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Old 09-16-2012, 08:17 AM   #1 (permalink)
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Why does The Template have to be a half body of revolution?

My understanding is that The Template is a half body of revolution to insure that air doesn't want to move laterally because of pressure differentials between the top and sides, which leads to vortex generation.

If that's the case, wouldn't a half-square cross section serve as well, given equal cross-sectional area? If air is not wrapping across the edge, would no vortexes be created?

Here are three aeroforms, a cylinder, a superellipse and a square. Would their Cd be equivalent?


(These are not dimensionally highly accurate; I just eyeballed the proportions against The Template)

The next question: If the height/width proportion changes, what happens to the respective tapers? If the half revolution form is 7' wide, then it is 3 1/2' high. If the height is proportionally greater, does the side taper increase or relax?

TIA

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Old 09-16-2012, 01:11 PM   #2 (permalink)
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quote: Here are three aeroforms, a cylinder, a superellipse and a square. Would their Cd be equivalent?

``````````````````````````````````````````````

The frontal areas are different the shapes are different,making it unlikely that they have precisely the same cd.
Why do you feel the model with its corners and sharp edges is as aerodynamic as the curved models? i feel the added mass will increase frontal area and the hard edges will create turbulence from mixing in the opposing airflow from the sides/top.
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Old 09-16-2012, 01:22 PM   #3 (permalink)
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Your logic looks sound to me until you throw in a cross wind.

I would guess the taper of the sides would relax with added height since air from the top is not helping fill in the void further down the sides.
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Old 09-16-2012, 04:29 PM   #4 (permalink)
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Quote:
Originally Posted by freebeard View Post
My understanding is that The Template is a half body of revolution to insure that air doesn't want to move laterally because of pressure differentials between the top and sides, which leads to vortex generation.

If that's the case, wouldn't a half-square cross section serve as well, given equal cross-sectional area? If air is not wrapping across the edge, would no vortexes be created?

Here are three aeroforms, a cylinder, a superellipse and a square. Would their Cd be equivalent?


(These are not dimensionally highly accurate; I just eyeballed the proportions against The Template)

The next question: If the height/width proportion changes, what happens to the respective tapers? If the half revolution form is 7' wide, then it is 3 1/2' high. If the height is proportionally greater, does the side taper increase or relax?

TIA
Nice graphics. I have been rolling some of that over in my mind since I have been trying to get some thoughts down on a tadpole shaped special with seperater wheel fairings in front. I am leaning toward that approach because I can register it as a motorcycle. My basic shape would probably be half of a pumpkin seed, as Phil calls it, with tandem seating and a canopy. Would kinda look like a streamlined CanAm 3 wheeler. Still working out the front suspension and the seating layout.
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Old 09-16-2012, 04:39 PM   #5 (permalink)
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Would kinda look like a streamlined CanAm 3 wheeler. Still working out the front suspension and the seating layout.
Jim,

Ohh, this sounds like an interesting project!!

I spent quite a bit of time investigating this same approach about 4 years ago, and my best advice would be......

.........Whatever components you decide upon for the vehicle, make sure they are all DOT approved components.........

The more "custom" components you put on the creation, the harder it is for the inspector to get a "comfort" level about your skill level and so on. He has to sign off on your design, and in case the machine has a "failure" in the future that causes injury to someone else, the inspector could still be held partially liable in certain situations for passing questionable construction methods.

I had many email exchanges with my state inspector about this issue, and to make it "easy" for him/her to inspect and give approval for road use, it's imperative that components are either from a previously DOT approved motorcycle, car, truck, you name it. Keep in mind that ATV's are not road legal in the normal sense of the word. Yes, you see them on the road now and then, but they are not DOT approved for normal road use.

Hope this helps, and good luck with your project.

Jim.
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Old 09-16-2012, 04:44 PM   #6 (permalink)
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Quote:
The frontal areas are different the shapes are different,making it unlikely that they have precisely the same cd.
I shrunk the box shaped one to approximate equal frontal area:
Quote:
...given equal cross-sectional area...
and I know the single-curve shape will have more skin drag. No doubt there would be a measurable difference, but would it be a practical difference?

Quote:
Your logic looks sound to me until you throw in a cross wind.
This is where I thought the conversation would go. I read, somewhere, that radiused edges improve crosswind performance. The superelliptic shape strikes me as a middle ground with the 'boxfish' shape:


But it's all compound curves and loses the construction simplicity of the square cross-section. The Tropfenwagen looses the taper in side view almost completely:


I find The Template is kind of like a straight jacket, and almost nothing practical looks completely, exactly like it (the 1939 Schlörwagen comes close). I'm just looking for the most respectful way to slowly drift astray from it. There must be fifty ways...
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Old 09-17-2012, 06:12 PM   #7 (permalink)
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Originally Posted by freebeard View Post
I find The Template is kind of like a straight jacket, and almost nothing practical looks completely, exactly like it (the 1939 Schlörwagen comes close). I'm just looking for the most respectful way to slowly drift astray from it. There must be fifty ways...
Yes, I agree and I don't think anyone would contemplate actually building to that optimized shape. But the template is based on sound measurements of Cd for various shapes, so it is completely ignored at your own peril. I found the VW L1, which you can google, to be an acceptable deviation. It has a pretty and practical body shape and still maintained a very low Cd.
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Old 09-17-2012, 07:05 PM   #8 (permalink)
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I have been trying to get some thoughts down on a tadpole shaped special with seperater [sic] wheel fairings in front.
Aptera-like? The always entertaining Autospeed website has a wealth of useful articles. For example:
Zero Cost Modelling of Space-Frames
Blows my mind. I see there're new articles since the last time I visited.

Quote:
I found the VW L1, which you can google, to be an acceptable deviation.
When that car hits the market, it will seriously kick some Prius butt. Did you know the first concept (not the 2nd one) had hollow gears in the transmission to save weight?
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Old 09-17-2012, 10:54 PM   #9 (permalink)
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The square one would definitely higher Cd. When you include the ground plane (and the wheels) in the picture, that changes things a lot -- the air flow is not symmetrical. That is why the template is a half revolution.

The frontal area is a separate issue from the Cd -- greater frontal area increases the CdA, but it does not change the Cd, per se.

Edit: I may be misunderstanding your question. All three of the shapes you are proposing are "flat bottomed", right? Elsewhere in this forum, someone did a CFD study of the round shape with a slightly curved underside (I think, and air comes out from under the bottom an spirals up and around the sides, if I recall correctly? So, the template is a general guide for shaping the upper side, but the interface of the wheels and the ground with the car chassis have to be worked on more.

That Boxfish model had an amazing Cd of just 0.095, if I am not mistaken.
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Last edited by NeilBlanchard; 09-17-2012 at 11:06 PM..
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Old 09-17-2012, 11:24 PM   #10 (permalink)
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box fish

dang you! I came on here to post my pics of my latest CFD testing. I'm trying to replicate the box fish, which MB used to shape the bionic car. My very first iteration, which was 80" long from peak to the kamm back, came to 0.143. My second iteration is 120" long, and came to 0.101. MB said the box fish came to around 0.005 with excellent yaw stability.

I used a convex rectangle section with rounded corners, approximating Morelli's later designs, instead of the "X" type shape of the box fish (I actually took a video of one at the local aquarium today). I was surprised how easily it obtained such a low Cd, considering how much more work I did to get Aero's template to 0.092. The body falls away from the streamlines much quicker, yet there's no delamination.

This answers my question of whether we "need" the tear drop shape for a low Cd. A box is certainly roomier, and easier to fit/hide wheels, and it's much shorter than the template.

Note that any body will work better with a rounded bottom - not flat - and generous fillets on the rocker panels, and a raised tail. Anything to equalise pressure around the perimetre of the body helps prevent vortices. Morelli discusses elliptical shapes to minimise wetted area, hence, skin drag.

discuss amongst yourselves

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