Irregularly High CD. What went wrong?

Project Support
1

Hi everyone,

I’m working on CFD simulations for STEM Racing CO₂-powered cars and consistently getting an unusually high drag coefficient.

Similar STEM Racing models from other teams typically produce a Cd of around 0.3–0.45, while my models are producing values around 1.0–1.1. My designs are very similar and arguably more aerodynamically efficient, so the difference seems too large to be physically realistic.

I have also tried using ANSYS, but the CAD models continue to show geometry issues and fail during meshing, even after using the available repair and healing tools. I’ve attempted repairs in Fusion 360, Blender, ANSYS, and SimScale, but the problems persist.

Could hidden CAD defects, incorrect frontal area calculation, mesh quality, or the simulation setup be artificially increasing the Cd? I’d really appreciate any advice on how to determine whether the issue is caused by the geometry, mesh, or boundary conditions.

If anyone is willing, I’d also greatly appreciate it if you could take a look at my model, make any geometry repairs you think are necessary within SimScale, and run a simulation to compare the results. It would help me determine whether the issue is with my CAD or my simulation setup.

I can share the CAD model, mesh screenshots, and simulation setup if needed.

Thanks in advance!

3

Hi @nfelice,

Welcome to the community!

A C_d jumping from 0.3 to over 1.0 is a massive leap, but the fact that you are experiencing meshing failures and geometry errors across multiple platforms is a huge clue. Here is a preliminary breakdown of what is likely causing this massive drag spike:

1. The Reference Area (Most Common Suspect)

Before diving into complex CAD issues, double-check your Result Control > Forces and Moments settings. The solver calculates C_d based on the Reference Area you input. For a car, this must be the projected frontal area. If you accidentally left this at the default value (often 1 m^2) or inputted the total surface area, your calculated drag coefficient will be wildly inflated even if the physical forces are correct.

2. The “Blender” Flag (Faceted Geometry)

You mentioned attempting repairs in Blender. Blender works with polygonal meshes (like STL or OBJ files). If you are importing an STL into a CFD solver, the software doesn’t see a smooth, aerodynamic curve; it sees thousands of flat, tiny, faceted faces. This geometry acts like aggressive sandpaper, prematurely tripping the boundary layer into massive turbulence and generating huge amounts of pressure drag. For CFD, it is critical to export a clean mathematical surface model (like a STEP or Parasolid file) directly from a parametric CAD program like SolidWorks or Fusion 360.

3. Internal Flow Leaks (CAD Defects)

If your CAD model has tiny gaps, unstitched surfaces, or hollow internal cavities, the high-pressure air at the front of the car will force its way inside the body. This essentially turns your sleek car into a parachute, destroying the aerodynamics and spiking the drag coefficient.

4. Boundary Conditions

Are your wheels set as rotating walls, and is your ground plane moving at the same speed as your inlet velocity? If the floor is stationary, you are creating a massive, unnatural shear layer underneath the car that doesn’t exist in reality.

To give you a definitive answer, it would be best to look under the hood. Could you please share the link to your SimScale project? (Make sure the project permission is set to Public).

And you can also drop a few screenshots of your mesh (specifically around the wheels and curves) and your boundary condition setup, we can quickly figure out if this is a geometry defect or a simple math error. Happy to help you get this fixed!