SURFACE MESHING OF A BMW M6

OBJECTIVE:

The given CAD model of BMW M6 is to be eliminated from it\'s topological errors & render surface mesh, expel the errors occuring during the wholesome process.

PREREQUISITES:

CAD CLEAN UP & ITS USES:

Meshing for FEA and CFD is simple: Just start with your CAD model, break it up into a bunch of small pieces, and you’re all set to go. Provided, of course, that you don’t mind getting totally bogus analysis results.

It’s more fair to say that the general concepts of meshing are simple, but that the actual practice of creating meshes that give good analysis results requires quite a bit of knowledge and experience. It’s not something you can learn from reading a short magazine article, or watching a webinar. What you can learn from an article are enough of the basic terms and concepts, so that you’ll be able to at least understand what meshing is about.

The first step in meshing is idealization of the CAD geometry. In most cases, this involves simplifying the model, removing details that are not relevant to your analysis, or that are likely to have a marginal impact on the results. It also involves cleaning-up or healing any defects in the CAD model.

Dirty CAD—models that are overcomplicated, or have defects that must be repaired, has long been one of the biggest impediments to using CAE software, particularly early in the design process, where it can do the most good. It used to be common to hear anecdotal claims that as much as 80% of the time spent on a typical CAE simulation was spent preparing and repairing the simulation model. Tools have gotten better, but the time to prepare the simulation model is still highly variable.

Once the simulation model is ready, the next step is to use it to create the mesh. In the case of FEA, the mesh is embedded within, and fitted to, the body of the simulation model. In the case of CFD, the mesh is created within the flow area defined by the body of the simulation model.

The actual mechanics to create the simulation model and the mesh will depend on whether you’re using CAD-integrated CAE software, standalone CAE software with integrated meshing, or a specialist meshing program. With CAD-integrated CAE, there’s likely to be a lot of automation, and only a few options. With standalone CAE, or with a specialist meshing program, you’ll have less automation, and a lot more control.

The end result of the process, no matter the tools used, will be a mesh made up of one or more types of geometric elements.

TYPES OF ELEMENTS:

Mesh elements can be 1D (lines), or 2D (triangles, or quadrilaterals.), but most of the time, when starting with 3D CAD data, the elements will be tetrahedra (tets; 3-sided pyramids), hexahedra (hexes or bricks), polyhedra (with any number of sides), prisms, or pyramids (with 5 sides.)

If a mesh is built from hexes, it can be structured (with all elements connected in a regular addressable pattern.) Meshes composed of hexes that are not connected in a regular pattern, or of tets or polyhedra, are unstructured. It’s possible to have hybrid meshes, with a combination of both structured and unstructured elements.

Historically, hexes have been more accurate than tets, but it’s been easier to create tet-based meshes than hex-based meshes. But, at this point, it seems like no more than a generalization. The choice of element type is more often a function of the capabilities of the meshing software and the requirements of the analysis code.

MAKING A GOOD MESH:

There’s no silver bullet that will assure you of good meshes. There are only some guidelines.

KNOW YOUR PHYSICS: A good mesh must resolve the physics to be studied. It needn’t incorporate any detail in the CAD model that is not relevant to the physics. The meshing software doesn’t know the physics; it’s up to you to know it.

THINGS CAN CHANGE: Even if you spend a lot of time preparing the simulation model, refining the mesh size, and using their meshing software’s validation tools, it doesn’t mean the mesh is right for the physics problem you’re trying to simulate.

Consider, as an example, that you have good mesh for an airfoil, and it captures the flow and forces accurately. If you change the angle of attack from 0 to 45°, it’s pretty likely that you’re mesh is not still going to be good.

When you change boundary conditions, loads, or analysis type, what was a good mesh is likely to become a bad mesh. A good mesh is tuned to the needs of your problem.

TYPES OF MESHES:

HEXES ARE NOT ALWAYS BETTER THAN TETS:

It’s common wisdom that a hex mesh is better than a tet mesh. But it’s not always true anymore.

Historically, people have preferred hex meshes because, at one time, most CFD codes could only use structured meshes (or their unstructured mesh support was immature.) Also, a hex mesh uses fewer elements, and so saves memory and CPU time.

Solvers have gotten better. The accuracy difference between hex and tet meshes is not significant for most engineering problems. And any savings in resources you might gain by using a hex mesh is offset by the time saved in creating a tet mesh.

Certainly, for some applications, hex meshes have advantages (for example, where they can be aligned with flow direction.) And there are some cases where it’s standard practice to use a hybrid mesh, using structured hexes with a tet boundary layer. But there are also plenty of applications where it’s just fine to use tets.

AUTOMATIC MESHING SOFTWARES CAN DO A BETTER JOB:

Advanced meshing tools provide a high level of control, and can be very powerful—in the right hands.

If you’re an inexperienced or average user, you’re likely to get better results with automatic meshing software (assuming it’s good quality automatic meshing software) than you would trying to fiddle with all the controls on an advanced meshing program. Of course, if you’re a power user, you’re probably going to want to use advanced meshing tools, to squeeze every bit of accuracy you can out of your simulations.

GOOD MESHES DON\'T NEED TO BE LARGE:

Just because you have a big computer, or a cluster, doesn’t mean you should be making meshes with massively large element counts. You need match the mesh to the physics of the system you are simulating. It only needs to be of good enough resolution to provide as accurate a result as you need for your project requirements.

Many models have symmetry. In these cases, you can often use ½ or ¼ of the full 3D CAD model, and get faster and better results. If the problem is axis-symmetric, you can get more accurate results with a 2D simulation than with 3D.

VIEW CONTROL:

CTRL + LEFT MOUSE BUTTON: It is used to rotate the surfacein 360 degrees with respect to you mouse movement. Selections of any entities, PIDs and surfaces, CONS, edges, points and every other important selections.

CTRL + RIGHT MOUSE BUTTON: Angle of alighnment with straightness as the only main feature of this view control. Deselecting entities, PID regions and delete the lines while using cut options.

CTRL + SCROLL:To zoom in and out, move the CAD model on a whole inside the workspace.

TERMINOLOGIES IN THE TOPOLOGICAL MODE:

BRIEF PROCESS INVOLVED:

1. BREAK UP OF CAD

2. GEOMETRY & TOPOLOGICAL ERROR IDENTIFICATIONS

3. PATCHING HOLES, OPEN SURFACES, UNEVEN GEOMETRIES

4. ELIMINATING TRIPLE CONS, SINGLE CONS

5. INTEGRATION OF CONTROL VOLUMES

6. DELETING THE OTHER HALF OF THE CAD TO MIRROR THE SYMETRICAL & CLEANED UP GEOMETRY

7. ASSIGNING MESH SIZES FOR INDIVIDUAL PIDs

8. MESH PARAMETERS

9. QUALITY CRITERIAS

10. CHECKS MANAGER - NEEDLE FACES, OVERLAPPING SURFACES  SINGLE CONS, DOUBLE CONS, TRIPLE CONS, PENETRATION                                    INTERSECTIONS, TRIAS ON CORNER.

11. FREE MESHING

12. MESH INCONSISTENCIES

13. MINIMUM LENGTH, MAXIMUN LENGTH, SKEWED SURFACES, ANGLES.

14. ELIMINATING THE ELEMENTS THAT ARE OFF THE PARAMETERS

15. PERFORMING SYMMETRY

16. CREATING NODES RELATIVE TO THE NODES ON THE MODEL

17. RENDERING PLANES FOR THE TUNNEL SET UP

18. INTERSECTING THE SURFACES FOR CLEAN SURFACES

19. RESIZING THE MESH OF THE TUNNEL FOR REAL TIME SIMULATION PARAMETERS TO EXTRACT FIGURES FOR AERODYNAMICS AND                          AIR FLOW BEHAVIOUR AROUND THE MODEL.

20. MAKING UP YOUR MIND TO PATIENTLY HANDLE THE STRESS & CHALLENGES WHILE ELIMINATING THE MESHING ERRORS                                        ACCORDING THE SET PARAMETERS.

TOOLS USED FOR RENDERING & ERADICATING ERRORS TO OBTAIN CLEAN GEOMETRY:

HOT POINTS:

The main tools used under this feature are INSERT & DELETE. These are primarily used for inserting a radom hot point on a con or a surface in order to perform various functions like cutting a con joining a broken con or a urface to patch it up.

CONS:

The tools under this window mainly deal with cons that are independant and that are in between two intersecting surfaces. The gears that are mainly put to use from this toggle are PASTE, PROJECT & RELEASE.

You can perfom pasting of two single cons and obtain a double con, which is what is desirable in the field of FEA; project a con on to a surface to intersect the con with the surface; ocassionally when two surfaces or a con and a surface are intersected there arises a triple con which can be released and pasted again to eradicate the triple con.

 

FACES:

The major part of the CAD clean up is being contributed with the tools offered by this window. They offer major gears such as NEW, CUT, DELETE, SET PIDs, TOPO, ZONE CUT, PROJECT CUT & EXTEND.

The NEW tool enables us to create a new face by various options based on coons, fitted, planarand surfaces. Using the entity button and selecting any kind of entity using left mouse button or by box selection.

Any face can be CUT with the many optional functions can be performed using this tool on a face, that has improper geometry, cutting a surface with respect to the hot point on a con or a hot point that is inserted on to a surface.

DELETE can be used to eliminate & delete part specific faces rather than using the UNIVERSAL DELETE on the UTILITIES bar.

Assigning part specific PIDs to the newly created surface or of the CAD model for better manouver in the PROPERTY LIST and the PARTS MANAGER.

Rather than choosing cons by clicking on them individually, TOPO enables us to select them using box selection, by dragging onto the PID or the ENTITY where single cons are present to paste the mand convert it to double cons.

ZONE CUT & PROJECT CUT are used in a minor varying conditons where cutting a face from one hot point to its projection on a face or con, to cutting by selecting a path of cons to be offset along selected facinf and then performing the cutting in that zone.

PLANE CUT is performed by picking two or three points to define the cutting plane and then cutting it.

EXTENDing a face to connect onto another face that is perpendicularly near.

SURFACES: 

The repair of the CAD can be carried out with these two mechanisms under the surfaces window.

Creating a COONS face from the selected cons or face & creating surfaces by extruding selected cons or 3D curves.

TOOLS USED DURING MESHING & ERADICATING THE FAULTY OFFSETS IN THE MESH PARAMETERS:

PERIMETERS:

The mesh generation limits with respect to the sie of the mesh encompassing each element is been assigned by this LENGTH tool, with respect to the cons or the macro area, whichever the problem demands.

 The SPACING gear while developing a progressive mesh, the nodal lengths can be controlled along the perimeter of the mesh with dmin & dmax numbers.

MACROS:

For the major part of the mesh repair, the CUT tools is a dynamic usage tool that which cuts the tria elements with effecient macro area cut and heals it. ou can also delete an element perimeter with the right click when this tool is engaged.

GRIDS:

MV SURF or the MOVE SURFACE gizmo is an effecient tool to move the single mesh cell or a macro area along the perimeter of that particular mesh cell with the nodes connected by the perimeter. This tool releases and heals the skewness and min length errors in the mesh parameters by moving the mesh element with respect to the angle, min length & max length.

MESH GENERATION:

The tool under this window called the FREE mesh generation uses a robust algorithm to generate mesh on the visible entities, the selected surfaces and renerating the mesh that is lost.

SHELL MESH:

The mesh RECONSTRUCT tool helps us to reconstruct the mesh lost during the healing process of the mesh parameter error.

ELEMENTS:

SPLIT gear under this window helps us to split a shell element into two different shells by selecting the nodes or the edges. 

An element can be swapped with the element that ehich is connected to an element by selecting the common edge that is shared by the elements.

 

RESULTS:

The resulting differences with the CAD model is vastly clean & geometrically sound.

fig 1. (single cons, triple cons & topological errors)

BEFORE

AFTER

fig 2. (eliminating overlapping surfaces, volume elements inside the car, repairing the uneven geometry eligible for error free mesh generation)

BEFORE

AFTER

fig 3. (segregating the PIDs)

CHASSIS

LIGHTS & GRILLS

WINDSHIELD & MIRRORS

fig 4. (checks manager)

GEOMETRICAL ERROR REDUCTION

fig 5. (mesh generation)

BEFORE MESH GENERATION

MESH MODE AFTER SMMETRY

SNAPSHOTS

ONE OF THE MOST PROMINENT CHALLENGES IN THIS PROJECT IS THIS SIDE VENT CORRECTION & CLEAN UP;

BEFORE AND AFTER GEOMETRY CLEAN UP OF CHASSIS UNDERBODY

CAD WITH MISSING PARTS OF THE TYRE;

 

THE CENTRALITY OF SURFACE MESH & ITS APPLICATIONS:

The surface mesh is widely used in the computation of aerodynamic behaviour and effeciency of the car. In contrast to the volume mesh, this method focuses on the volume outside the car and its characteristics when interacting with the surface of the car and its effects on it.

Hence to create a real time simulation in computing the air flow around the car ina closed control volume, a wind tunnel is created and in it the simulations are run to obtain virtual results.

WIND TUNNEL:

The wind tunnel is built around the car using the lengths of the car, meshed accordingly with respect to the intensity of the volume interacting eith the surfsce of the car, reducing the size of the mesh and making it fine in the bottom part of the wind tunneland gradually increasing the size and spacing of the mesh parameters on to the top.

 

CONCLUSION:

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

Rendering hollow surfaces on the inside with completely covered and closed surfaces on the outside to compute the flow characteristics on the outside of the car alone.

Mesh is generated according to the given parameters and sizes, the errors eliminated and off elements fixed.

Refining the mesh, assigning individual PIDs to ease of access.