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 Piwoslaw 09-04-2009 07:39 AM

"Intelligent" Kammback

On our trip to Romania, I had a friend drive at speeds of 130-140km/h on the highways (we switched since I knew she'd have more fun than I would at those speeds), so I was free to observe how the car's aeromods behave at those speeds. First, I noticed that the Kammback is stable, that is it doesn't wobble or flap in the wind. That's good. But at speeds above 120km/h I noticed that the center seemed to curve upward. I have two posssible explanations for this. The first is an updraft from below pushing up. I'll add that the Kammy is of the open back design. The second is separated airflow over the top sucking from above. The second hypothesis would imply that the angle is too steep. This may be the case, since when the Kammback was being constructed the car was empty, while during the trip it was packed, and the rear was around 10-15cm lower, changing the angle. Also, maybe at higher speeds a flatter angle would be better than at 70-90km/h? I guess that I would find most of the answers if I did some tuft testing.

All of this gave me the idea of an "intelligent" Kammback, which would adjust its angles depending on speed and whether the trunk is full. I guess this would be very complicated with not too much to be gained?

 Daox 09-04-2009 08:26 AM

Quote:
 Originally Posted by Piwoslaw (Post 125778) I guess this would be very complicated with not too much to be gained?
I'd agree. While reading Hucho, he explains that Cd of a car changes pretty drastically depending on the angle of a car vs the wind (going over bumps, cross wind, etc.). You just can't accommodate for every circumstance.

 gone-ot 09-04-2009 09:48 AM

...design compromise = optimization of minimization

 2003protege 09-04-2009 10:03 AM

I think more than an updraft it's the low pressure created by the velocity of the air flowing over the top of the Kammback. I don't think it's separated flow at that point - [attached] flow still produces lower velocities with increased airspeed.

But back to the question, I think you answered your own question and daox concurred. I'd say just boattail the beast!!! :thumbup:

 vtec-e 09-04-2009 10:10 AM

That upward curve gives me the impression that lift was being generated. At high speed that would be a Very Bad Thing!
Perhaps if it was offset with a belly pan and rear diffuser. More work i know but....

ollie

 COcyclist 09-04-2009 10:23 AM

The proper angle has to do with the relationship between the roof of the car and the Kammback and that doesn't change whether loaded or not. The fact that it is bowing up in the middle suggests that you have created an airplane wing and you DO have attached flow. As others have suggested you are creating lift, but also reducing the size of your trailing wake. Are you driving to set a land speed record or driving for economy? Driven conservatively, gravity will keep your tires in contact with the road

 chuckm 09-04-2009 01:48 PM

The center was flexing upward... but were the sides flexing inward? Depending on your construction and geometry, the transition from the sides of your kammback to the top may be a high pressure point. Rather than being a case of the center of the top lifting, the sides may be squeezing in and forcing the top up.

 2003protege 09-04-2009 03:49 PM

Quote:
 Originally Posted by chuckm (Post 125838) The center was flexing upward... but were the sides flexing inward? Depending on your construction and geometry, the transition from the sides of your kammback to the top may be a high pressure point. Rather than being a case of the center of the top lifting, the sides may be squeezing in and forcing the top up.
the entire outside of the kammback is almost definitely at a low pressure with [attached] flow at the speeds discussed. So the sides probably want to bow out as well as the top.

And concerning the fear of dangerous amounts of lift, I would think that unless the kammback is made from heavy guage metal, such amounts of lift would tear it off the car before it caused a huge control issue.

 Bicycle Bob 09-04-2009 04:52 PM

Could we please stop using "laminar flow" where we mean "attached flow" and "kammback" for "roof extension?" We only make things difficult for newcomers, especially those with a little knowledge, if we develop a new jargon here.

If a panel bulges at speed, a pressure difference has built up. We don't really need to decide if the high pressure or the low pressure is the "problem" area. In general, the panel is there to separate pressure zones, and requires appropriate stiffness.

I think that load-levelling arrangements on the chassis will be easier and more productive than adaptive aerodynamics. It is possible to get a drag reduction by "sailing" in crosswinds even with a rigid, fixed shape, and still have the shape stall and go into high-drag mode if the side force becomes a problem. Such tricks are only worthwhile with a limited speed range and good traction, though.

 chuckm 09-04-2009 04:58 PM

Sorry, I was intending to say that the top corners were squeezing in. I wouldn't be surprised that the sides would try to bow out as well, but it would not be as dramatic as the top (since the sides taper into the car, the free length is shorter than that of the top).

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