Week 4 Challenge : CFD Meshing for BMW car

Aim: CFD meshing in BMW car

Procedure:

To perform CFD meshing in BMW car by following steps:

1) Clear the geometrical errors/topo clean up

2)Assign PIDs

3)Surface meshing

4) Create Wind tunnel

5) Volumetric meshing

Steps -

Resolving Geometrical Errors -

Import the Geometry. You are in the "Topo Deck" by default. Turn the geometry to "Shadow display" mode from the "Wireframe display" mode so as to perform further operations on it. At first, the resolution of the geometry would not look proper and we need to change its resolution. 

Tools > Settings > Resolution/Tolerances/Units > Resolution > CONS > Perimeter Length = 1

Note - Always use "Topo" over the geometry after fixing the errors. The Topo module helps to auto-fix errors that aren't much significant in the geometry. It joins the faces which are not joined properly after we perform an error clean-up. 

Here, before proceeding toward the geometry cleanup of every part, we may find that the given geometry of the BMW car is axially symmetrical. Hence, to ease the entire process we would delete either half part of the geometry and would later mirror that part to produce the other half.

Universal Delete > select either half of the geometry > middle click

Note - In case, if some parts aren't symmetrical or cannot be mirrored, we may isolate them for time being and proceed with the further operation. We will start the geometry clean-up by resolving the three grills which are present on each half.

Grill 1 -

Isolate the grill and lock the hidden parts.

Delete the external faces which are creating closed surfaces inside the grill. Unless you do not project any surface you cannot delete it. 

Unnecessary hot points should be deleted here as they will be the cause of extra unwanted faces. Also, during meshing, they will try to create extra node points.

Once, the clean-up is done, save the geometry. 

Grill 2 -

Isolate the grill and lock the hidden parts.

When looking through the inside of the grill, we found that the grills aren't properly aligned. We will cut a specific volume of the grill from the backside by inserting hot points and making a new face there.

• Hot Points > Parametrical > put 0.15 > select the cones on the external faces > OK
• Faces > Cut > select all the hot points > OK
• Faces > New > select all the cones newly created > select Fitted > OK

Thus, a new face is created at a distance of 15% of the total length from the end surface of the geometry.

We will temporarily create and assign PIDs as;
1. Grill 2 - grill (excluding the new face)
2. Grill Temp 2 - only the new face 

Use the "Intersect" module button over both the temporarily assigned PIDs and delete all the external surfaces which are unnecessary. Remember to keep the faces of the newly created face.

Once, the clean-up is done, save the geometry.

Grill 3 -

Isolate the grill and lock the hidden parts. 

Accomplish the necessary geometry clean-up and save the geometry.

Other geometry components like the logo, mirror, floor, tyres, and rims need to be error-free using a variety of tools and techniques for geometry clean-up.

Note - Depending on the strategy a user takes, there are many ways to clean up the entire geometry. The only goal of cleanup is to ensure that there are no intersections or other geometrical errors.

Thus, all the errors have been resolved with warnings only which could be ignored as they are insignificant. 

Assigning PIDs -

Components to be assigned with PIDs are;

1. Body 
2. Lights 
3. Grills 
4. Rims and Tyres 
5. Optical Rear view mirror 
6. Window 
7. Silencer 
8. Logo

Mesh Generation -

Click "Classic Mesh".

Switch ON the quality criteria for a better quality mesh generation;

Skewness = 0.5, Minimum length = 0.1 mm & Maximum length = 07 mm. Click the "Hidden" tool to keep the visibility of quality parameters on the screen. 

Target lengths specified for various components are -

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

Here, we can either mesh each component one after another, or else we may mesh components with the same target length at once. When we mesh the components using either of the two ways, we may find that each individual meshing has been generated very properly according to the given target lengths.

However, when we look at the mesh generated over the entire geometry, we may find some unmeshed surfaces. These unmeshed surfaces are generated due to the varying target lengths of each individual component.
These unmeshed surfaces are generated at the perimeters of the components of different target lengths.

To avoid this we have two methods;

1. As soon as we finish the mesh generation of the entire geometry and find some unmeshed surfaces, we need to use the "Free" module button in the "Mesh Generation" module button group.

Classic Mesh > Mesh Generation > Free > Visible

2. As soon as we finish the mesh generation of each component(s) with the same target length, use the "Freeze/Unfreeze" module button in the "Macros" module button group to freeze the mesh generated over the selected geometry so that it won't be erased or deleted when we mesh the other components.

Classic Mesh > Macros > Freeze > Left click > select the visible geometry

Repeat this tool after the mesh generation of every component(s). After finishing mesh generation all the components use the same "Freeze/Unfreeze" module button in order to further process the geometry.

Classic Mesh > Macros > Freeze > Right click > select the visible geometry

Among the two methods, the former is a shortcut tool to fix unmeshed surfaces. However, the latter method is a professional way to generate mesh over the components with varying target lengths.

To generate mesh, we need to follow three steps:

1. Click "Mesh Parameters" in the Toolbar. Under the "Basic" menu, select mesh type as "CFD Mesh" & element type as "Tria". Enter the minimum target length and maximum target length. Click OK.

Here, we used CFD mesh instead of General mesh because the former gives more accurate results. One can select General mesh as well, there will be no error in the final results.

2. Perimeters Module > Spacing > Auto CFD > Perimeters > Select entire geometry > middle click > OK > Macros > Select entire geometry > middle click > OK

3. Mesh Generation Module > CFD > Visible

Note - Always mesh the components having lower target length first, going in ascending order to the components having upper target length at last, so that the geometry adapts the mesh transition well and helps to properly capture the body. 

In the case of off elements (bad elements):

Right-click "Off elements", click "Improve" and try to 'Reconstruct' or 'Reshape' the elements so that the off elements get improved and turn into desirable elements. One may use the 'Grids Move' option to move the grids of off elements. 

Thus, Mesh generation is done on the geometry. 

Now it's time to mirror the geometry to produce the other identical half of BMW. 

Transform > Copy > Entities > select entire geometry > select 'Symmetry' > select '3 Points Plane'  select any three random points on the edge of the geometry > OK > Apply > Finish

Also, it is asked to construct a wind tunnel as a fluid domain for the external flow analysis over the BMW. A wind tunnel will have one top face, one bottom face, and four side faces (two at the rear and front ends & two at the left and right ends). Let 'x' be the length of the car (from the rear end to the front end). 

The distance between the car and the bottom of the wind tunnel should be such that they do not intersect. The distance between the car and the top of the wind tunnel should be greater than or equal to '3x'. The distance between the car and the two left and right ends should be greater than or equal to 'x'. The distance between the front end of the car and the wind tunnel should be greater than or equal to '4x' & that of between the rear end of the car and the wind tunnel should be greater than or equal to '6x'. When measured the value of 'x' came out to be '1310 mm' approximately. Thus, a wind tunnel was created using various tools such as Points, Curves, etc. 

This wind tunnel needs to mesh (without any quality criteria) with target lengths as; 

Bottom Face - 20 mm,

Top Face - 100 mm,

Side Faces with 'Growing Mesh' from 20 mm to 100 mm :- Perimeters > Spacing > Manual > Growth Factor = 1.2, Dstart = 20, Dlimit = 100 > select all the four edges of side faces > OK

Thus, surface mesh generation of geometry, along with fluid domain construction and meshing, is done.