Force plot deviation

Hello!

My project is to find the sources of aerodynamic drag of an automotive trailer, and improve its efficiency. I would like to do CFD simulations of different models (each model has some kind of improvement), and later on compare the results, to find the most efficient solution.

I have run into several problems, that I have had solved with other forum posts, but now I cant get my head around plot deviation problem. As seen on the force plot below, the forces and moments are not diverging, instead they are very swirly. I have changed some settings, simplyfied the model, enlarged the flow volume and so on. My model is 3 times smaller than the actual thing, but to get the forces correct I increased the air density 3 times.

The latest is Run 12
https://www.simscale.com/workbench/?pid=2898658840671785296&rru=7f00b70c-48fb-4d16-bb54-2998927f9cc7&ci=d0ac4b0e-a8a0-4ce3-bbc6-5ab6a80feb30&ct=PLOT&mt=SIMULATION_RESULT

What changes should I make, to get the right aerodynamic drag force for my model?

Dear @KasparKaasan

It is not appropriate to “increase the air density 3 times”, but to calculate a drag coefficient in terms of a characteristic length, and compare them.

Regarding the very “swirly” results, I have discussed it in other forum posts, i.e: Convergence issue - #8 by jairogut