Wind tunnel testing is limited by the FIA to 60% models with 60m/s wind speed (216km.hr = 135mi/hr), i.e. they can only simulate the air flow around the car at the equivalent to 36m/s = 129.6km/hr = 81mi/hr for the full scale car. So they could be having issues with the car at speed - there's a separation which isn't occurring at the low Reynolds numbers of the wind tunnel, or a boundary layer is transitioning earlier or something. They are also limited to using air at atmospheric pressure - so they can't increase the Reynolds number in the tunnel by compressing the air or using a different gas.bill shoe wrote: ↑Mon Jun 25, 2018 10:25 pmI understand on a basic, literal level what scale and compressibility are in a wind tunnel, but I don't know what you really mean by scale and compressibility as the problem. What is it that wind tunnels can't simulate (accurately?) due to scale and compressibility effects? Or, how do some teams handle that limitation better?
The speed of sound is 343m/s = 1234.8km/hr = 771.8mi/hr - in dry air at sea level - normally for subsonic aero you consider the air to behave as an incompressible fluid, but that's only true up to ~Mach 0.3. Under the floor and front wing in particular air will be travelling between 2 and 2.5x the speed of the car (Cp>-4). At 60m/s in the wind tunnel that air is travelling at 120-150m/s or Mach 0.35-0.43 so the air is going to be compressible. But at the dynamically similar speed (36m/s) on the track the air under the car is only at Mach 0.2-0.26... so the air in the tunnel is compressible but with dynamic similarity on track (at the same Reynolds number) it isn't. Including flow compressibility in CFD simulations has quite a big effect - especially under the floor, which is where most of the downforce comes from so it's an important area.
Now no car will travel at 81mi/hr for long - and they do their constant speed aero testing at ~240km/hr = 150mi/hr to more closely match to the average speed for the car where maximum downforce is required - which also more closely matches the Mach number of the wind tunnel, but not the Reynolds number. So you can get some things right but others wrong or the other way around - best to test on the track at an appropriate speed.
In CFD is easier to match everything (scale/Reynolds#/Mach#) than wind tunnels but there are limitations there too.
This is a long answer and I'm not sure if it answered the question....