DrivAer Car Tutorial
The DrivAer car is available after registering at this URL:
Once the download is complete, this tutorial uses the following parts:
· 01_Body.stp
· 02_Underbody_Detailed.stp
· 03_RearEnd_Fastback.stp
· 04_ExhaustSystem.stp
· 06_Wheels_Rear_Smooth.step
· 05_Wheels_Front_Smooth.step
The CAD Model
Loading the parts listed above will give the following model in Ennova:
CFD Domain
Add a box as CFD Domain.
Note: We will keep the domain small for the purposes of the tutorial, but a CFD domain of 1-2 car lengths upstream and 6-8 downstream would be a normal minimum.
Repair
Set repair tolerances listed below and Run Full Repair:
• Merge: 1.5
• Defeature: 0 (we will not defeature until later)
• Angle: 0.784
Further Manual Repair
Almost the entire repair of the car has occurred automatically. Ennova has largely been able to understand the full topology. Looking at the tree for hints we see the mirrors have not been successfully created as volumes.
On inspection there is a double face where the mirrors attach to the car. Like the exhaust, had we removed these surfaces before the merge they would have been treated correctly by the automatic routines. Sometimes it is easier to simply help the automatic repair ahead of time. Or just delete the surfaces.
Use RMB -> Remove to delete the surfaces.
After deleting, notice that there is a tiny extra surface at the lower front as well. If you miss this Ennova indicates it by displaying it found “single edges “ in the tree.
We do the same for the other mirror, but now that we know there are two surfaces to delete we can be more careful and use Select Attached Manifold to select all the surfaces at once. Often Select Attached Manifold is safer when there are large and small entities together.
Locate Single Edges
The next step is to locate the remaining single edges. For a closed volume that has no baffles we cannot have single edges.
The easiest way to do this is to turn off all nodes, edges, and surfaces and turn on only single edges. Find them in the Display by pressing the home icon. If they are still hard to see, use RMB -> Select All Visible. This will highlight them.
Exhaust Detail
Once selected, we see an issue in the exhaust at the rear of the car.
After inspection we see that on both exhaust pipes there is not an intersection with the car muffler. We need to manually split these surfaces.
The green arrow indicates the surface that needs splitting. Note both the Black and the Red edges need to be used for splitting.
Highlight the correct surface and edge and use RMB -> Split Face at Edge for both exhausts.
The yellow edge is now a closed loop. Rather than look for Red edges (single) we could also have searched the model for open Yellow edges indicating a problem.
Fix both exhausts.
Fig. Rear Exhaust After Fix
There is still one more open yellow loop in the model. It is harder to find, as it is a very small gap. One way to research a model is pick a black edge that is on a manifold that should be able to be displayed easy. For example the inboard end of the exhaust header should be one manifold for just until it crosses the body.
Selecting the manifold highlights the whole exhaust, so this yellow loop is leaking.
On close inspection there are four surfaces intersecting here and one of these did not split. Use RMB -> Split Face at Edge to fix as usual.
The model is now complete. Use the Find Volume command to reveal 17 volumes.
Volume 1 is the CFD Domain we want. Use RMB -> Delete Group and Contents on all other volumes.
Select and Move boundaries to appropriate groups.
The model is now ready for meshing.
Mesh Control
Set maximum sizes:
• Car 12 mm
• Wall 500 mm
• Floor 200 mm
Set Blayer height to 0.1 mm with 15 layers.
Use the Mesh parameter spreadsheet and dropdown box.
Basic Mesh Parameters
Create Density Box around Car and Wake
Defeature Model
Now the model is a valid volume. Set the defeature parameter to 5 mm and run the Defeature command. This will remove any CAD artifact that did not mesh well.
Note: We are only defeaturing the topology of the mesh. The underlying CAD geometry is unaltered.
A final surface mesh x, y, z point always lies exactly on the original CAD geometry.
Surface Mesh – Diagnostics 1
At this stage, try a surface mesh. Ennova will come back reporting that a partial mesh has been created and ask whether you want to load it. Answer No to this, and No to viewing the unmeshed faces. Instead, look in the check mesh and there will be an unhappy face. Click on this and it will take you to the model face that could not mesh.
There is no tutorial for this as it is a sliver surface, extremely thin and hard to view. Track it down by turning everything else off except the smiley face.
Delete the face and there are two red edges on the left close together. Merge those nodes together using Merge Adjacent Edges. Remesh again.
Use the unhappy face and the Home icon to find the next unmeshed face. It is easier to see and is on the exhaust.
As this is in a non-critical location, simply removing the face and using RMB -> Create Face at Single Edges will fix the issue. The face itself is overlapped and not trimmed correctly.
Repeat this procedure again to find one more sliver surface. Then a complete surface mesh can be loaded.
Surface Mesh Diagnostics
After running the mesh diagnostics we find there are some duplicate faces. Use the find small entity tool to find them.
In this case an edge is not meshing/projecting properly. Simply split the edge with a vertex and re-mesh. A valid mesh will result.
Final Volume Mesh with Prisms
At this stage a valid mesh should result. Similar to before mesh diagnostics can be used to verify the quality of the mesh. Using the Ennova button the mesh export dialogue box can be used to export the mesh to the desired format. This concludes the tutorial.
