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Old 12-09-2006, 02:41 PM   #11
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Quote:
Originally Posted by ZugyNA

Yet supposedly...the CRV on this page...sharp top...rounded corners..

...claims 7-10% mpg gain with airtabs on sides and top?



Been seeing factors relating mpg with CD values.
Airtabs! This dead horse won't go away! 7 to 10 %??? Do the numbers....

Aero drag comes from two sources 1) Disturbing smooth airflow... ,and 2) moving the air out of the way (displacing) as you penetrate it.
(please note: Cd is covered in source 1 ... as is boundary layer disturbances )
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Old 12-09-2006, 02:56 PM   #12
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Quote:
Originally Posted by theclencher
That is one of the main premises behind Kamm's theory and all Kamm-backed vehicles.

We might be getting into a semantics issue here.
I would say that the bumpers are somewhat rounded but not really rounded, but they do stray somewhat from the Kamm ideal.
Ah! the Kamm Effect! It has been years since I've seen this in print! Remember how he was ridiculed for his idea? As was Jim Hall with his rear "wing" chaparall (did I spell this correctly?)....One of the big advantages of the Kamm Effect was (is?) it makes the cars shorter! Stylists love this!
The more we play with aero mods...the slicker we become...even in our claims. I knew a guy (who was an auto body shop genius) who's last name was Kombach...boy, did he hear about his "square tail"! LOL
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Old 01-09-2007, 02:17 AM   #13
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Quote:
Originally Posted by MetroMPG
I've realized aero is a much more complicated subject since I decided to make an effort to go beyond "lay" or "pop" aerodynamics. Perhaps time to track down an expert and ask directly.
I hope you succeed, since the car industry is probably not sharing its cutting edge knowledge on this subject. And besides, most companies don't consider it to be terribly important.
A question, and a theory for the discussion:
1. the rear angle of the EV1 (as for the Probe V) is rather close to the 30 degrees forbidden by theory. Why is that?
2. My gut feeling (not tested, sorry) is that the aerodynamic qualities of a car vary according to speed. The theory goes like this: air has mass; mass is slow (or how do you say this in English). The mass is pushed aside by the moving car, and once the car has passed, the mass comes back, attracted by the low pressure area behind the car. My point is this: because of the slowness of mass, the airflow picture will be different for a car depending on whether it's going 50 or 150. With 150, the wake will be longer, and the 'bow wave' will be stumper in shape. If you take one individual air molecule, and a car at 150, (as compared to car travelling 50) it goes like this: the molecule gets closer to the car before it starts being pushed to the side (by other molecules), it probably travels further outward since it hits a higher pressure area, and takes longer to travel back in the wake of the car - or would the lower pressure (stronger vacuum) cancel out this last effect?

In other words, the air behind an Insight travelling at 50 would be back to normal pressure at a smaller distance behind it, than with an Insight travelling at 150.
Or: does it make a difference for the Cd figure which wind speed is used in the wind tunnel that tests it?

There might be a link between the answer to my two points: namely, that at low speeds (say up to 50 MpH), the rear third of the EV1/Probe is perfect, since air flow remains well attached to the car, from start to finish. But the rear third would not be perfect if the EV1/Probe were to travel at a speed of 130 MpH. At that speed, the air flow would not remain attached to the rear third since the car would be going to fast: and a smaller angle would be needed, like 10 or 15 degrees.

This stuff is not necessarily based on existing (known) theory, but there may be a parallel (in the water, not in the air) that I know of: in windsurfing, the rear fin slices through the water, and prevents the tail from gliding sidewards. There is more pressure on one side of the fin than on the other. At low speeds, no problem. At high speeds however, you may encounter spinout: the pressure difference causes the angle of the fin (in comparison to the direction) to become too large, the water can no longer remain attached to the low pressure side (the wind side) and air bubbles start to form. The back of the board slides away to the leeside.
Now of course the properties of water and air are much, much different, but I see no reason why a small similar effect does not exist for air.
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Old 01-09-2007, 07:15 AM   #14
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Quick point here - will write more later...

Quote:
Originally Posted by 86Celica
A question, and a theory for the discussion:
1. the rear angle of the EV1 (as for the Probe V) is rather close to the 30 degrees forbidden by theory. Why is that?
I measured the EV1 rear slope to be 18 degrees. The Probe V is harder to measure because I haven't found an image with a true profile view, but the closest 2 I have found measured 17 and 20 degrees.

So not quite the 30 degree angle of death.

One reason both of these cars can use relatively steep rear angles is the shape of the rear is dramatically more conic than wedge like. There is a very generous curve from the sides to the backlight/decklid, and this would help avoid the formation of lift induced vortices formed at the rear of a more conventional wedge-like rear end shape with the same backlight angle but a sharper side-to-deck transition.

The EV1 side-to-top rear radius is most obvious when seen from above:

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Old 01-09-2007, 02:21 PM   #15
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Quote:
Originally Posted by theclencher
That's boat-tailing.
I think the EV1 went beyond typical boat-tailing. Maybe a distinction is called for:

A pyramid is "boat tailed". But a cone is far more aerodynamically efficient owing to the elimination of the sharp angles where the pyramid's planes meet.

I would say that most cars - even those which employ boat-tailing - retain distinct side & top "planes", like a pyramid.

The EV1 did "conic" boat-tailing. Better.

(EDIT: of course it's not really a cone, since the edges are curving in 2 dimensions, but the analogy still works.)
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Old 02-06-2007, 05:27 AM   #16
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Quote:
Originally Posted by theclencher View Post
This is the reason slower airplanes like Cessnas and such are aerodynamic with their relatively blunt noses (airflow remains attached) while the faster stuff like, say, a Concorde requires a pointy nose due to it's much higher operating speeds.
The effects that make a pointy nose more aero than a blunted one are associated with traveling at Mach numbers near or over 1. Has alot to do with the speed of the vehicle in relation to the speed that pressure waves travel (ie: mach 1).

Not very applicable to highway speeds though, or even speeds attainable by street-legal cars. At speeds any of us are likely to travel in a car, the classic teardrop shape is supposed to be the most aerodynamic as mother nature has been telling us for years.
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Old 02-06-2007, 06:49 AM   #17
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re sharp vs blunt, all I can tell you is that on rc sailplanes they prefer NOT to round off the trailing edges, if you cant bring them to a point then leave them square. rounding will create control surface flutter and other noises. Granted the back of a car isn't exactly a trailing "edge". A boat too has a sharp transition to verticle at the back, if any verticle, don't know if it's for the same reasoning.
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Old 02-06-2007, 06:57 AM   #18
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Good link. Thanks. One day I will find an aerodynamicist to talk this over with.
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Old 02-06-2007, 07:43 AM   #19
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Get a block of foam and a big fan - shape the foam to the teardrop shape first and place in the fan air stream and measure the force on the shape . . . repeat as you cut the rear taper off.
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Old 02-11-2007, 03:59 PM   #20
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MetroMPG,
I haven't seen this referenced to anywhere on this forum yet.

http://www.nasa.gov/centers/dryden/p...ain_H-2283.pdf

Big file, 3.35 MB. It does give hard experimental evidence of whether radii help or not, at least for a van. It ought to rank at least "for kicks and giggles".
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