Quote:
Originally Posted by kach22i
My first car was a 1966 Mustang coupe, automatic with a Sprint Six.
Owned from spring of 1979 to fall of 1982, more bondo and fiberglass than I want to think about. 
Your photo however brings back fond memories.
Back on topic, I would think that like positive pressures, negative pressures take energy to create and therefore the smaller the deviation from neutral the better.
Then again, we want to establish attachment at the bow/front so that we can maintain a resemblance of attachment along the aft body, right?
That's all going to take energy, question is, what is the magic ratio for energy optimization?
The path to least resistance. |
The magic ratio is a 'streamlined' body.
There's extreme latitude in the forebody, as this is a location of 'favorable' pressure gradient, with air flowing in the direction of low pressure. No 'magic' radii are necessary. The flow is literally being held in place by the attacking flow.
Once beyond the body's maximum cross-section, flow is 'leeward' of the 'source', in a now unfavorable pressure gradient, moving towards higher pressure, lower momentum, and there's no reason for the flow to remain attached at all, unless the cross-section allows for only a gradual pressure rise ( streamlined) contour, otherwise it will trigger separation, turbulence, low pressure, low base pressure, and higher pressure drag.
Rising out of your seat on a descending roller coaster might serve as an analogy for what the turbulent boundary layer experiences during separation, as the 'downhill' slope becomes so radical, gravitational attraction can't compensate for it any longer. Valve float, with too radical a cam grind overwhelming the valve spring's tension might be another example.
The closer the separation line is to the suction peak near the windshield, the faster the air, lower the pressure, larger the wake, lower the wake ( base ) pressure, and higher the pressure drag.
The energy difference can be calculated directly from the difference in road load power between the sample vehicle, and that of the same vehicle with the minimum drag body.