[aerohead]

Reynolds number has to do with a bodies size in relation to its velocity,moving through a viscous fluid and boundary layer conditions,whether the boundary layer is laminar or turbulent is dependent upon it.
The number is named after Osborne Reynolds who introduced it in 1882.
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For road vehicle work the Reynolds number is defined by:

Length X Velocity / Kinematic viscosity

L X V / v

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Kinematic viscosity is the 'coefficient of viscosity',also 'absolute viscosity',also 'dynamic viscosity',and also just 'viscosity', divided by density,or

viscosity / density ( Mu over Rho )
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EXAMPLE: Toyota Prius,approx. 14-foot length,at 60-mph ( 88 feet per second ),on a 50-degree F day,( v = 0.00015 ft-squared/second )
Then,--------------- 14 X 88/ 0.00015 = 8,213,333 = Reynolds number

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Kinematic viscosity can be found in tables.You can directly measure your vehicles length,and you just pick a velocity in feet per second.
P.S. v at standard temp and pressure is 0.000158.
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Knowing the Reynolds number allows calculation of surface-friction drag,or the 'skin-friction' component of your vehicles profile drag.Subtracting the skin friction component from the profile drag reveals the 'pressure drag' of your vehicle which is governed by flow separation.A perfectly streamlined vehicle will have zero pressure drag and only a bit of skin friction which is unavoidable to the viscous qualities of the air.