Quote:
Originally Posted by ACEV
What still takes me by surprise is that no one seems to notice that the air on the surface of a moving object is what creates drag. Therefore it is reasonable that breaking that up would decrease the drag of attached air. Let's discuss the basics first.
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From:
Turbulence - Wikipedia, the free encyclopedia
Flow over a golf ball. (This can be best understood by considering the golf ball to be stationary, with air flowing over it.) If the golf ball were smooth, the boundary layer flow over the front of the sphere would be laminar at typical conditions. However, the boundary layer would separate early, as the pressure gradient switched from favorable (pressure decreasing in the flow direction) to unfavorable (pressure increasing in the flow direction), creating a large region of low pressure behind the ball that creates high form drag. To prevent this from happening, the surface is dimpled to perturb the boundary layer and promote transition to turbulence. This results in higher skin friction, but moves the point of boundary layer separation further along, resulting in lower form drag and lower overall drag.
" . . . flows at Reynolds numbers larger than 5000 are typically (but not necessarily) turbulent."
Now reread post 46, paragraph 5, above, by Aerohead:
"5. since Reynolds number is a function of a bodies length,as compared to it's velocity in a specific fluid,when calculated for automobiles it is found that all production motor vehicles will achieve critical Reynolds number near 20 mph,and from there on,their drag coefficient will be constant up to transonic flow velocities,where compression effects begin to enter the picture."
Get it now?