[aerohead]

*Aerodynamic coefficients of drag depend upon the type of boundary layer/Reynolds number.
*For road vehicles,due to ambient atmospheric turbulence,the boundary layer will transition from laminar,to turbulent at around 20-mph.
*From 20-mph,to about 250-mph,compressibility effects are non-existent and the drag coefficient is constant and stable within this velocity regime.
*So,for 'normal' driving velocities,automotive coefficients of aerodynamic drag can be considered constant.
*During SAE coastdown testing,the initial and terminal velocities are held within this turbulent boundary layer spectrum.
*Model wind tunnels have turbulence screens to introduce turbulence into the airstream,as would be found in the 'real world.'
*Full-scale wind tunnels will generate normal turbulence as a consequence of viscous shearing of the infinite strata of air layers as they move away from tunnel boundaries.