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Old 02-01-2010, 05:00 PM   #37 (permalink)
aerohead
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Quote:
Originally Posted by ChazInMT View Post
OK, Got it, clock template thing is a strict section from a circle, your Fig 2 template is not, it is relatively flat towards the back. Still, when you apply even this 2.5:1 teardrop template, it flies in the face of reason to use it by putting it "under the wheels". I have overlaid it (Gold shape on top of the BMW) onto the Beemer as an example.



What do you have to say about scaling these templates down to match the curves of the roof?

It seems to me that the limiting factor is getting the air to try and pull past 22, the end of your tear drop template is very close to that.

It still seems you are creating a far to conservative "go-no go" by scaling the template to go under tires/top of roof.

Also, I'm not rebelling, I just want to educate myself as much as possible. I am also inherently a skeptic, meaning if it doesn't quite make sense to me, I really want to get to the bottom of things and understand it in principle. I really don't take "That's just the way we do it." as an answer. I'm sure you understand that.

I am so grateful to have found this forum where these aerodynamic ideas are bantered about. I have learned a ton in the past month. I really want to understand why you don't think the template should be scaled to match the roof line.
I'm looking at the gold template and it's centerline should be right at the bottom of the tires which should reduce the overall height of the upper curve,bringing it more in line with the BMW.
Scaling and lining the template up with the bottom of the wheels,top of the roof camber is the premise behind Jaray's "Ground-Reflection" technique.It's the only parameter that can be shared with all vehicles.
The concept is,given the height of the car,and selecting the roof peak,from there back,the template defines the top and sides of a vehicle which will exhibit zero separation.
Models are horrifically expensive to build and test.Building a full-scale prototype even more.
By engineering a "conservative" roof and aftbody,we can get that part of the build behind us and move on to new business.
Cd 0.04 is the lowest drag form I know of for a structure which can deliver Cd 0.08 in ground-effect.I couldn't find a body of revolution of Cd 0.04 with attached flow shorter than 2.5:1.Hucho demonstrates it to be the form of lowest drag.And,just as a coincidence,it also possesses the 22-degree exit exo-duct architecture which Mair's research determined to provide the lowest drag/minimum structure.
If we had unlimited resources we could pick a styling design and then optimize it to death as an automaker can,and perhaps eventually arrive at an acceptable drag figure.
But I don't know anyone at this site who's in that position.
To go after really low drag,in turbulent boundary layer,the pumpkin seed appears to be the ever-repeating solution for this challenge.And the pumpkin seed is a derivation of the 1/2 body-of-revolution.Hucho states that the target for low drag is a body fineness ratio of 5.0:1,after which if made any longer,will have only higher drag.This form becomes a full-boat-tail 2.5:1 teardrop in "reflection."
This infers that the 2.5:1 teardrop is the only structure which can satisfy this limitation.This is why I believe it to be without peer as far as a starting point.I've never found any science to disqualify it.
Of course,reality intrudes,and for road vehicles many compromises are thrown into the mix,and practicality rears it's ugly head.
So we're left to make our way,best we can.
I'm headed for sub-Cd 0.10 territory and the 2.5:1 teardrop will be my A-Ticket to get there.And not because I wish it to be,it's because all the folks that dedicated their entire careers to aerodynamic research say it's so.
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