Week 4 Challenge : CFD Meshing for BMW car

CFD MESHING ON A BMW CAR USING ANSA

(WEEK-4 CHALLENGE)

1. Aim

Our aim is to mesh the volume between the Wind Tunnel and a BMW car in ANSA v22.1.0 using tria element type and suitable given target length.

2. Theory/ Equations/ Formulae Used

ANSA is a computer-aided engineering tool for Finite Element Analysis and CFD Analysis widely used in the automotive industry. It is developed by BETA CAE Systems. The software is distributed worldwide by a number of BETA CAE Systems subsidiaries and business agents. In the United States, it is distributed by Beta CAE Systems, USA, based in Farmington Hills, Michigan.

ANSA maintains the association between CAD geometry and the FE mesh. This means that the FE meshes are better representations of their geometric parents. Also it is easy to maintain and update any changes in the geometry by simply reworking the updated area instead of recreating the FE from scratch. It carries several proprietary algorithms for meshing suitable for both CFD and structural models. ANSA was initially standing for 'Automatic net generation for structural analysis', but the software has gone beyond that very quickly.

3. General Procedure

• Firstly the software’s are downloaded and installed in the
• Geometry is imported in ANSA, firstly the diagnosis is carried out to check for any errors, then the errors are rectified, Property ID’s are defined and necessary surfaces, coons and hot points are created in topology
• Mesh Parameters such as target length, Perimeters and macros length, Quality parameters such as Min and max mesh length are defined before surface mesh is created using various algorithms available such as free mesh, Spot mesh etc. as required in Mesh Check the quality and refine the mesh as required.

4. Preprocessing

• Geometric Model

The 3D geometry file of a BMW Car is imported in ANSA as per the figure given below.

Fig. Wireframe - BMW Car

Fig. 3D Geometry - BMW Car

Meshing Process

• After the model is imported, the draw mode is changed to PID and the available perimeter points, cons are checked.

• As it can be observed that there are lot openings in the model, diagnosis is carried out using checks manager for any errors such as unchecked faces, collapsed cons, triple cons, single cons, needle faces, cracks, overlap faces, [Check > Geometry>Execute]

• As expected there is lot of errors that needs to be rectified before heading for meshing and most of it is single cons and some triple cons as well. Also not all triple cons should be considered as error.

• The openings/ errors are rectified by applying topo, creating new faces in order to convert the single cons into double cons and filled up the cracks as well by various surface type options such as existing surface, planar, fitted, coons and fill hole. [TOPO > faces > new], [TOPO > cons > fill hole].

• Wind Tunnel Volume Meshing is optional. Mirror option can be used if the model is symmetry.
• Cons resolution is improved in order to get the proper curvature of edges at critical locations. [Auxiliaries > Fine]. Also it’s very difficult to use the above mentioned option individually over a complex model, so instead the alternating method is to specify the mesh length. [Mesh > Length].

• As the plan is to work on only exact half of the model. Clear all the single cons and triple cons wisely where not required as single cons will be present at the cut section.
• After the surface mesh is done on the half model, it can be mirrored to get the complete surface meshed model of the BMW Car.
• Do all the necessary clean ups of the surfaces in order to make a single volume of whole model except the tires and rims.
• Once done with all the cleanups such as single cons, duplicates etc., the properties ID (PID) are created in order to identify each surface and named as body, lights, grills, rims and tires, optical rear view mirror, window, silencer and logo accordingly as shown in the figure below. [Properties > New > PSHELL > PSHELL]

• More importantly the unwanted surfaces and parent PID are deleted by checking the surfaces in order to avoid
• Now do cleanup of surface PID-wise in order to accommodate the given minimum length of the cells.
• Once all the errors are rectified and PID is created, now move onto Mesh deck for surface mesh.

• Specify the mesh length in both perimeter and macros as [MESH > Perimeters > Length]
• Define the target mesh length and element type such as quad, tria, mixed etc as [MESH Perimeters]

Fig. Given Target Mesh Length

• Set the min and max mesh length in quality parameters as per the [Quality Perimeters]

• Once everything is set, create the surface mesh for each PID one by one as per it’s given target length by using spot mesh algorithm by selecting individual PID [MESH > Mesh Generations > Spot Mesh]
• Also check the model using entity and hidden option.
• Various options such as reconstruction, move, split, smooth, improve can be used to get rid of off- elements
• Finally, disable the perimeter and double cons option to see the surface mesh clearly for all cases.
• We can see the Surface mesh is not created in all the surfaces as the target length for some connected surfaces are
• So the Length Options such as Perimeter and macro should be defined accordingly so that the transition of mesh happens smoothly. Also made sure that all the surfaces are

• Create surfaces in order for the wind tunnel and surface mesh accordingly.

• Once the surface meshing is done then moved onto the volume
• In Volume mesh deck, volumes comprising the thickness between BMW car surface and wind tunnel surface are isolated from other volumes such as inside volume of the car surface, four tires and deleted. [VOLUME MESH > Volumes > Define]

• Then the flow domain volume is meshed using tetra element [VOLUME MESH > Unstructured Mesh > Tetra CFD]

• As the volume mesh of the domain is computationally expensive, so it’s left as optional.

CONCLUSION

1. The flow domain for a flow over a BMW car in a wind tunnel is created in ANSA v22.1.0 using the Graphical User Interface (GUI) commands and surface mesh is carried out using the given target mesh lengths for each
2. Surface topology is cleaned by rectifying various geometrical errors such as cracks, single cons, etc by closing the holes, creating new surfaces to convert the single cons to double cons and also by deleting certain cons which are very closer to each other in order to avoid any error while meshing.
3. Surface meshing is carried out using tria element type and appropriate given target mesh
4. In each of the part the mesh length is maintained in such a way that there is smooth transition between PID surfaces of different mesh length.
5. Surface Mesh is checked for off elements and rectified those off elements using various options such as reconstruction, move, split, smooth, and improve.
6. It can be seen that there is an increase in number of element with decrease in target mesh
7. It can be observed that different target mesh length for different PID surface is important in order to capture the surface curvature and in turn the flow domain volume especially with better accuracy.