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Old 12-30-2008, 05:36 PM   #22 (permalink)
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Originally Posted by NeilBlanchard View Post

Velocity means that air is being deflected, or pushed up and over the front of the hood, and this is where drag is most easily created. The air velocity is almost nil at the base of the windshield (I have seen maple seeds just sit there) and so while it may not be generating lift, I don't think that there is much pressure there.

So, the diagram makes sense if you think about velocity (changing direction), and this also means pressure against the surface of the vehicle. Low velocity is closer to still/stagnant air, and therefore is lower pressure against the surface of the car (because it is closer to the way it was before the car pushed it out of the way).

The vectors are showing lift, I guess. They only shade the parts that have a positive Y value. I guess it's the X values that I have a problem with -- why are the vectors below the neutral point (on the front bumper) going the right (positive X) and the vectors above the neutral point go to the left? Is this area of the car pulling the car forward?

I'll draw a picture later that is more helpful, to me at least.
The velocity will be imparted to stagnant air as it is displaced by the moving vehicle,going from rest,and atmospheric pressure,to some local velocity determined by the time/displacement function,and lower static pressure.---------------- Drag will be imparted by viscous shearing forces of the air of the boundary layer,induced from positive or negative lift generated by the body of the car, along with attached- vortices ( all from pressure differentials ),and primarily from form-drag ( profile-drag ),associated with separated flow and it's attendant turbulent wake,with low base pressure acting against the back of the vehicle.--------------- The drag is not created by the impact of the vehicle against the air,but rather the pressure differential "across" the vehicle.
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