create triple cons and delete the unwanted surfaces by doing that.
• 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.

Wheel and Base region:

• Isolate the wheels to perform geometry clean-up, to rectify the triple cons, hide the surface blocking the view and delete the overlappng surfaces causing triple cons.
• Create larger surfaces after deleting smaller surfaces which are parallel to eachother.

• The base of the car is intersecting with the tyre, 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.

• Therefore we need to clean the entire base and create new surfaces accordingly having smooth edges.Before deleting faces, we have to convert cons into curves.
• These curves act as guiding curve to create the faces.
• After making the surface changes, we need to check if the base surfaces has no intersection with the tyre region.

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

Note : Single cons shown here are only along the symmetry of the car as shown below.

Once the geometry is checked and has no errors, we assign PIDs to different components of the car.

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

Using the measure tool we can calcualte 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 failed mesh.

Surface meshing:

We need to mesh the components individually since the given mesh targets are different for each PID regions.

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.

Logo and grills region:

These off elements can be reduces or removed by using improve option of mesh and then reconstruct. Even after that, off elements are there, we can modify the mesh for those off elements by move option in grids and finally will have no off elements.

After this, freeze the mesh so that it will not unmesh at the boundary of two different mesh parameters.

Similarly we will mesh all the parts of BMW Car.

Rims and tyres, window and rear view mirror surface mesh:

Body and Silencer :

Lights:

Note : Since body PID region is in contact with various other PID, the mesh length variation at the boundary region of 2 different PID cause the mesh to erase or cause mesh failure. Therefore, While entering the perimeters under length in mesh drop down. Unselect 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.

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.

Final surface mesh of car model:

Performing symmetry on the car model:

• Perform symmetry under transform option to create a exact same opposite body of the car to create a full body car.
• Use the copy option and select the half body car and diselect the logo sice we dont 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, middle click twice and the symmetry is performed, click middle click again to confirm the geometry and complete the process.
• There will be few single cons present near the middle section, use paste or other tools to solve the probem.

Creating wind tunnel:

a. First, we need to create reference points to create wind tunnel surfaces.

b. Select relative under points in topo mode, create a point below the car taking one reference point and entering the correct values.

And calculating the wind tunnel dimensions appropriately, we can create the wind tunnel accurately.

d. The wind tunnel dimmensions are: ( standard distance of the tunnel wrt to car model)

• car model: 1310mm (approximate) .................. (x)
• Front surface distance(from reference points): 590mm .................. (4x)
• Back surface distance(from reference points): 8510mm .................(6x)
• Side Surface distance (from reference points) : 1610 mm ..................(x)
• Top surface distance(from reference points): 4480 mm ..................(3x)

Note: Some approximations in length is considered.

e. After creating the bottom surface use the curves for having the reference of cons, we can delete all the hot points and curves.

f. Create all the other wind tunnel surface using the new surface option.

Surafce meshing of wind tunnel:

a. We need to apply surface meshing depending 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 spacing option to have a mesh length varying from the small mesh length size to larger mesh length.

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

Volumetric mesh of wind tunnel-car volume:

a. Since we are interested only the volume between the car model and 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, volumetric mesh is not performed as given by the challenge question.

Below is the volumentric region :

• Learning outcomes:

a. Learnt the complete CFD meshing skill from skratch.

b. Performed various topo clean-ups using different tools.

c. Understood the need of double cons and removing unecessary triple and single cons.

d. The use of symmetry helps in reducing the work time and helps in having accurate results with less work.

e. Create surfaces using the reference hot points by using relative points.

f. Surface meshing for different components and their varying mesh lengths.

g. Using spacing option in surface meshing helps in having varying mesh lengths along the surfaces.

h. Using re-construct option to reduce off elements to a level where we can edited it manually.

i. Using the mesh modifying tools like move, split and smooth options.

CONCLUSION:

The BMW car model has been meshed with the given criteria. The generation of 3D elements for this model requires huge power resources and time. Hence the volumetric meshing of this model can't be done.

Model Link : https://drive.google.com/drive/folders/10F11rzI8D5ChQgQ50qKXsqifBzWNV7NP?usp=sharing