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Old 02-03-2010, 04:53 PM   #40 (permalink)
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Originally Posted by ChazInMT View Post

So you're saying we can't scale the template to match a feature on a vehicle to determine if it will maintian attached flow (Lime). You say that if the roofline of a vehicle is steeper than the back curve of the 2.5:1 Pumpkin Seed Template which is scaled to match the bottom of the tires and the peak of the roof (Gold), it will not have attached flow, and create drag.
The spirit of the template is to serve as a guide for the design of any add-on device to clean up an aft-body already demonstrating separated flow,like a spoiler for a coupe/sedan,or aeroshell for a pickup.
The template might also give insight to potentially "dirty" designs which might require remedy.
Paul Jaray invented the "ground-reflection" technique for auto design.When the "upper" half of his teardrop was used to design his 1922 "pumpkin seed",it produced a "car" with Cd 0.15.By adding a chin spoiler,the drag fell to Cd 0.13.
W.I.Kamm basically did the same thing during development with Fachsenfeld of the famous K-cars.
The Jaray form and Kamm form passenger cars have identical Cd 0.21 with full boat tails.
From Fachsenfeld's "Aerodynamics of Road Vehicles," 1951,the pumpkin seed development models for Jaray and Kamm,in "reflection" create teardrops of 2.28:1 and 2.4:1 respectively.
During WW-II the DVL at Berlin ( Germany's NASA ) came up with a streamline body of revolution of Cd 0.04 with L/D = 2.5.
Hucho depicts this form in Table 2.1.This is the "shortest" structure I've ever seen which can produce Cd 0.08 in "reflection" within ground-effect.
Hucho also demonstrates with Table 4.119 that an ellipsoid of L/D 2.5 also produces the minimum Cd beyond critical Reynolds Number.
This is why I settled on the 2.5 L/D as the "minimum"
To get even Weirder,if you take Mair's wind tunnel model for boat tails and lose the cylinder portion,leaving only the nose and boat tail,you end up with a teardrop of 2.52:1 L/D,and this form embodies the 22-degree exit architecture demonstrated as the maximum angle which will sustain attached flow in an aft-body.
So the short answer to your question is yes.Theoretically,and confirmed by wind tunnel study,if the aft-body roof-line or sides of a car taper in, any more aggressively than the 2.5:1 teardrop/pumpkin seed form, one could have little confidence in it's ability to support attached flow.
A "back door" reverse-logic confirmation of the notion comes from Hucho when he chastises Mercedes-Benz "short" length,with it's C-111-III which fell below the "ideal" 5.0 L/D,something Volkswagen capitalized on with it's ARVW concept car,which in "ground-reflection" produces the 2.5 L/D form.
Of course,it's simplistic,and with coupes,sedans,SUVs,Vans,etc.,it's asking a lot for the template to always perform.
However,since it's origins are steeped in good science,it may be considered as a go-no-go quick assessment tool.
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