Perform Surface Mesh on BMW M6 model by clearing the Topological errors & Create wind tunnel.

OBJECTIVE

      The objective of this project is to clean up the topological errors in the parts of the BMW M6 model and then perform the surface mesh on the BMW model. Create a wind tunnel for the BMW M6 model to simulate the movement of the air over the vehicle.

PROCEDURE

       The original model of the BMW M6 which contains numerous errors.

Topological Errors

      1. Styling lines - The lines which are used to create the outer frame of the model. Macros under Topo mode is deactivated which is used to hide all the PID. The styling lines are used to create the frame of the car are deleted since it doesn't have any effect on the CFD analysis.

      2. The frame of the car is isolated and topological errors are corrected.

The BMW model is symmetrical about the longitudinal axis, one half of the geometry is deleted.

Different options used:

Paste - Function is used to paste two or more single cons together which at the boundary of different faces.

Release - Function is used to decompose triple or double cons into single cons.

Extend - Function extends faces according to a specified distance or until they reach a target face or target group of faces, by adding new faces as extensions to the source ones. The new faces surfaces are extensions of the surfaces of the sources.

Project - The project is used to project a line onto a surface.

Extrude - Function is used to create a surface by selecting a curve and the path for the curve is provided.

         3. Symbol BMW is not symmetric and also has less effect on the flow analysis so the part is replaced by a circle.

        4. Windows of the model are isolated changed into a single volume by deleting the inside layers of the windows (i.e) flow analysis is done on the outer part of the model so inner parts just increase the meshing elements. The same procedure is repeated for the headlights PID.

       5. Grills at the front of the model has more effect on the flow analysis and has more elements, convert these elements into a single volume element.

       6. Tires part doesn't have many errors, the intersection at the rims and tires are made.

These are topological errors in the BMW M6 model and error in these are corrected. The geometry check is performed to check the crack, needle face overlap face. Since only one half of the model is used the single cons errors cannot be neglected.

                                                Geometry check after corrections

Set PID & Parts

       The BMW M6 model is by default a single part (BMW M6 Ensemble) is separated into different parts like body, tires, grills, windows, lights, exhaust. The separate part is selected using ! not tool and the new part is created by giving an apt name.

                                       Model browser before & after corrections

The PID is not grouped properly, the set PID option is used to create the properties for the entities. The color for the PID is provided. The compress tool is used to delete the extra PID.

                                                   Properties before corrections

                                                   Properties after corrections

Surface Mesh 

      The element length for the different PID is given. Click the mesh parameters icon and choose general and provide values.

Mesh parameters dialog box                                                Element size of various parts

  Click checks --> checks manager --> surface mesh checks to checks for errors in the geometry.

  Geometry unmesh macros - used to check whether all the surface has meshed or not.

  Penetration intersection - used to check whether mesh intersects along its edges.

  Mesh sharp edges - Angle between two mesh (i.e) angle between trias & value of sharp edge is provided.

  Penetration proximities - Check proximities between two distance of the edges.

  Mesh trias on corner - Check trias at the corner of the faces.

  Mesh quality - Check the property of the trias elements various parameters are set.

  Duplicate - If the same space is occupied by a different element.

                                          Surface mesh checks dialog box

CUT - Function cuts an existing Macro Area. Two Hot Points or nodes on the Macro Area perimeter are needed at the positions where it is to be cut. Select remesh macros & enable all options. After all the surface mesh errors were corrected then mesh quality is checked.

 Quality Criteria

Skewness - Irregularity in the trias mesh.

                                       Green element represents skewness

Min length - The length which is less the minimum length set.

Max length -  The length which is greater than the maximum length set.

                               Red element represents min & max element length

Mesh distortion - The change in the actual geometry due to the mesh.

Min & max angle -  The minimum & maximum angle within the mesh.

Tools used to correct the Quality criteria

Reconstruct -Function is a powerful tool for the creation of a high-quality surface mesh. It examines a given mesh and performs re-meshing to correct problematic areas and optimize the overall quality of the mesh according to the mesh parameters. For Geometry Mesh (Macro Areas) all modifications are made on the underlying CAD surface.

MV surf - Function moves a selected mesh node on the Macro Area's surface or a perimeter node along the perimeter segment on which it lies. Also, it moves a node belonging to the FE-model mesh on the surface which is defined by the adjacent shell elements.

Proj cut - Function Cuts an existing Macro Area from one node or Hot Point perpendicular to another selected perimeter.

                                            Topo mode after all corrections

Symmetry

Transform --> copy --> symmetry, choose the entities & select mirror three points. Three points on the symmetry plane are chosen. The same procedure is followed for all the parts, The paste option is used at the intersection of the body to convert single cons to double cons. Lines on the tires are different so the translate option is used to copy it.

                                                 TOPO mode of BMW M6 model

                                                  MESH mode of BMW M6 model

Wind Tunnel

Dimension

Relative - Function defines new 3D-points that lie on a given relative position from a number of selected 3D-points, Hot Points, Weld Points, etc. The input of a (dx, dy, dz) vector or an indication by the mouse is required for the definition of the relative position.

New under curves - function defines smooth 3D-curves by selecting a number of positions. At the moment of position indication, if the Working Planes are visible the left mouse-button defines new positions on the active working plane, while the right-mouse-button picks existing positions to be projected on the active working plane.

Planes - Function can be used to define a planar surface. In order to define a new planar surface, an active working plane has to be specified. This working plane may be defined by selecting an existing working plane or by defining a temporary working plane.

                                                      Wireframe model

                                                         TOPO mode model

We give a top length segment as 100 and the bottom length segment as 20. The spacing tool is used for constant length for the sides. Start the bottom length with 20 and then gradually increase the length to 100 at a constant increase rate. The complete model of the mesh tunnel with the meshed car. The mesh of the tunnel with the meshed car. The mesh of the tunnel is not done as it is a huge model.

                                                           Mesh mode

CONCLUSIONS

     1. The topology of the CAD model is been cleaned and geometrical errors are fixed.

     2. The PID and parts were assigned to the individual entities.

     3. The surface mesh is done with the required element size and off elements errors, were fixed using different topo tools and symmetry option to create the whole model and wind tunnel for the model is created.