Negative Drag Coefficient for two out of three (plus other weird data)

Hi!

I am trying to test different modeled sedans in a compressible simulation (the sedan model is quite rough, but the basic shape is the most important) and see how for example a dimpled roof or back will affect the different coefficinet, but my focus is on the drag coefficient.

For the back-dimpled on I think I get quite a reasonable graph, with a drag coefficient of around 0.2, but for the other two, the normal sedan and the roof dimpled sedan, the drag coefficient is negative, but still around 0.3. And the other forces and coefficients, like the lift coefficient is around 40 on the roof dimpled, 1.3 on the normal and 0.06 on the back dimpled. Is there something that I am missing, a minus sign where it shouldn’t be or something else? Woud gladly accept any feedback or explanation. :slight_smile:

The links (focus is on the latest simulations, 29 m/s with good area):
Back dimpled: SimScale Login

Roof Dimpled: SimScale Login

Normal: SimScale Login

Hey there!

Are you aware of these articles?

Please let me know if it helps or if you have further questions.

Hi @ggiraldof!

Sorry for the late reply. I didn’t know about the second article, so I thank you very much for it, yet even when looking through the article and checking I can’t rap my head around why give so different values, even if all of them converge, have the same reference lenght and area and are only slightly modified, and my lift, drag and pitch is set correctly, for all three of the simulations. There might just be something very easy that I am missing, but because of my lack of experience I don’t notice it. I would very thankful if you could maybe check the three simulations and maybe notice a discrepency that I didn’t.

Thanks again for the articles!