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Originally Posted by j-c-c
Then why are most tuff tests at multiple speeds faster on the road and/or in a tunnel?
It's IMO a lot easier to securely fasten a camera for 20? mph tests than messing with a drone and nearby traffic.
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The Cd will be 'fixed' at around 20-mph.
A laminar boundary layer becomes unstable at Rn 500,000.
Turbulent boundary layer is firmly established by Rn 1,000,000.
The Cd 0.495 VW Beetle, reported by Hucho/ Hoerner, Figure 4.119, Page-200, Hucho, 2nd-Edition, presents the VW at Rn 11,000,000, which is associated with 85-mph ( 137 km/h )
The Beetle achieves supercritical Rn at 7.738-mph ( 32.2-km/h ).
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Higher tunnel and CFD velocities are associated with pitch, yaw, and roll moment magnitude, and the lift and lift coefficients for each axle, all of which come into effect at higher speed. Overkill for Cd measurement.
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Everyone here at the Aero Forum would benefit from knowing how to calculate Reynolds number, and why it exists.
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A Go-Pro at 20-mph is just fine! Just practice situational awareness when you're testing. Calm conditions away from traffic is your friend.