Quote:
Originally Posted by kach22i
There is a typical pattern of the front wheels generating more drag than the rear wheels - on most cars. I'm sure that you will find an exception or two, but look for a majority pattern.
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Most textbooks I've read put this around 60-65% of wheel drag associated with the front wheels, and 35-40% with the rear; reverse those numbers for buses and tractor-trailers.
Bernoulli's equation posits a relationship between pressure and velocity due to conservation of energy: the static pressure, gravitational potential energy, and kinetic energy ("dynamic pressure") of a fluid must remain constant if no external energy is added to the system. "Dynamic pressure" is proportional to the square of fluid velocity, and as that term increases static pressure must decrease (and vice versa), assuming no or minimal change in height.
The relative velocity of the air around a car body is zero due to the no-slip condition at the wall. Because of the viscosity of the fluid--a result of molecular attraction between fluid particles--the relative velocity increases with distance from the body until you reach the edge of the boundary layer, where relative velocity is now equal to free stream velocity. The drag from the "sheets" of air in the boundary layer sliding against each other is friction drag, and yes, it is caused by velocity differential.
But: the majority of overall drag force acting on a car (>80%) is due to pressure drag. I haven't read much about airplane aerodynamics (and that link isn't opening for some reason), but it may be that they don't operate in flow with a no-slip condition at the body; in that case, velocity differential probably contributes the majority of drag since they are streamlined shapes. But on bluff bodies at the speeds we're interested in on this site (read: low), I would not say that velocity and not pressure causes drag.