Week 4 Challenge : CFD Meshing for BMW car

AIM :

CFD Mesing For BMW car .

To learn topology, geometry clean-up, and meshing for BMW car.

 Also, perform meshing with the given Target length and elements quality criteria. 

After meshing the half model, perform symmetry to the other side.

Given Information:

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

1. Body - 3 mm 
2. Light - 4 mm
3. Grills - 1 mm 
4. Rims and tyre - 2 mm
5. Optical Rear view mirror - 2 mm
6. Window - 2 mm 
7. Silencer - 3 mm 
8. Logo - 1 mm
9. create a wind tunnel for CFD  analysis using ANSA software.

Procedure:

-Geometry clean-up and Assigning PIDs

-Surface meshing and performing symmetry

-Creation of wind tunnel

1) Geometry clean-up and Assigning PIDs:

• Initially, we get a BMW car model as shown below figure.

 

• Enable all the cons and crosshatches to perform geometry clean-up and also go to >mesh>parameters> length> and keep as 1mm to have smooth curves.
• here we have seen the geometry of the car is symmetric hence we edit one-half of the model and mirror it to get a full model, once done with topology cleanup.

• Once the half surface of the car model is deleted we do the checks managers we find out numerous errors such as single con, triple con, intersections, and overlapping surface. These errors have to be solved one after the other in order to get clean geometry for meshing.
• First, the front headlights are edited since there are many triple cons issues as shown below.

• Topo cleanup for grills is done by deleting unnecessary surfaces from grills to remove triple cons, deleting un-wanted hot points, and single cons are removing by patching the new surfaces using the New-Cons/Fitted option

.

• Topo clean-up tyres and side mirrors and head and tail lights include triple cons by deleting the unwanted surfaces.

 

• For a logo, the overlapped surfaces were identified and separated by projecting the surfaces on the hood and deleted followed by removing small edges which may cause errors during meshing.

 

• The lower car body needs to correct by removing unwanted overlapped parts and the lower body intersecting with tyres needs to be deleted to make tyres as a free volume keeping track of different parts in the lower body 'con2curve' is used to have blueprint deleted surfaces with the help of which new surfaces are created.
• Geometric checks are performed to have no single cons present in the model and possible triple cons are removed by deleting it using the universal delete option.

• Before creating surface mesh the cons and small edges that have lengths less than the minimum mesh length are identified by measuring the distance between cons and solved using the 'cut' option in order to avoid mesh failing the mesh criteria at these small surfaces.
• PIDs are assigned using the PID manager which presents the top deck for all parts of the body.

 

2) Surface Meshing and performing symmetry: 

• We need to mesh the components individually since the given mesh targets are different for each PID region and 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. 
• Grills are isolated and give a mesh length of 1mm with minimum and maximum lengths of 0.5mm and 2mm with triangular elements and for Quality, the criteria provide appropriate mesh length. Here are the quality criteria and mesh element size for all the components.

BMW LOGO

GRILLS 

 

BODY

SILENCER

WINDOWS 

• The mesh is seen in Hidden Mode and ENT wise, which will show off elements which are violating the criteria. So we need to remove off elements by using options such as MOVE, SPLIT, RECONSTRUCT, and SMOOTH the grid respectively, from this we can improve our meshing.
•  Once the off elements' number is reduced to 0 then meshing is done successfully, and a Similar method is used for other PIDs.
• Symmetry needs to be performed on the other half of the car in which all the components with mesh are copied directly to the other side using the Transform option from the top side option list.

 

3) Creation of wind tunnel:

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

Data for the wind tunnel are as follows:

• The wind tunnel dimensions are, if x is the length of the car then
• The front of the wind tunnel is 4X (5875mm) from the car.
• The rear of the wind tunnel is 6X (8480mm) from the car.
• The top of the wind tunnel is 3X (4323mm) from the car.
• The side walls of the wind tunnel are at X (1306mm) from the side portion of the car

• The bottom of the wind tunnel is in close proximity to the wheels
• Points are created by using the 'Relative option' in points of Topo Deck.

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

• We need to apply surface meshing depending on the mesh length near the car and regions far away from it Therefore we need to apply different mesh lengths at the bottom and top wind tunnel surfaces We need to use the spacing option to have a mesh length varying from the small mesh length size to larger mesh length Surface meshing can be achieved by doing this accordingly.

• 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 

 

Conclusions:

• Topo clean up of BMW car & surface meshed appropriately.
• The wind tunnel is created and the surface meshed with reference to the car model size.
• Single volume is created of the car body so that it can be avoided at one go when creating a volumetric mesh.
• Required physics tests can be performed on the wind tunnel volume model after volumetric meshing successfully.