Week 4 Challenge : CFD Meshing for BMW car

1] AIM -

To generate volumetric mesh for BMW car for CFD applications.

2] OBJECTIVE -

* To create half model of the given geometry and solve all geometrical errors.
* To perform topology cleanup,generate PID's and generate surface mesh.
* To create wind tunnel.
* To generate volumetric mesh from the surface mesh and do symmetry.

3] 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.

We have been provided with a CAD model of a BMW car with some geometrical errors.As 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 meshing operation also ,we need to patch any open surfaces as in CFD analysis surfaces should be close to do 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 model for our purpose then will perform symmetry on it to reflect all our operations.

4] PROCEDURE -

4.1] 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 .Beside script tool there is arrow botton,click on it and select ckecks and select geommetry.Then from the bottom part of the window we will select the things which we wants to check then click execute.We can see many errors listed.Lets resolve them.

First we will topo tool.Topo is opposite of release.Release will create 2 single cons from a double con and 3 single cons from a
triple con.Topo will do the same in 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 are 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 appropriatly,as can be seen at edges around various commponents,i.e. our cons resolution in bad. There are many ways to increase cons resolution,like manually resolving each component or we can use perimeter option from mesh deck.
In Mesh deck => In perimeters => Length => Select whole model => Enter length 1 mm.

This will reduce the distance between two nodes,which evident from much improved cons resolution.As model is symmetric we will only use half of it ,will work on it,do topo cleanup, do mesh,then reflect half side on other 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 outside of the car and area between car outer surface and wind tunnel.Any extra surfaces will create unnecessary volumes utilising 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 CUT tool to create cons whereever necessary.Also delete any overlapped surfaces.Overlapped surfaces may cause an error while going in the volumetric mesh.Once we go in the volumetric mesh ,we may get generation error,that error is basically showing us overlapped surfaces.

4.1.1] Side Grill -

Front Upper Grill ;

Windows ;

 Front Lights ;

Rear Lights ;

Rear view mirror ;

Tyres ;

 Logo ;

 Side Aero Fairing ;

 Rear Bumper ;

 Other Components  ;

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 use in creating symmetric other side.

 PID Creation ;

Now lets create PID's which will come handy during meshing.To do that follow below procedure ,
Click Properties => right click and select New => select PSHELL => again select PSHELL => Give name as Silencer => ok.
Simillarly create body,lights,window,grills,rims and tyres,optical rear view mirror and logo PID's.

Now ,to assign each PID to it's respective part follow below steps,
Right click on PID name => Apply => select the respective part => click middle mouse button to confirm.
Simillarly assign all the PIDs.

 

 

We are making sure that at the end we only have 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 adeleted 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 arrow symbol beside the measure tool and select compress then press ok.This will
delete the unnessary PIDs.

 Meshing ;

Now here we need to create mesh for various PID's by using 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

Now from the top deck window select openfoam and also enable below quality criterias.

Quality Criteria -
Skewness - 0.5
Min angle quads - 45
Max angle quads - 0.7
Min angle trias - 30
Max angle trias - 0.7

0ur elements should follow above quality criterias,if they fail then they will be highlighted by the respective color chosen for them.As we have different length surfaces thus we need different quality citerias.Also examine PIDs for minimum length element to avoide mesh problems.If small elements are present,then use CUT tool and try to resolve it.Also check distance between two nodes.

We will setup min.length and max. length in such a way that it includes all traget lengths mentioned above.Thus we will set min. length = 0.3 mm and max. length = 5 mm.

 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 Spot Mesh => Select Visible.

Now enable PID and Hidden mode,this will show OFF elements or those elements that failed quality criteria if any present.

We can see may off elements are present.To resolve them we followed below steps,
In Mesh Deck => In Shell Mesh => Reconstruct => Select Visible.

This will try resolve off elements.If still aren't fully resolved,we can also try using below tool from 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 desired direction,may come handy with skewness and other errors.
Simillarly use above mentioned steps for other PIDs, to resolve any mesh quality criteria errors.

And if the elements error aren't getting resolved by using move ,we can also use Origin tool over those elements and then again use move tool.

Grills  ;

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 Spot Mesh => 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 Spot Mesh => 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 Spot Mesh => 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 Spot Mesh => 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 Spot Mesh => 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 Spot Mesh => 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 Spot Mesh => Select Visible.

 Final Model After Meshing With No Off Elements  ;

Symmetry ;

Now we will replicate all the topo cleanup and surface mesh to the other side using symmetry.To do that follow below steps,
From Transform => select Copy => select same part => select three points along the single cons and do not give any offset value => ok

 

Wind Tunnel ;

We will create wind tunnel using  points, then joining them bycurves and finally creating sufaces using those cons and also surface mesh it as follows.

 

 

 

Volumetric Mesh ;

Volumetric mesh is nothing but 3D elements,while surface mesh is mesh with the help of 2D elements which are tria elements .One important thing to do before volumetric mesh is to make sure that all of our surfaces are meshed.As surface mesh is pre-requisite 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 below steps,
Click on V.Mesh deck => Volumes => Define => Right click and select Auto Detect => Work on whole db => OK.

 

As we can ANSA auto detected 8 volumes.Now we need to figure out required and non required volumes. We will only consider the volumes where fluid may flow and we will try to 3D mesh it and delete the rest of the volumes.Thus will only consider volume between wind tunnel and car,and delete the volumes occupied by tyres. Will keep the volume as shown below.

To do volumetric mesh for the required volume follow below steps,
In unstructured mesh => select Tetra CFD => select the desired volume => middle click.

We have selected Tetra CFD as we want elements for CFD application.Always check type and status after finishing meshing of perticular component or PID.Also make sure not to have any unmeshed volumes.

Results and Outcome ;