Challenge

Week 4 Challenge : CFD Meshing for BMW car

Report on BMW

To generate a CFD mesh on a BMW car model for external fluid flow analysis

import the file, open it in shadow mode
As the design is symmetric from the center, for simplification let's delete (use universal delete) half of the design (except the BMW emblem).

now do TOPO and delete the unnecessary hot point
check the geometry check to find the errors
to get the design in good resolution change the perimeter length to as minimum as possible in resolution settings in Tools
As our aim is to mesh the car for external flow analysis. We do not need any internal volume at the surfaces
There are 3 grill structures in the front, so let’s start cleaning the errors from that.

Isolate the bottom grill, and check the edge connections and delete the internal volumes (thicker volumes) and create fully connected surfaces.


Misalignment of edges and individual volumes	Made as single volume and proper connections at edges

Clear the surfaces (delete) and create new surfaces using (projection of cons) and crating new cons.
Now select the other Grill, isolate and lock
We see the miss alignment at all edges, so we need to clear the misalignment for that we make a proper cut and make a surface and use an intersection to separate the entities.
Create parametrical points, join with cut, and make a surface (fitted give good surface).
Set PIDs and then do intersection and delete unwanted surfaces.


Before	After

3^rd grill and isolate, here we see the open volume that needs to be closed and cut and make surfaces so the design is closed and with a single volume


Create cuts and hot points and make new surfaces, delete unnecessary surfaces, use plane cut for planar cut through all the surfaces	After, all surfaces were closed and made it to single volume

After the 3 grill design, now check the geometric check and see any error in the grill.
Find the errors and triple cons on the car one by one and delete the inner volumes.
For example, headlight, as it is an external flow analysis no need for an inner light setup so delete.
At logo, it has double cons, so isolate and check geometric check, cause any two parts are in contact it’s better to check for the intersectional errors.
The logo has also some thickness, which is unnecessary, so delete the inner surfaces.
Also, an error (triple con) at the letter M, is edited by deleting and creating new surfaces
Side indicator, inner surfaces are deleted and made closed body using a command called extended ( extends a face to another perpendicular face, it happens only for red cons)
Then the side mirror and windshields, both have double surfaces which produce closed volumes
So delete the inner volumes and make a hole projecting the side mirror hole to the side window and make an open connection to the side mirror
Also, delete the internal portions at the rare end of the car bonnet.
The intersectional surfaces at tyres delete the surfaces and make a new surface accordingly.
Make edges into cons, so even after deleting the surface the boundaries don’t disappear.
Blue cons in tires are due to the inner surface, so delete the surface.
Similarly, complete all the errors and do it until no errors are shown except single cons at half part and convex cones at door handle.

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

By maintaining certain parameters such as skewness, aspect ratio, warping etc
In this assignment we are considering only the skewness, now by maintaining the highest valve of skewness for the triangular mesh (0.6), giving the meshing parameters and mesh quality
Mesh parameters are given individually to each PID, where the minimum and maximum target length will be set, which means the minimum and maximum length of a mesh.
In the quality of mesh, we give the range of min and max side length by considering all parts of the design. For this assignment of BMW, min and max lengths were 0.1 and 7 respectively.
While doing the meshing it gives “off” elements which are beyond the limits of the parameters mentioned (skewness, min and max target length).
These off elements should be cleared by reconstructing in (improve command right click on off) the off elements or by using the ‘move’ command under the grid and changing the position of the node till the error is cleared.
Use the freeze command to fix the mesh.
After meshing create the other half using the transform command (by hiding the logo)
And use the cons paste command to eliminate the single coons (it attaches the surfaces)
As we are creating for external flow analysis, need to create a block around the car.
With a gap of
1. Front 4x
2. Rear 6x
3. Top 3x
4. Sides 4x
5. Bottom more than the size of the car could be any. (so the plane should not intersect the car model)

Where x= length of the car.

For creating a plane, initially plot hot points accordingly using the relative command in points
Then create new surfaces by joining the curves.
Now mesh the bottom plane with 20 mesh length
And top with 100 mesh length
Now for varying mesh design to side faces use the spacing command that mention the sides and growth rate and limits
And do any of the meshing (spot generates less no of mesh elements)

Learning outcomes

Got to learn new commands such as

Finally created a perfect meshed model with an external shell for fluid analysis