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Old 02-25-2012, 01:11 AM   #1 (permalink)
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The body shape of minimum drag

I'm quoting Antonio Morelli's 1976 SAE technical paper #760186

"The historical development seems to indicate that an optimization of the body shape should be sought for through the reduction to a minimum of the sum of the profile and the induced drag, the third term of the sum expressing th total drag, the friction drag, being practically not variable and negligible (7).
In fact, till now, either a low profile drag was obtained (but not a low induced drag) as with the Jaray body, or a low induced drag (but not a low profile drag) as with the "Kamm" body or with the adoption of spoilers and dams.
...The scope of this work is to determine a shape, suitable for motor cars, having a low profile drag and zero induced drag.
The following conditions were imposed:
A) total aerodynamic lift equal to zero;
B) gradual variation of the area of the transverse sections of the body
C) gradual shape variation of the transverse section of the body.

In order to meet other minor requirements, the following additional conditions were also imposed:
a) aerodynamic pitching moment equal to zero, for stability reasons
b) an oval contour of the transverse sections of the body (mainly of the "basic body" which will be defined further on) in order to reduce the "wetted" area and, as a consequence, the friction drag."

I think it's most important to note that Morelli got close, but didn't achieve his goals. The final shape had a Cd of 0.23.

Figure 5 shows that a "drop shape" in free air has a Cd of ~0.09, while in close proximity to the ground (9-14% of the body thickness) it rises to ~0.14.

A beneficial shape is a "banana" with ~9% camber - the mass centroid peaks at 9% of the length of the body.

His shape is NOT "reverse aerodynamics". He states that a gradual pressure gradient is beneficial on the front and back of the body. You can mess up the air flow in a short distance, and spend the next 10 ft to settle it down, and Morelli believed that this method "increased the car length without contributing much to the usable space."

I learned that you can super impose various bodies and sum the Cd of the individual shapes.

Jaray's aero philosophies are 80 years old. Even this paper is 35 years old. Stay tuned.

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Old 02-25-2012, 05:32 AM   #2 (permalink)
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Old 02-25-2012, 02:35 PM   #3 (permalink)
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banana car

Quote:
Originally Posted by ERTW View Post
I'm quoting Antonio Morelli's 1976 SAE technical paper #760186

"The historical development seems to indicate that an optimization of the body shape should be sought for through the reduction to a minimum of the sum of the profile and the induced drag, the third term of the sum expressing th total drag, the friction drag, being practically not variable and negligible (7).
In fact, till now, either a low profile drag was obtained (but not a low induced drag) as with the Jaray body, or a low induced drag (but not a low profile drag) as with the "Kamm" body or with the adoption of spoilers and dams.
...The scope of this work is to determine a shape, suitable for motor cars, having a low profile drag and zero induced drag.
The following conditions were imposed:
A) total aerodynamic lift equal to zero;
B) gradual variation of the area of the transverse sections of the body
C) gradual shape variation of the transverse section of the body.

In order to meet other minor requirements, the following additional conditions were also imposed:
a) aerodynamic pitching moment equal to zero, for stability reasons
b) an oval contour of the transverse sections of the body (mainly of the "basic body" which will be defined further on) in order to reduce the "wetted" area and, as a consequence, the friction drag."

I think it's most important to note that Morelli got close, but didn't achieve his goals. The final shape had a Cd of 0.23.

Figure 5 shows that a "drop shape" in free air has a Cd of ~0.09, while in close proximity to the ground (9-14% of the body thickness) it rises to ~0.14.

A beneficial shape is a "banana" with ~9% camber - the mass centroid peaks at 9% of the length of the body.

His shape is NOT "reverse aerodynamics". He states that a gradual pressure gradient is beneficial on the front and back of the body. You can mess up the air flow in a short distance, and spend the next 10 ft to settle it down, and Morelli believed that this method "increased the car length without contributing much to the usable space."

I learned that you can super impose various bodies and sum the Cd of the individual shapes.

Jaray's aero philosophies are 80 years old. Even this paper is 35 years old. Stay tuned.
*The banana car achieved Cd 0.161 without wheels in ground proximity.
*23% of the tail was chopped off with no measurable drag increase.
*When wheels were added the Cd jumped to 0.35.
*The rest of the time at Pininfarina's wind tunnel was spent anguishing over detail optimization to shave the drag finally achieving a Cd equal to Kamm's K-3 of 1939.
*And I believe it is 'Alberto' Morelli.
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Old 02-25-2012, 02:56 PM   #4 (permalink)
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It is indeed Alberto...I must have had Banderas on the brain

Morelli's figures for the basic shape are:
Cx 0.071
Cz -0.044

The wheeled body:
Cx 0.177
Cz 0.166

The 0.23 figure is an estimate to include surface and underbody details.

This is all just a preface to his more recent work.
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Old 02-25-2012, 03:43 PM   #5 (permalink)
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figures

Quote:
Originally Posted by ERTW View Post
It is indeed Alberto...I must have had Banderas on the brain

Morelli's figures for the basic shape are:
Cx 0.071
Cz -0.044

The wheeled body:
Cx 0.177
Cz 0.166

The 0.23 figure is an estimate to include surface and underbody details.

This is all just a preface to his more recent work.
What's really tough,is that he pours his soul into this work ( it was an absolute coup in its day),throws it out there for everyone's benefit,and is rewarded with little or no enthusiasm from automakers,maybe until Aptera comes along.
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Old 02-25-2012, 09:24 PM   #6 (permalink)
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Are we talking about this? It's so difficult to understand many things on here without images.


http://ecomodder.com/forum/showthrea...hape-3746.html
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Old 02-25-2012, 10:23 PM   #7 (permalink)
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That's the one Sven. I have been busy doing simulation for the last week. When I took a break to post this I was unable to post pics. Soon!
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Old 02-26-2012, 09:08 AM   #8 (permalink)
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Quote:
Originally Posted by aerohead View Post
What's really tough,is that he pours his soul into this work ( it was an absolute coup in its day),throws it out there for everyone's benefit,and is rewarded with little or no enthusiasm from automakers
The shape might be fine from an aerodynamic viewpoint, it's a dog from a practical PoV.

Adding wheels is problematic - both from an aero viewpoint (see above) as well as from a practical viewpoint (see Aptera).
The shape is very complex to build.
The shape has little internal volume, and even less practically useful volume.
The shape is awkward in everyday vehicle use.
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Old 02-28-2012, 11:16 PM   #9 (permalink)
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Is a 20' long 2 passenger vehicle practical?

The basic shape is an idealisation. The better the basic shape, the better the final car. One of Morelli's stipulations was that it have practical internal volume for passengers and their baggage (his main reason for avoiding the tear drop). I've uploaded the basic shape (Cd 0.071) and the wheeled car (0.177).

I want to reemphasize that this paper is 35 years old. I'll get to posting his 2000 paper. I'm intrigued by his "fluid tail technique".

I did some CFD with a shape inspired by his latest work. I got a wedge shape within a few counts of a 2.5:1 ellipse (0.174). however, I've come to believe that the results were invalid. I'll post a full report when I can.

Like the MB bionic car pointed out, current frames are inefficient. Neural networks and genetic algorithms are whittling away at frame/body to decrease weight, maximise internal volume, and reduce drag. Technology that has not yet been created will shape the cars of the future. Reading over recent SAE papers, they're still struggling to model rotating wheels, and how air moves within internal cavities. It will happen within a few years imo.
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Old 02-29-2012, 05:09 PM   #10 (permalink)
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a few years

Quote:
Originally Posted by ERTW View Post
Is a 20' long 2 passenger vehicle practical?

The basic shape is an idealisation. The better the basic shape, the better the final car. One of Morelli's stipulations was that it have practical internal volume for passengers and their baggage (his main reason for avoiding the tear drop). I've uploaded the basic shape (Cd 0.071) and the wheeled car (0.177).

I want to reemphasize that this paper is 35 years old. I'll get to posting his 2000 paper. I'm intrigued by his "fluid tail technique".

I did some CFD with a shape inspired by his latest work. I got a wedge shape within a few counts of a 2.5:1 ellipse (0.174). however, I've come to believe that the results were invalid. I'll post a full report when I can.

Like the MB bionic car pointed out, current frames are inefficient. Neural networks and genetic algorithms are whittling away at frame/body to decrease weight, maximise internal volume, and reduce drag. Technology that has not yet been created will shape the cars of the future. Reading over recent SAE papers, they're still struggling to model rotating wheels, and how air moves within internal cavities. It will happen within a few years imo.
I agree on the timeline.Theoretical mathematicians worldwide are networking and building off the state-of-the-art as soon as it is published or lectured.
Just a few years seems to make quantum leaps.Snow-balling!
And wind tunnels are very expensive to operate when the same work can be done in a virtual domain.
It's a really exciting time to be around.

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