Project 1 : CFD Meshing for Tesla Cyber Truck

AIM -

To generate volumetric mesh for Tesla Cyber truck for CFD applications.

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.

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 the Tesla Cyber truck 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 on other side.

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 . 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 use 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 when 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,i.e. our cons resolution in bad.There are many ways to increase cons resolution , like manually resolving each component using smooth tool or we can use length option under perimeter tab 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.

Now here there are two ways we can go,either resolve errors for whole geometry one by one or delete half of the model since it is symmetric,will go with the later one.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 the other side.

First will create cut around the whole truck so that we could select half the model then delete half car model using universal delete.

Now will delete all unnecessary surfaces from inside the car model,as we are only concerned about outside of the car and the area between wind tunnel.Any extra surfaces will create unnecessary volumes utilising memory and time.As shown in the below images 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 where ever 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.

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 Front light => ok.
Simillarly create body, back lights,window,rims and tyres 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 unused 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 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
Front and Back Lights - 2 mm
Rims and Tyres - 2 mm
Window -2 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

Our 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 any minimum length elements to avoide mesh problems.If small elements are present,then use CUT tool and try to resolve it.Also check distance between two nodes and make sure that it fits minimum and maximum length criteria.

This is the important step,we will select minimum and maximumm length criteria in such a way that it includes each and every target lengths for our model and also these has to be the values which satisfies minimum distance between cons and nodes. Thus we will use 0.2 mm as minimum lenght and 5 mm maximum length.

Front Light -

* CLick on Quality Criteria =>minimum length 0.2 mm and max. lenght 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.

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.

Back Light

* CLick on Quality Criteria =>minimum length 0.2 mm and max. lenght 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.

Window -

* CLick on Quality Criteria =>minimum length 0.2 mm and max. lenght 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.

Rims and Tyres -

* CLick on Quality Criteria =>minimum length 0.2 mm and max. lenght 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 0.2 mm and max. lenght 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.

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 by curves and finally creating sufaces using those curves and also surface mesh it.There are some standards which we need to follow while creating wind tunnel,they are as follows,

Front of Wind Tunnel - 4 * Car Length from front surface of the car.
Back of Wind Tunnel - 6 * Car Length from back surface of the car.
Top of Wind Tunnel - 3 * Car Length from top surface of the car.
Bottom of Wind Tunnel- Close to the wheels
Sides of Wind Tunnel - Car Length from side surfaces of the car.

Car Length - 5906 mm
Front of Wind Tunnel - 26150 mm
Back of Wind Tunnel - 38661 mm
Top of Wind Tunnel - 19659 mm
Bottom of Wind Tunnel- 460 mm
Sides of Wind Tunnel - 5906 mm

To create wind tunnel follow the following steps,
First we will create reference point from a hot point at the truck's bottom,
Topo Deck => Points => Relative => Select Node => Select hot point => Enter the lenght -460 mm => ok.

Now using this point and the same steps mentioned above we will create points as shown in below image.

Lets join these points using create option from under the curves tab and then create new surface using these curves.

Simillarly we will create top surface as follows,
Surfaces => Offset => select the above surface => moddle click => keep the original surface.
Now using these surfaces create side,front and back wind tunnel surfaces.

Wind Tunnel Meshing -

As we are concerned about area near the car,thus we will have finer mesh near car and away from it will be coarser.

Mesh deck => Perimeter => Length => Select top 4 cons => Enter 100 mm.If there's any error regarding minimum and max.length,just disable min. and
max. length from quality criteria.

Also,Mesh deck => Perimeter => Length => Select bottom 4 cons => Enter 20 mm.

Now at top we have 100 mm length and at bottom we have 20 mm,as our length is varying from top to bottom,thus our verticle cons should also have varying mesh length.For this we will use spacing option.

Mesh deck => Perimeters => Spacing => Manual => Biasing Geometri => Factor 1.1 => Dstart 20 => Dlimit 100 => Select all 4 verticle cons and make sure that all arrows points from bottom to top => ok.

By doing this we are conforming that distance between two nodes,starting from bottom will go upto 100.We do not have to give quality criteria and mesh parameters as our target length is not constant.It is varying from bottom to top.As our mesh varying from top to bottom,we don't have to give mesh parameters as there is only one target length.

Now for top and bottom surfaces we have to give target lengths.

Isolate bottom surface => In Quality Criteria => Min length 15 mm and max. length 25 mm => Target length 20 mm and select Tria elements => Spotmesh.

Isolate Top surface => In Quality Criteria => Min length 80 mm and max. length 120 mm => Target length 100 mm and select Tria elements => Spotmesh.

Select Front Face => Spot Mesh.

Select Back Face => Spot Mesh.

Isolate Side surfaces => in Mesh Paraneters => select target length as Local => Spotmesh.

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 6 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 the truck,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.

5] Results and Outcome -

* Form 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 criterias.

* By using symmetry we can see that topo cleanup and mesh gets successfully replicated to the other side.

* ANSA successfully detected volume between wind tunnel and the truck model and this will be use to generate volumetric mesh, which can then be used in CFD simulation.

* By using half model we have significantlly reduced the memory usage and also computation time.

6] Conclusion -

* We have successfully performed topology cleanup on the given Tesla Cyber truck model and resolved single cons and removed all overlapped surfaces.
* Successfully generated surface mesh,which is pre requisite for volumetric mesh and created wind tunnel and meshed it.
* Also learned what is volumetric mesh and how to performe volumetric mesh.
* Thus we have successfully generated volumetric mesh for the give Tesla Cyber truck model for CFD applications.

Link for the Model -https://drive.google.com/file/d/1E9YcMH7UAeWLjuckUBuYCnmJWaKzJfvP/view?usp=drivesdk