Week 4 Challenge : CFD Meshing for BMW car

In this project, a BMW car in a wind tunnel will be meshed in ANSA. The geometry has multiple errors, these errors will be fixed. The mesh's element's lengths are different for each component of the geometry, and the mesh will be generated accordingly. The mesh length specifications are:

• Body - 3 mm
• Lights - 4 mm
• Grills - 1 mm
• Rims and Tyres - 2 mm
• Optical Rear view mirror - 2 mm
• Window - 2 mm
• Silencer - 3 mm
• Logo - 1 mm

 

The geometry was imported into ANSA and this is the initial geometry:

Since the car is exactly symmetrical along the central plane, the car can be cut into two parts and it can be copied along the symmetry plane after meshing.

Note: during deletion of the other half, the logo is kept intact as the logo is not symmetrical.

Switching to wireframe view,

Upon further inspection, the errors are primarily of three types: single CONS, overlapped surfaces and extra surfaces.

These errors were rectified using the TOPO sidebar and all errors were rectifie.

After all issues were fixed, the geometry checking tool was run (Checks -> Geometry). The default checks are chosen, and the following errors are checked. The checking process is started by clicking the Excecute button.

The results of the geometry checking tool are below:

Since only one half of the model is being worked on, there are single cons along the symmetry plane. Running the geometry checking tool again without checking for single cons shows the following results:

These are the double and triple CONS: (there are none)

The model came with existing PIDs, these were created or modified/renamed to the following:

The empty PIDs with the default names were removed by going to: Compress -> Ok

Surface Meshing

The preproccesing to the actual meshing process is complete. The surface mesh will be generated first under Classic Mesh Model. The PIDs were grouped by mesh size for convenience. For each size: perimeters, macros, mesh parameters and quality criterion were defined as follows.

Mesh Size: 1 mm

Parts of geometry/PID: logo and grills

The quality of the mesh is defined in this step, this is done by going to Quality Criteria

The minimum length is chosen from analysising the geometry in detail.

This is where the quality of mesh will be defined, this is done by going to: Mesh Parameters. The target length is entered as 1mm and the element type is set to Tria.

Go to Perimeters -> Length -> Perimeters, the whole geometry can be selected with the box select tool. After selection, the middle mouse button is pressed and mesh length (1mm) is entered.

Entering mesh length in mm.

The same process is repeated for Macros as well.

The mesh is generated for the following parameters using the spot mesh, this is done by going to Mesh Generation -> Spot Mesh -> Visible

The quality criteria of the meshed elements are checked by switching over to the hidden view:

The off/bad elements are fixed using mesh tools such as: Join, Split, Move, Reconstruct and Paste.

After no off elements are found in the meshed PIDs, the next set of PIDs are meshed.

Mesh Size: 2mm

Parts of geometry/PID: windows, wheels_and_rims and side_view_mirror

The quality of the mesh is defined in this step, this is done by going to Quality Criteria

The minimum length is chosen from analysising the geometry in detail.

This is where the quality of mesh will be defined, this is done by going to: Mesh Parameters. The target length is entered as 2mm and the element type is set to Tria.

Go to Perimeters -> Length -> Perimeters, the whole geometry can be selected with the box select tool. After selection, the middle mouse button is pressed and mesh length (2mm) is entered.

Entering mesh length in mm.

The same process is repeated for Macros as well.

The mesh is generated for the following parameters using the spot mesh, this is done by going to Mesh Generation -> Spot Mesh -> Visible

The quality criteria of the meshed elements are checked by switching over to the hidden view:

The off/bad elements are fixed using mesh tools such as: Join, Split, Move, Reconstruct and Paste.

After no off elements are found in the meshed PIDs, the next set of PIDs are meshed.

Mesh Size: 4mm

Parts of geometry/PID: lights

The quality of the mesh is defined in this step, this is done by going to Quality Criteria

The minimum length is chosen from analysising the geometry in detail.

This is where the quality of mesh will be defined, this is done by going to: Mesh Parameters. The target length is entered as  and the element type is set to Tria.

Go to Perimeters -> Length -> Perimeters, the whole geometry can be selected with the box select tool. After selection, the middle mouse button is pressed and mesh length (mm) is entered.

Entering mesh length in mm.

The same process is repeated for Macros as well.

The same process is repeated for Macros as well.

The mesh is generated for the following parameters using the spot mesh, this is done by going to Mesh Generation -> Spot Mesh -> Visible

The quality criteria of the meshed elements are checked by switching over to the hidden view:

The off/bad elements are fixed using mesh tools such as: Join, Split, Move, Reconstruct and Paste.

After no off elements are found in the meshed PIDs, the next set of PIDs are meshed.

Mesh Size: 3mm

Parts of geometry/PID: exhaust

The quality of the mesh is defined in this step, this is done by going to Quality Criteria

The minimum length is chosen from analysising the geometry in detail.

This is where the quality of mesh will be defined, this is done by going to: Mesh Parameters. The target length is entered as  and the element type is set to Tria.

Go to Perimeters -> Length -> Perimeters, the whole geometry can be selected with the box select tool. After selection, the middle mouse button is pressed and mesh length (mm) is entered.

Entering mesh length in mm.

The same process is repeated for Macros as well.

The same process is repeated for Macros as well.

The mesh is generated for the following parameters using the spot mesh, this is done by going to Mesh Generation -> Spot Mesh -> Visible

The quality criteria of the meshed elements are checked by switching over to the hidden view:

The off/bad elements are fixed using mesh tools such as: Join, Split, Move, Reconstruct and Paste.

After no off elements are found in the meshed PIDs, the next set of PIDs are meshed.

Mesh Size: 3mm

Parts of geometry/PID: body

The quality of the mesh is defined in this step, this is done by going to Quality Criteria

The minimum length is chosen from analysising the geometry in detail.

This is where the quality of mesh will be defined, this is done by going to: Mesh Parameters. The target length is entered as  and the element type is set to Tria.

Go to Perimeters -> Length -> Macors, the whole geometry can be selected with the box select tool. After selection, the middle mouse button is pressed and mesh length (mm) is entered.

Entering mesh length in mm.

The same process is repeated for Perimeters as well. Note: While selecting edges for perimeters, the edges which are in contact with other PIDs are deselected.

The mesh is generated for the following parameters using the spot mesh, this is done by going to Mesh Generation -> Spot Mesh -> Visible

The quality criteria of the meshed elements are checked by switching over to the hidden view:

The off/bad elements are fixed using mesh tools such as: Join, Split, Move, Reconstruct and Paste.

 

After no off elements are found in the meshed PIDs, the next set of PIDs are meshed.

Now, the surface has been meshed for the half body. The body has to be mirroed now. This is done by going to: Transform -> Copy -> Entities

Shift + left click drag to select all entities:

Note: deselect the logo as it is not symmetrical.

Click the middle mouse button to open the copy menu. Select three points on the symmetry plane of the car (or on the single cons)

 

The points selected in this project were near the windshield. Middle click again to confirm selection, and choose the following transformation options.

Click on OK, and after verifying the mirror is correct. Click on Finish to generate the copy.

The single cons are not fixed yet, they can be fixed by pasting the single cons together. This is the final geometry after pasting:

After pasting, a final geometry check is performed and no errors were found:

Surface meshing for the car is complete. A wind tunnel is used for CFD simulations, and has to be created in ANSA. The wind tunnel is a cuboid, and the car is placed inside it. The inlet of the wind tunnel is 4 times the charecteristic length of the car from the front of the car, the outlet of the wind tunnel is 6 times the charecteristic length of the car from the back of the car. The height of the wind tunnel is 3 times the charecteristic length of the car from the top of the car. While, the width of the wind tunnel is 1 time the charecteristic length of the car from the sides of the car.

For this BMW car, the charecteristic length was measured to be ~1300mm.

Using the above information, multiple vertices of the wind tunnel were created using: TOPO -> Points -> Relative

These points were joined to form a linear curve using: TOPO -> Curves -> Create

The curves were used to create surfaces using: TOPO -> Faces -> New

This is the resulting wind tunnel:

Hiding the front to see the car:

The curves and points created were then removed. PIDs were created for all surfaces of the wind tunnel.

The surfaces of the wind tunnel have to be meshed now.

Mesh Size: 100mm

Parts of geometry/PID: windtunnel_top

The quality of the mesh is defined in this step, this is done by going to Quality Criteria

This is where the quality of mesh will be defined, this is done by going to: Mesh Parameters. The target length is entered as 100mm and the element type is set to Tria.

Go to Perimeters -> Length -> Perimeters, the top plane's edges are selected. After selection, the middle mouse button is pressed and mesh length (100mm) is entered.

Entering mesh length in mm.

The same process is repeated for Macros as well.

The mesh is generated for the following parameters using the spot mesh, this is done by going to Mesh Generation -> Spot Mesh -> Visible

The Mesh quality was not checked as quality of elements at the boundaries do not matter as no data is obtained near the planes.

Mesh Size: 20mm

Parts of geometry/PID: windtunnel_base

The quality of the mesh is defined in this step, this is done by going to Quality Criteria

This is where the quality of mesh will be defined, this is done by going to: Mesh Parameters. The target length is entered as 20mm and the element type is set to Tria.

Go to Perimeters -> Length -> Perimeters, the bottom plane's edges are selected. After selection, the middle mouse button is pressed and mesh length (20mm) is entered.

Entering mesh length in mm.

The same process is repeated for Macros as well.

The mesh is generated for the following parameters using the spot mesh, this is done by going to Mesh Generation -> Spot Mesh -> Visible

The Mesh quality was not checked as quality of elements at the boundaries do not matter as no data is obtained near the planes.

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Mesh Size: variable

Parts of geometry/PID: windtunnel_left, windtunnel_right, windtunnel_front and windtunnel_back

This is where the quality of mesh will be defined, this is done by going to: Mesh Parameters. The target length is selected as local and the element type is set to Tria.

A variable element length is required for these edges, as they join the lower, more finer mesh region with the upper, less finer region of the wind tunnel. Hence, mesh lengths are uniformly increased from the lower surface (20mm) to upper surface (100mm), with a factor of 1.1. This is done by going to: C. Mesh -> Perimeters -> Spacing -> Manual -> with the following settings, and selections.

Click on OK and verify the steps and labels.

Note: the lower part of the vertical edges have to be selected.

 

The mesh is generated for the following parameters using the spot mesh, this is done by going to Mesh Generation -> Spot Mesh -> Visible

The Mesh quality was not checked as quality of elements at the boundaries do not matter as no data is obtained near the planes.

 

 

 

The surface mesh is now complete for both car and the wind tunnel, there are about 1.46 million elements, all of them are trias.

The car has about 1.09 million elements, all of them trias.

Another angle,

Another angle,

This is the inlet/front of the wind tunnel, notice how the element size increases as we move upwards.

 

Volumetric Meshing

Go to V.Mesh tab,

Volumes -> Define ->

Click on OK, and ANSA will generate volumes automatically. These are the autogenerated volumes:

Click on each volume and view its preview, the required volumes (volumes where fluid flow will occur) are renamed and the other volumes are deleted.

After deletion, these volumes remain:

Double click on the volume and change "Mesh with" to Tetra CFD. Tetra is the shape of the mesh's elements and CFD is the application.

Right click on the volume and click Remesh, this will mesh the volume with the selected element type (Tetra CFD)

Note: the volumetric mesh was not generated for this project as it requires a lot of computational resources.

ANSA file: https://drive.google.com/file/d/1GCqzBq_K_uoWCifKI9YG8BB1K7NVWHMr/view?usp=sharing