Week 4 Challenge : CFD Meshing for BMW car
AIM:
To generate mesh for BMW cars for CFD applications.
OBJECTIVE:
Clean up the geometry till there is no topological error.
Assigning relevant PIDs.
Surface meshing.
Perform symmetry.
Creation of wind tunnel.
For the mesh, we have to give the following length of element size as follows
- Target lengths for the different parts of a model are as follow:
- 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
INTRODUCTION:
ANSA is a computer-aided engineering tool for Finite Element Analysis and CFD analysis widely used in the automotive industry. It carries several proprietary algorithms for meshing suitable for both CFD and structural models.
Enabling the PID mode.
We have been provided with a CAD model of a BMW car with some geometrical errors. From the model we can see there are many single coons(in red) and glitchy surfaces are present, which we need to resolve before performing the meshing operation also, we need to patch any open surfaces as in CFD analysis surfaces should be close to doing volumetric mesh. There are also some default PID's which we will delete to create our own PID's. Also, our model is symmetric thus will consider only half the model for our purpose and then will perform symmetry on it to reflect all our operations.
PROCEDURE :
Topology Cleanup:
As we open our model in ANSA, first we will perform checks on the geometry to see all the errors that are present in the model. Besides the script tool there is an arrow button, click on it and select checks and select geometry. Then from the bottom part of the window, we will select the things which we want to check and then click execute. We can see many errors listed. Let's resolve them.
First, we will topo tool. Topo is the opposite of release. The release will create 2 single cons from a double con and 3 single cons from a
triple con. Topo will do the same in the opposite manner i.e. from 3 surfaces it will create triple con and from 2 surfaces will create
double cons. Topo is useful when we see triple cons but there is no overlapped surface then we can release and do topo again just to show that there aren't any triple cons present or any overlapped surfaces present.
Not many features are captured appropriately, as can be seen at edges around various components,i.e. our cons resolution is bad. There are many ways to increase cons resolution like manually resolving each component or we can use the perimeter option from the mesh deck.
In Mesh deck => In perimeters => Length => Select whole model => Enter length 1 mm.
This will reduce the distance between two nodes, which is evident from much-improved cons resolution. As the model is symmetric we will only use half of it, will work on it, do topo cleanup, do mesh, then reflect half side on another side. Delete half car model using universal delete, except car logo as it's not symmetric.
Now, will delete all unnecessary surfaces from inside the car model, as we are concerned about the outside of the car and the area between the car's outer surface and the wind tunnel. Any extra surfaces will create unnecessary volumes utilizing memory and time. As shown in the below pictures will perform topology cleanup on our model and will try not to have any single or triple cons except, at locations around which symmetry will be performed. Will remove any extra hot points. Also will use the CUT tool to create cons wherever necessary. Also, delete any overlapped surfaces. Overlapped surfaces may cause an error while going in the volumetric mesh. Once we go into the volumetric mesh, we may get a generation error, that error is basically showing us overlapped surfaces.
Clean up the geometry
Middle Grill:
Before
After
Front Grill:
Before
After
Side Grill:
Before
After :
Side mirror:
Before
After
Rims and Tyres
Before:
After:
Logo:
Before
After
Silencer
Car Body
Before
After
Final Geometry After Topo Cleanup
Final Checks After Topo Cleanup
From the above image, we can see that only single cons are left which will be used in creating a symmetric another side.
PID Creation -
Now let's create PID's which will come in handy during meshing. To do that follow the below procedure,
Click Properties => right click and select New => select PSHELL => again select PSHELL => Give name as Silencer => ok.
Similarly create the body, lights, window, grills, rims and tyres, optical rearview mirror and logo PID's.
Now, to assign each PID to its respective part follow the below steps,
Right click on PID name => Apply => select the respective part => click middle mouse button to confirm.
Similarly, assign all the PIDs.
We are making sure that at the end we only have a larger body PID, thus we have assigned all the small PIDs.Now all remaining components will be in Body PID.
Now if we try to delete the shell properties we got an error as "Selected ID is used by a deleted face and can not be deleted".But there aren't any surfaces showing after selecting show only for Shell_Property PID. Thus we will delete it using brute force. Thus select the arrow symbol beside the measure tool and select compress then press ok. This will
delete the unnecessary PIDs.
Symmetry -
Now we will replicate all the topo cleanup and surface mesh to the other side using symmetry. To do that follow the below steps,
From Transform => select Copy => select same part => select three points along the single cons and do not give any offset value => ok
Meshing -
Now here we need to create a mesh for various PIDs by using the below data,
Target Lengths -
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
We can see many off elements are present. To resolve them we followed the below steps,
In Mesh Deck => In Shell Mesh => Reconstruct => Select Visible.
This will try to resolve off elements. If still isn't fully resolved, we can also try using the below tool from the mesh deck.
Split
Mesh => Elements => Split => Edges => This will split the selected edge and may help with maximum and minimum length error.
Move
Mesh => Grids => Move => This will move the node in the desired direction, and may come in handy with skewness and other errors.
Similarly use above mentioned steps for other PIDs, to resolve any mesh quality criteria errors.
And if the elements error isn't getting resolved by using the move, we can also use the Origin tool over those elements and then again use the move tool.
Logo
Click on Quality Criteria =>minimum length 0.3 mm and max. length 5 mm.
Mesh Parameters => Target Length 1 mm and select element type as tria.
In Perimeter and Macros set length as 1 mm.
In mesh generation => Select CFDMesh => Select Visible.
Grils
Click on Quality Criteria =>minimum length 0.3 mm and max. length 5 mm.
Mesh Parameters => Target Length 1 mm and select element type as tria.
In Perimeter and Macros set length as 1 mm.
In mesh generation => Select CFDMesh => Select Visible.
Windows
Click on Quality Criteria =>minimum length 0.3 mm and max. length 5 mm.
Mesh Parameters => Target Length 2 mm and select element type as tria.
In Perimeter and Macros set length as 2 mm.
In mesh generation => Select CFDMesh => Select Visible.
Optical Rear view mirror
Click on Quality Criteria =>minimum length 0.3 mm and max. length 5 mm.
Mesh Parameters => Target Length 2 mm and select element type as tria.
In Perimeter and Macros set length as 2 mm.
In mesh generation => Select CFDMesh => Select Visible.
Rims and Tyres
Click on Quality Criteria =>minimum length 0.3 mm and max. length 5 mm.
Mesh Parameters => Target Length 2 mm and select element type as tria.
In Perimeter and Macros set length as 2 mm.
In mesh generation => Select CFDMesh => Select Visible.
Body
Click on Quality Criteria =>minimum length 0.3 mm and max. length 5 mm.
Mesh Parameters => Target Length 3 mm and select element type as tria.
In Perimeter and Macros set length as 3 mm.
In mesh generation => Select CFDMesh => Select Visible.
Silencer
Click on Quality Criteria =>minimum length 0.3 mm and max. length 5 mm.
Mesh Parameters => Target Length 3 mm and select element type as tria.
In Perimeter and Macros set length as 3 mm.
In mesh generation => Select CFDMesh => Select Visible.
Lights
Click on Quality Criteria =>minimum length 0.3 mm and max. length 5 mm.
Mesh Parameters => Target Length 4 mm and select element type as tria.
In Perimeter and Macros set length as 4 mm.
In mesh generation => Select CFDMesh => Select Visible
Final Model After Meshing
Fluid Domain
We will create a wind tunnel using points, then join them by curves and finally create surfaces using those cons and also surface mesh it as follows.
Meshed Fluid Domain model
Bottom
Top
Front, Rear and Sides
Volumetric Mesh
Volumetric mesh is nothing but 3D elements, while the surface mesh meshes with the help of 2D elements which are tria elements. One important thing to do before the volumetric mesh is to make sure that all of our surfaces mesh.As the surface mesh is a prerequisite for volumetric mesh. Click on mesh deck and make sure there aren't any unmeshed surfaces and off surfaces.
First, we need ANSA to detect all volumes present. To do that follow the below steps,
Click on V.Mesh deck => Volumes => Define => Right click and select Auto Detect => Work on whole db => OK.
Since we are interested only in the volume between the car model and wind tunnel, we can delete other volumes of the car model which is not necessary Since performing the volumetric mesh takes 2-3 days, the volumetric mesh is not performed as given by the challenge question.
Results and Outcome
- From the meshing images, we can clearly see the smooth transition of elements from one PID to another. Also, there aren't any unmeshed surfaces, also off elements present. All elements follow the given element quality criteria.
- By using symmetry we can see that topo cleanup and mesh get successfully replicated to the other side.
- ANSA successfully detected volume between wind tunnel and car model and this will be used to generate volumetric mesh, which can then be used in CFD simulation.
- By using the half car model we significantly reduced the memory usage and also computation time.
Conclusion
- We have successfully performed topology cleanup on the given BMW car model and resolved single cons and removed all overlapped surfaces.
- Successfully generated surface mesh, which is a pre-requisite for volumetric mesh and created wind tunnel and meshed it.
- Also learned what is volumetric mesh and how to perform volumetric mesh.
- Thus we have successfully generated volumetric mesh for the given BMW car model for CFD applications.
Drive Link:
https://drive.google.com/file/d/1nEq4pnBT7WTmSpG2gs61llSN_xkrobs-/view?usp=sharing