Quote:
Originally Posted by aerohead
1)
A) ' it is possible to relate geometry to flow pattern clearly for the fastback and the squareback.' Hucho, page- 155
B) ' On a prismatic body, the base pressure in real flow is dependent upon the fineness ratio.' Hucho, page- 140
C) ' The base pressure also depends upon the angle at which the flow separates from the contour.' Hucho, page-141
D) 'With a long diffuser, a notable reduction in drag can be achieved with a very small angle.' Hucho, page- 144
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2)A) ' The shape for minimum drag can be calculated for a given fineness ratio and volume, and the theory can be used to translate results from scale models to full-size bodies.' Hucho, page- 106
2)B) ' Basic bodies with drag figures of less than 0.15 can be ... used as initial shapes for shape optimization.' Hucho, page- 198-9
2)C) It's conceivable that a template may allow for zero ' changing the detail in question until the flow no longer separates around it.' Hucho, page- 124.
So in regards to your comment, I could not disagree more.
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3) absolutely!
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4) If the aft-body cross-section contracts progressively in the flow direction such that only a 'moderate' pressure increase is produced, the boundary layer will be protected from separation, flow will remain attached for the entire length of the existing body, turbulent boundary layer will be the thickest, and pressure will be the highest possible, when separation does finally occur at the trailing edges; yielding the highest base pressure.  |
I looked up each of your quotes and they're generally either taken out of context, refer to 'shapes' (not real cars), are a bit simplistic or are rather tortuously applied to support your theories.
All the professional car aerodynamicists that I asked to comment on the topic of 'what influences base pressures?'
on real cars said there was no single answer, and the influences of various mechanisms were poorly understood.
My summary thus remains:
1. There is no simple, pat answer as to what influences base pressure on different cars.
2. Therefore, without testing, you're working very much in the dark as to the effectiveness of modifications.
3. Higher pressures on the side of the car, especially towards the rear, will likely increase base pressure. (And my testing clearly shows that higher side pressures occur with better flow attachment.)
4. Thick boundary layers towards the back of the car will likely increase base pressure.