Geometry cleanup and surface meshing of a BMW M6 model in ANSA

Objective:

      The goal is to clean the surface geometry to avoid errors and create a mesh that accurately captures the input domain geometry(A BMW M6 model) with high-quality  cells so as to make subsequent calculations intractable.

      The above said goal is achieved by using the most widely used Pre - processing software named ANSA.

What is ANSA?

      ANSA is an advanced multidisciplinary CAE pre-processing tool that provides all the necessary functionality for full-model build up, from CAD data to ready-to-run solver input file, in a single integrated environment.

      ANSA is the user\'s preference due to its wide range of features and tools that meet their needs. The list of productive and versatile features is long and the alternative tasks and processes to be completed using them are countless.

Mesh generation:

        Mesh generation is the practice of creating a mesh, a subdivision of a continuous geometric space into discrete geometric and topological cells. Often these cells form a simplicial complex. Usually the cells partition the geometric input domain. Mesh cells are used as discrete local approximations of the larger domain.

Purpose of meshing:

        Meshes are used for rendering to a computer screen and for physical simulation such as finite element analysis or Computational fluid dynamics.

        Finite element calculations cannot be done directly on solid parts. We know to perform those calculations only on points created by the mesh generation process. Thus generating mesh is a must and most important step for any CFD or FEA calculations to be performed.

Steps involved in geometry clean-up by ANSA:

1.The solid part made using any cad modeling software is imported into ANSA. The Procedure is File>Open and select the required solid part.

2.The solid part is initially imported as a wireframe model in ANSA and it is changed into a solid model by clicking on the surface icon as shown below.

3.The geometry is checked using the checks button and selecting the geometry checks option as shown

4. The geometry check dialog box displays various errors in the geometry.

5.You can highlight any of the problematic areas by left-clicking on it, or isolate it on the screen by right-clicking on it and selecting “Show only”. Depending on the type of the problem ANSA also offers the option to automatically fix it.

6.Close the Checks Manager window. Click “All” from the focus toolbar to make all the faces visible again.

7.De-activate “Shadow” mode and the visibility flag button for “Double” CONS. You will now isolate a region of your part to work on.Activate the “Or” function located in the Focus Toolbar. Make a box selection of the area shown on the left using the left mouse button. The “Or” function leaves visible only the selected Faces. (If you accidentally select wrong Faces, press the All function of the same Group to bring all the entities back to visible).

8. Activate the “Lock” flag button in the Focus Toolbar to lock the currently visible entities. Now when you press the “All” button, only the selected and locked entities will become visible.

9. To view the complete Faces that you have left visible, activate again the “Double” CONS visibility flag. Activate also the “Cross Hatches” visibility flag. This allows the visualization of two green dashed lines along the center isoparametrics of the Faces. Apart from visualization purposes, the crosshatches are also used for selecting Faces.

10. After visualizing the surface to be cleaned use various topo corection tools provided in topo mode.Some of the visualized errors are shown below

11. The topo mode provides following menus to perform geometry cleanup

TOPO menu

In this menu the user can create, modify and clean up the geometry. The menu is divided in groups of functions according to the entities that they refer to.

Hot Points

These functions are used to create or delete Hot Points or Weld Spots.

CONS

Functions that are applied on CONS (Curves ON Surfaces).

Faces

Functions to create or modify Faces.

Surfaces

Functions that create Faces and modify Surfaces.

Curves

Functions to create 3D Curves.

Points

Functions to create 3D Points.

Auxiliaries

Functions to create Working Planes in order to draw in 2D mode.

12. After performng cleanup the geometry is again checked for error using geometry checks.

Steps involved in setting property ID\'s in ANSA:

1. Next step is to assign property id to the solid part. It is very similar to that of assigning boundaries. In order to assign property to the solid part first we have to change from entity mode to the PID mode

2. After Changing into PID mode go to utilities>quality>presentation parameters and click on view cons as in entity mode button to view the cons  and also by clicking on cross hatches icon the cross hatches are visible.

3. To assign property id to the part click on set pid option under the faces menu in the TOPO module window and select by faces option in the next window.

4. Select each regions and click the middle mouse button and create a new property.The selected regions can be assigned to a particular property ID by double clicking on it.

5. The geometry after assigning the property ID looks as like below.

Steps involved in mesh generation by ANSA:

1.Now, to generate the mesh, change from TOPO mode to MESH mode. In Mesh Generation select Spot Mesh and define the element type as tria.

2. After this in Mesh module, under Perimeter, in length option, the size of the mesh is given after selecting the edges. here different mesh sizes are generated for different PID\'s.

3.  Now, Individual pid\'s are selected and assigned the mesh length as shown above.

4. After meshing the surface mesh checks is performed as shown below.

5. The above seen errors are rectified by performing surface checks where we rectify the errors for the model by cut option (in case for the rims), delete the surfaces in case of sharp edges and change parameters for intersection pid.

6. After that we define mesh cleaning parameters i.e.skewness,warping,min & max length, min & max ang.

7.In order to clean the geometry , we use the reconstruction feature, for some elements we MV free or paste , we rectify some elements by using the cut option.

8.Thus by this all the errors for mesh parameters are cleaned.

9.We create the symmetry of the half of the body by using 3 points.

Wind tunnel creation:

1. After the surface mesh is performed a wind tunnel is created around the BMW model in order to perform CFD flow analysis over the car body.

2. The wind tunnel of following dimension is created around the model.

3. To create the wind tunnel we use the relative to create points and then create the curves .

4. To create the plane , we must points at x,y,z co-ordinates so we use the same relative to create points and then create the plane.

Conclusions:

1. All the topological errors in the geometry have been cleaned.
2. Surface mesh & wind tunnel have been created successfully.