Quote:
Originally Posted by AeroMcAeroFace
And that is with zero ground clearance, lifted off the ground it is higher than that. I think I remember it as 0.15.
If there is far lower drag coefficient shapes then why are we chasing non-optimal? (non optimal in the sense of not in free air and close to a ground plane, basically where cars are)
Even if the template can predict separation, it would be able to do so only exactly on the centreline, because that is the only place where the flow is parallel to the direction of travel.
I think chapter 4 in Aerodynamics of road vehicles says it best
"In spite of its comparatively low drag, the passenger car is closer to a
rectangular box in terms of fluid mechanics than it is to a body of
revolution, though with refinements in aerodynamics progress is towards
the body of revolution. The flow round a car body is characterized by
separation (Figs 1.1 and 1.2) and its drag is primarily pressure drag.
Attempts to relate drag to primary shape characteristics (see section
1.2.3) have been unsuccessful. The number of parameters describing the
geometry of a car is too large and the interaction of the individual flow
fields too complex."
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* Cd 0.15 is for Klemperer's 1922 half-body with wheels.
* Cd 0.10 without wheels, maintaining the same ground clearance.
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* The 1981 Volkswagen 'Flow' body long-tail, by Buchheim et al., is Cd 0.14.
Its half-body, at maintained ground clearance is Cd 0.0913.
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* Jaray's 'pumpkin seed' of 1922, with diffuser ( his invention ) is Cd 0.13.
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* The baby 'template' car measured Cd 0.121 with 'loose' wheel fairings and chopped tail.
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* Cambridge University's CUER solar racer is 'template'-esque, and with knife-edge trailing surfaces measured Cd 0.11.
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* The flow on the sides of the 'template' is essentially the same over the top. It's a half-body. That's what half-bodies do.
* ' The main contribution to the drag force originate from the rear part of the body.' Hucho, page-61, 2nd-Edition.
* ' [I]t is very important to design a rear body surface which brings the divided streamlines smoothly together. Optimum shapes are ' streamlined' bodies having a very slender rear part.' Hucho, page 61, ditto.
* ' [T]he optimum shape in terms of drag is a half-body, which forms a complete body of revolution together with its mirror image- produced through reflection from the roadway.' Hucho, page 15, ditto.
* ' [L]ow drag can only be achieved when the separation at the rear is eliminated.' Hucho, page 16, ditto.
* ' [A]n effective fineness ratio in free air of 2.27..... approaches the drag minimum recognizable.' Hucho, page 210, ditto. ( 'template' is 2.5:1 )
* The pressure recovery... provides for the reduction of the drag.' Hucho, page 144, ditto.
* ' [P]ressure drag is the largest component in the aerodynamic drag. Its minimization is the true objective of motor vehicle aerodynamics.' Hucho, page- 119, ditto.
* ' A closer approach to the value of the basic body without wheels is only achievable through further integration of the wheels into the body.' Hucho, page- 201. ditto.
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All the above are defining the 'template.'
* Hucho shows it in Table 2.1, page 61, from Horner's book of 1951. Cd 0.04 in free flight ( actually lower ). Half-body would technically be Cd 0.08.