Time scales and Fluent's unsteady solver issue

Freeman

Hi all.

After making dozens of steady state simulations with my 2D problem (bluff body, side view of a car), I am starting to think that my case is unsteady; when I switch to 2nd order schemes, solution starts to oscillate (I mean, residuals, Cd, Cl... oscillate)

I've tried to run unsteady simulations with the Automatic Time Stepping (and the Non-Iterative Time Advancement option enhabled) and after each step, Fluent ALLWAYS reduces time step: it gave me a step of 1e-40 and continued reducing it!! (I started from a step of 0.001s)

Now I want to try with fixed stepping, but I'm not sure about my problem's time scale. How can I calculate it? In bibliography I allways see Kolmogorov time scale, or viscous diffusion time scale, but in their formula it appears the characteristical Lenght, and this lenght in my opinion can be arbitrary ANY! (from tunnel's dyameter to a screw of the car xD), so time scale is not really defined.

Could anyone enlighten me? Or suggest me a time step, or how to fix my ATS-NITA problem?

Thank you very much!