Week 4 Challenge : CFD Meshing for BMW car

AIM : To perform CFD Meshing for BMW car in ANSA.

OBJECTIVES :

• Loading up the Geometry in ANSA.
• To do the Geometry Preparation using TOPO Tools in ANSA
• To perform surface meshing Geometry for the CFD.
• With different given mesh target lengths for the PIDs
• Target lengths for the different parts of a model are as follow : - Body - 3 mm, Lights - 4 mm, Grills - 1 mm, Rims and Tires - 2 mm, Optical Rear view mirror - 2 mm, Window - 2 mm, Silencer - 3 mm, Logo - 1 mm
• Create Respective domain for the CFD analysis
• Then generate Volume mesh for the CFD.

INTRODUCTION :

ANSA is a computer-aided engineering tool Used for Pre-Processing for finite element analysis and computational fluid dynamics analysis widely used in the automotive industry. It is developed by BETA CAE Systems. It is used to Prepare Geometry and do meshing before doing the Computational Analysis.

STEPS / PROCEDURE :

1) Loading Geometry

The Following is the geometry given -

Selecting the PID view -

2) Geometry Cleanup -

Before proceeding to meshing we will improve the geometry using tools in TOPO deck.

The first thing to do is to check the single cons and triple cons in the model. These are identified using geometric check tool.

As Shown below we got this much singles cons after perfoeming the the gemortric check.

We can see from the Topo check and following image, there are many Single and triple cons.

Also, we can see from the following image we can see the surfaces are looking very jagged and sharp edges.

We can apply length perimeters and macros from mesh section -

Now we can the surfaces look much nicer.

We the car is symmetric therefore we can delete one half of the car, which can be recreated later.

Deleting Overlapped unnecessary surfaces. Also, for the separated cons as shown below using Paste option.

Following we can see the surfaces at the Grille. Deleting the surfaces which are conflicting. We need to create a closed watertight geometry of surface to have just single volume.

Resolving Single and triple cons. and Surfaces around the Grilles.

Following you can see the completed lower grille surfaces.

Now solving the same for the upper grille. Using to delete and create new surfaces where needed.

Following we can see the double cons i.e. all single and triple cons are resolved.

Now we go for the surfaces logo as shown below.

We can use intersect tool in order to solve overlapped and enlarged surfaces. Also, we will delete and create surfaces such that it will have one closed volume.

After completing, the surfaces of the logo, we move forward to the Wind shield light's glass. We can see the triple cons and Redundant surfaces. We can easily resolve by deleting the respective surfaces.

After doing that, we can proceed to other surfaces. The Following are the interfering surfaces.

We can use the intersect tool from the TOPO in order to cut the extra part of the surfaces and deleting those surfaces. And the other surfaces can be resolved by using cutting and surfaces tool.

Above you can see the perfected surfaces of the body. Now in the following we have the rearview mirror.

We can delete the overlap surfaces. and also deleting and recreate surfaces to avoid multiple volumes.

Now going to the Wheels of the car.

Here we perform the similar tools in order to solve triple and single cons. now since the wheel surfaces are not directly connected to the car body, therefore we while create a single closed volume of a wheel and its rim.

Now we come to the main car body surfaces. Similarly, like above we have to check for the single cons, triple cons, interfering surfaces.

We can see from the following images how the surfaces are extending and interfering with the wheels of the car.

we will cut and delete the extended surfaces and create new surfaces for the wheel fender by taking the reference of the correct surfaces.

Same way we will rectify the surfaces of the rear fender. we can use intersect tool of the from the topo deck and delete irregular and unwanted surfaces.

For the places where cons are irregular or missing, there we can use the curves tool in order to create reference for generating the respective surfaces.

same way we will do the surface cleanup for the exhaust.

After doing the topo cleanup for all the pid surfaces, we will perform a geometry check. As you can see below the only single cons remaining are the ones, which will get attached to the other symmetrical half of the car body.

Hence all major geometry cleanup is done.

Following is the complete geometry looks like after geometry cleanup-

After completing with the topo cleanup, we can assign the PID to the set of surfaces which designated part of the components

After Completing with the Topo cleanup and PID assignment we move towards the Meshing.

3) Meshing -

Before proceeding to meshing we can also improve the surface geometry for getting a good optimal mesh. We can see from the following images there are few areas where the cons/edges are too near and forming very narrow width surface, in the those areas we can remove cons by selecting cut tool from topo and right click on that edge. This way we can reduce the very small size mesh elements there and can have an uniform mesh over the surface as the target length.

from the following you can see, from cut tool how we can create changes in the geometry so we can have more optimal mesh.

similarly, we can remove cons in other areas where the cons are very close and creating very small mesh elements.

From the Classic Mesh deck, we can use the length tool in order to assigning target mesh length size to the geometry.

Following are the mesh lengths given for the different parts of the turbocharger -

• Body - 3 mm
• Lights - 4 mm
• Grills - 1 mm
• Rims and tires = 2 mm
• rearview mirror = 2 mm
• window - 2 mm
• silencer - 3 mm
• logo - 1 mm

Before proceeding to assigning length to cons and macros, we can manually measure the min. distance for each and every PID.

Now from the measurement have got an idea a max. and min. range of mesh length for every PID, and Target length we already have.(The target length will behave as the average length which will be between the min. and max. length)

For doing the meshing ,

First we assign the meshing criteria and then assign the mesh parameters.

Starting with the PID- Logo

In Length tool, we assigned 1 mm size to all faces in both parameters and macros.

With min and max length criteria be 0.4 and 2 units respectively.

In the Mesh Parameters Section, we will assign 1 mm to the target length.

And setting the Mesh element to Triangle elements.

Then, we will proceed to generate mesh using Spot meshing tool.

After generating spot mesh we will click on the hidden option in toolbar below. we will be able to the see the mesh quality and no. of off elements.

Now after doing spot mesh, we can now perform a Reconstructing tool on the surface from the classic mesh deck. After applying that you can see there are no off elements.

For PID - Grills

Target length = 1 mm

Min. size = 0.4 mm

Max. size = 2 mm

For PID - Windows

Target length = 2 mm

Min. size = 1 mm

Max. size = 3 mm

For PID - Lights

Target length = 3 mm

Min. size = 4 mm

Max. size = 5 mm

Even after applying reconstruct operation we can see from the following image some off elements aren't resolved. Now from the Grid move tool from the mesh deck we can move the nodes in order to resolves the off elements and satisfy mesh criteria.

For PID - Rims and Tires

Target length = 2 mm

Min. size = 1 mm

Max. size = 3 mm

For PID - silencer

Target length = 3 mm, Min. size = 2 mm, Max. size = 4 mm

For PID - Rearview Mirror

Target length = 2 mm, Min. size = 1 mm, Max. size = 3 mm

For PID - Body

Target length = 3 mm,

Min. size = 1 mm, Max. size = 5 mm

After, doing the Surface/shell meshing on all the PIDs and doing the resolving the off elements of the mesh. We can check the Mesh of all surfaces together and see the total mesh count.

We can see that the mesh length of the element ranges from 0.3 mm to 5mm. And the total no. elements after surface meshing are 542297.

Now as we can see we have created the perfect shell mesh for the one symmetric half of the car.

The Next part is to create other symmetric half of the car. We can do this by using Copy entities tool, in that there is Symmetry .

From the Single cons on the center of the car we can select 3 points, and create symmetry by choosing the option of Mirror points plane. Which will create mirror copy of the selected surfaces, and select no-offset option to create surface just over the cons.

you can see red single cons, now we can use topo and paste tools in order to solve these. After this we can perform a geometric check.

As we can see there are no single cons and triple cons remaining in the check.

After completing the surface mesh, we can go to volume mesh deck for proceeding volume meshing.

Before creating the Domain and volume mesh, we will measure the length of the car from the front to the back.

First we will create respective reference points around the car, and creating 8 corners of the Wind tunnel domain we need to create.

The wind tunnel is designed with respect to the dimensions of the model. If the model has ‘D’ mm length, then the front face of tunnel must be at ‘4D’ mm distance, the rear face must be at ‘6D’ mm distance and the top face must be at ‘3D’ mm distance from the top point, the bottom face must be below the model with a considerable gap and in the top view, the side faces of the tunnel must be ‘D’ mm on either side.

Select ‘relative’ option in ‘points’ menu and select a point at the base and in Y-axis set an arbitrary distance of -75 mm.

Using these reference points we will create the Curves.

Now from these curves we will create surfaces of the wind tunnel domains, top, bottom, front, back and sides.

After creating the domain surfaces and assigning them PID, we will proceed to the shell/surface meshing of the these domains.

For surface meshing we will assign the lengths to the surfaces. The length will depend on how we want to generate volume mesh cells. For the Current case of BMW car simulation, the mesh cells near the track(Bottom surface) will be Finer than the mesh cells on the top face. Therefore we will assign mesh lengths to be -

For Top face = 100 mm

For Bottom Face = 20 mm

After Doing that we will create the gradient size of mesh lengths for the volume mesh, by creating and applying the gradient mesh length on the four edges between top and bottom face. So the mesh length will be increased 20mm to 100mm with factor of 1.1 mm from bottom to Top face.

Now Moving to the Volume Meshing

By define option we can check/scan for the volumes from the surfaced mesh geometry.

Following are the detected volumes

Now We will delete the Solid volumes which are the body of the Car and just keep the fluid volumes.

After deleting solid volumes. From the properties tab we can see the mesh status.

By clicking over the unmeshed tab, we can see different meshing option and we gonna use Tetra rapid option for optimal meshing.

After that we have to just click and generate the volume mesh.

Note: The volume mesh was not generated because of the high computational requirement and log computation time.

After generating Volume Mesh, the process is the same we need to check the mesh element quality of the Volume mesh.

CONCLUSION :

We have successfully created the mesh for the CFD of the BMW Car.

In this project we have used different new tools in the ANSA in order to generate the volume mesh.

We have seen different tactics and ways for the surface correction and geometry optimizations in order for the optimum meshing.

Used tools like symmetry copy, points, curves etc. for creation of the surfaces.

We have seen reconstruct, grid move, etc. tools in order to improve the off quality elements.