[aerohead]

Reynolds number determines whether a structure will have a laminar or turbulent boundary layer.
At 20-mph and above,because of their length,automobiles will transition to turbulent boundary layer (TBL),which helps the air to stay attached (like a dimpled golf ball).
If the TBL does stay attached,then the outer,inviscid flow will be laminar all the way across the car,and the only penalty will be the bit of TBL in the wake which cannot be converted to a pressure rise.
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Reynolds number Rn = Length (feet) X Velocity (feet per second)/ v (kinematic viscosity [ feet-squared per second])

or, Rn = L X V X 1/v

Standard kinematic viscosity is 0.00015 ft-square per second

1/v = 6380

So,simplifying, Rn = L X V X 6380
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Due to legal constraints on camera systems and interest in the drag of smaller structures on a car (mirrors/tires),knowing the Rn may mean a better understanding of whether or not,these smaller structures might benefit from some modification for boundary layer control (dimples/VGs/grooves/stubs/jets/etc.)
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*With no outside turbulence the TBL transition occurs at Rn = 500,000.
*Earth's TBL can have a thickness of 5-kilometers on any windy day.
*Typically,with any wind at all,automobiles will be fully immersed in the Earth's TBL and have no chance of any LBL and are at full TBL by Rn = 75,000.
*A Prius is at full TBL by 4-mph.
*One MIRA aerodynamicist reported that in any given year,they might have only 7-calm days in which they could conduct outdoor testing in England.
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NACA 6-Series airfoils on aircraft experience minimum-drag flight Rns of 20,000,000-40,000,000.