Negative drag coefficient value Near Zero

Hi, Im simulating a Small RC Aircraft for Aerodesign to obtain significant values such as Cl, Cd and Cm. Everything is fine except for one simulation in the negative angles of attack (specifically Alpha -12) where the drag coefficient reaches a negative value. Everything else seems fine. Already checked the usual settings that could be wrong and lead to a wrong value. My velocity vector is correctly orientated in both initial and boundary conditions and my reference vectors for lift and drag are OK too.

Link to the proyect: https://www.simscale.com/projects/rrothe/txv-m/

Hello,

The velocity & drag/lift coefficient orientations indeed look correct. I’d still not expect a negative drag coefficient, as you have pointed out.

From the results, it looks like the x component of the drag force is in the positive x direction (which is expected), however the forces in the positive y direction are too large, and this is causing the resultant force in the drag direction to be negative (which is not expected).

A few questions:

  • I don’t think this is the cause of the issue, but do you happen to have a cleaner model, with a single face for the airfoil profiles? In case you are using Onshape, this profile generator script is very handy.

Cheers

Hi, I do have a cleaner model but for some unknown reason gave a lot of issues at the end of wing airfoil, (most of the simulations done diverged in various points of the wings around this location) Even rebuild the model around 4 times and still happened so I decided to use this approach that gave me no issues in past simulations. I would try to run it again with potential flow initialization and see if gives me a better result. Thanks for the help !