Go Back   EcoModder Forum > EcoModding > Aerodynamics
Register Now
 Register Now
 

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 09-04-2009, 08:39 AM   #1 (permalink)
aero guerrilla
 
Piwoslaw's Avatar
 
Join Date: Oct 2008
Location: Warsaw, Poland
Posts: 3,541

Svietlana II - '13 Peugeot 308SW e-HDI 6sp
90 day: 58.1 mpg (US)
Thanks: 1,056
Thanked 629 Times in 396 Posts
"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?

__________________
e·co·mod·ding: the art of turning vehicles into what they should be

What matters is where you're going, not how fast.

"... we humans tend to screw up everything that's good enough as it is...or everything that we're attracted to, we love to go and defile it." - Chris Cornell

Piwoslaw's Peugeot 307sw modding thread

  Reply With Quote
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 09-04-2009, 09:26 AM   #2 (permalink)
Administrator
 
Daox's Avatar
 
Join Date: Dec 2007
Location: Germantown, WI
Posts: 11,031

CM400E - '81 Honda CM400E
90 day: 58.68 mpg (US)

Daox's Grey Prius - '04 Toyota Prius
Team Toyota
90 day: 49.53 mpg (US)

Daox's Insight - '00 Honda Insight
90 day: 64.33 mpg (US)

Swarthy - '14 Mitsubishi Mirage DE
Mitsubishi
90 day: 56.69 mpg (US)
Thanks: 2,354
Thanked 2,422 Times in 1,465 Posts
Quote:
Originally Posted by Piwoslaw View Post
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.
__________________
Current project: Adding regenerative braking to the Mirage
  Reply With Quote
Old 09-04-2009, 10:48 AM   #3 (permalink)
...beats walking...
 
Join Date: Jul 2009
Location: .
Posts: 6,191
Thanks: 179
Thanked 1,521 Times in 1,122 Posts
...design compromise = optimization of minimization
  Reply With Quote
Old 09-04-2009, 11:03 AM   #4 (permalink)
EcoModding Enthusiast
 
Join Date: May 2008
Location: Austin
Posts: 155

Silver Bullet - '03 Mazda Protege LX
90 day: 29.51 mpg (US)

Silver Bullet - '03 Mazda Protege LX
90 day: 37.06 mpg (US)

Blaze - '17 Ford Escape
Thanks: 23
Thanked 2 Times in 2 Posts
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!!!
__________________

Last edited by 2003protege; 09-04-2009 at 06:42 PM.. Reason: semantics
  Reply With Quote
Old 09-04-2009, 11:10 AM   #5 (permalink)
Master EcoModder
 
vtec-e's Avatar
 
Join Date: May 2008
Location: Ireland
Posts: 507

De Yaris - '04 toyota yaris T2
90 day: 69.55 mpg (US)
Thanks: 111
Thanked 32 Times in 22 Posts
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
__________________
  Reply With Quote
Old 09-04-2009, 11:23 AM   #6 (permalink)
Aero Wannabe
 
COcyclist's Avatar
 
Join Date: Dec 2007
Location: NW Colo
Posts: 573

TDi - '04 VW Golf
TEAM VW AUDI Group
90 day: 54.33 mpg (US)
Thanks: 403
Thanked 128 Times in 100 Posts
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
__________________
60 mpg hwy highest, 55+mpg lifetime
TDi=fast frugal fun
make NOx not Carbon


Quote:
Originally Posted by freebeard View Post
The power needed to push an object through a fluid increases as the cube of the velocity. Mechanical friction increases as the square, so increasing speed requires progressively more power.
  Reply With Quote
Old 09-04-2009, 02:48 PM   #7 (permalink)
Master EcoModder
 
chuckm's Avatar
 
Join Date: Aug 2008
Location: Monroe, LA
Posts: 308

Exploder - '02 Ford Explorer xlt

Rolla - '02 Toyota Corolla ce
Team Toyota
90 day: 44.43 mpg (US)
Thanks: 11
Thanked 12 Times in 11 Posts
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.
__________________
"Jesus didn't bring 'Natty Lite' to the party. He brought the good stuff."
  Reply With Quote
Old 09-04-2009, 04:49 PM   #8 (permalink)
EcoModding Enthusiast
 
Join Date: May 2008
Location: Austin
Posts: 155

Silver Bullet - '03 Mazda Protege LX
90 day: 29.51 mpg (US)

Silver Bullet - '03 Mazda Protege LX
90 day: 37.06 mpg (US)

Blaze - '17 Ford Escape
Thanks: 23
Thanked 2 Times in 2 Posts
Quote:
Originally Posted by chuckm View Post
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.
__________________

Last edited by 2003protege; 09-04-2009 at 06:42 PM.. Reason: semantics
  Reply With Quote
Old 09-04-2009, 05:52 PM   #9 (permalink)
EcoModding Apprentice
 
Bicycle Bob's Avatar
 
Join Date: May 2008
Location: N. Saskatchewan, CA
Posts: 1,537

Appliance White - '93 Geo Metro 4-Dr. Auto
Last 3: 42.35 mpg (US)

Stealth RV - '91 Chevy Sprint Base
Thanks: 72
Thanked 301 Times in 212 Posts
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.
  Reply With Quote
Old 09-04-2009, 05:58 PM   #10 (permalink)
Master EcoModder
 
chuckm's Avatar
 
Join Date: Aug 2008
Location: Monroe, LA
Posts: 308

Exploder - '02 Ford Explorer xlt

Rolla - '02 Toyota Corolla ce
Team Toyota
90 day: 44.43 mpg (US)
Thanks: 11
Thanked 12 Times in 11 Posts
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).

__________________
"Jesus didn't bring 'Natty Lite' to the party. He brought the good stuff."
  Reply With Quote
Reply  Post New Thread


Thread Tools


Similar Threads
Thread Thread Starter Forum Replies Last Post
Accordion Kammback for big rigs ? Cd Aerodynamics 36 02-27-2012 06:03 AM
Minivan Kardboard Kammback boosts MPG +3.7% (6.6%, counting roof rack delete) MetroMPG Aerodynamics 31 05-20-2010 01:23 PM
Sedan Kammback? KJSatz Aerodynamics 10 09-28-2009 05:58 AM
Prius Removable Kammback ENERGYprof EcoModding Central 20 08-04-2009 07:52 PM
Non-invasive mounting for kammback wagonman76 Aerodynamics 4 03-24-2009 07:15 PM



Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2020, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright EcoModder.com