Getting negative drag coefficients on my incompressible runs

Project Support
1

I’ve been testing a few shapes and keep getting negative drag coefficients which can’t be right. not sure why this is happening or how to fix it. Can you help?

This is one of my runs (Incompressible 3)

2

Hello, and thanks for posting your question on the Forum!

Seems like the project is private - could you please share your project by making it public so other community users can access it as well?

Thanks!
Kaan

3

So sorry :sweat_smile: just did it

4

No problem at all! :slight_smile:

I noticed that the drag coefficient is defined in x direction, while it should be defined in negative y direction. I believe this is the main cause of the unexpected values.

5

Ok so I changed the drag direction to dy = -1, but now my drag coefficient is way to high :sob:. What did I do wrong?

6

Hi, make sure to have a look at this page, more specifically the pointer related to poorly estimated reference area/velocity.

Cheers

7

I read through that. it sounds like my reference length and area is off. How can I fix that?

8

The page that I linked contains instructions on that, still in the troubleshooting section.

Cheers

9

I looked through the article and I am still unsure of how to fix my mistakes. How do I find the correct reference area for my shape?

10

There are some workflows available, but they usually require a CAD tool. Project/sketch the outline of the vehicle on a plane and measure its area. E.g.: Aerodynamic Flow Behavior Around a Vehicle Tutorial | SimScale

11

I use Fusion 360 to make my models. Is there a way to get the numbers I need from there?