24A Full Car Sim Notes

Owned by Michael Chang
Last updated: Oct 30, 2024
4 min read

This document contains all of the notable actions and helpful tips that brought to fruition the first full car CFD simulation in ANSYS.

Geometry (Solidworks)

We first create a CFD specific assembly that excludes irrelevant details/parts. The important parts to include are:

  • Chassis

  • Bodywork

  • Aero parts

  • Major suspension members

  • Wheels

  • Simplified rear end components (diff, motor, etc.)

image-20241023-041900.png

Chassis tubes are hollow. As such, when we create a surface and volume mesh, the interiors are meshed as well, even though they do not affect the simulation. This leads to greater simulation complexity and cost without any gain in accuracy. To keep this from happening, a custom structural member profile was created (in the weldment profiles->simulation folder in PDM labeled CFD_1OD or smth like that). This way, we only have to edit the structural member feature that is used to create the chassis members instead of modeling every one from scratch.

During meshing and body combine commands, both Solidworks and ANSYS have an issue with some trim/extend features creating zero-thickness geometry. Therefore, the trim/extend features were simply suppressed. This leads to some janky looking joints in the model, but Solidworks and ANSYS both appear to be markedly happier with this.

The trailing edges of the wings were given a 1/8” blunt face for easier meshing.

image-20241023-042426.png

In addition:

  • the wheels were given a fillet at the contact patch; bodywork was created as a solid body

  • a very basic driver model was included

  • major powertrain and suspension components were represented with very simplified geometries.

 

The assembly was then saved as a part file. Every connected body was then combined. The end result were four bodies:

  1. Body, chassis, driver, suspension, and wheels

  2. Powertrain components, which were just floating in the rear box

  3. Front wing

  4. Rear wing

Not sure combining all of the parts was necessary- it was only done out of sheer desperation since Solidworks was complaining of a zero-thickness error when we tried to combine the bodies, and ANSYS gave an error that was scoped to the same regions. Since removing the trim/extend features fixed this, it might be possible that we don’t have to combine the bodies in solidworks in the future.

For those confused, the troubleshooting sequence for the trim/extends looked something like this:

  • Car part file included like 800 bodies. Enclosure failed to include a few chassis members.

  • Went back to solidworks and tried to combine as many bodies as possible within the part file. Solidworks gave zero thickness geometry error on the same chassis members ANSYS was having trouble with

  • [insert a lot of desperate troubleshooting here]

  • Figure out to remove the trim/extends, then can combine into one body in part file and mesh properly in ANSYS

The part file was then exported as a Parasolid file.

Geometry (ANSYS Spaceclaim)

The Parasolid was then imported into ANSYS Spaceclaim for meshing preparation.

Make sure that the enclosure includes all of the features by visually checking (make sure to hide all the imported bodies so you only see the enclosure) and confirming no errors. Due to some trim/extend feature weirdness in the chassis, ANSYS did not throw a critical error but instead gave a warning and excluded the body from the enclosure subtraction.

Each time you reimport the model into Spaceclaim, double check coordinates for rotating components like the wheels. For some reason, they changed- and so when the xyz coordinates are not updated, the wheel boundary condition will be very fucked.

Meshing

Meshing did not take long. In fact, it was remarkably faster than our previous attempts, which was a very good indication that our model quality was sound. The surface mesh finished in >10 minutes, and after an “Improve Surface Mesh” task, had a quality value of 0.7. The volume mesh finished in <15 minutes, and after an “Improve Volume Mesh” task, had a quality value of 0.14[something something something].

 

Solver

The greatest issue with running the Fluent solver on a personal laptop or computer is that it gobbles up any available RAM and pagefile space very quickly. For 8 parallel cores, it took about 70Gb of RAM and pagefiles combined.

The most common solution is to increase pagefile space to provide more memory. If that doesn’t work, decreasing the number of parallel cores used by the solver will also decrease the required memory.