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

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 03-26-2010, 03:43 PM   #1 (permalink)
aero guerrilla
 
Piwoslaw's Avatar
 
Join Date: Oct 2008
Location: Warsaw, Poland
Posts: 3,691

Svietlana II - '13 Peugeot 308SW e-HDI 6sp
90 day: 58.1 mpg (US)
Thanks: 1,268
Thanked 721 Times in 458 Posts
Stagnation point

Here is a drawing from Piechna's book. It is based on a paper by Buchheim et al.[1]. It shows how the coefficient of drag (Cx) is influenced by the ratio of the height of the stagnation point (Hsp) to the height of the car (Hnadw).



[1] Buchheim R., Leie B., Luckoff H.J., Der neue Audi 100- Ein Beispiel fur Konsequente Aerodynamische Personenwagen-Entwicklung, ATZ 85, 1983, pp.419-425.

Attached Thumbnails
Click image for larger version

Name:	pktsp.jpg
Views:	1296
Size:	69.2 KB
ID:	5856  
__________________
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


[Old] Piwoslaw's Peugeot 307sw modding thread
  Reply With Quote
The Following 4 Users Say Thank You to Piwoslaw For This Useful Post:
aerohead (03-26-2010), California98Civic (07-06-2014), chaster (03-27-2010), PaleMelanesian (03-30-2010)
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 03-26-2010, 04:24 PM   #2 (permalink)
Master EcoModder
 
NeilBlanchard's Avatar
 
Join Date: May 2008
Location: Maynard, MA Eaarth
Posts: 7,907

Mica Blue - '05 Scion xA RS 2.0
Team Toyota
90 day: 42.48 mpg (US)

Forest - '15 Nissan Leaf S
Team Nissan
90 day: 156.46 mpg (US)

Number 7 - '15 VW e-Golf SEL
TEAM VW AUDI Group
90 day: 155.81 mpg (US)
Thanks: 3,475
Thanked 2,950 Times in 1,844 Posts
So, if I am reading the chart right: if the height of stagnant point is less than 10% of what the height of the vehicle, then it has a *negative* affect on the Cd? And if the ratio is between 10% and 20% it has adds to the Cd, and above 20% it starts to drop off again?

Is that what it is showing?
__________________
Sincerely, Neil

http://neilblanchard.blogspot.com/
  Reply With Quote
Old 03-26-2010, 04:41 PM   #3 (permalink)
aero guerrilla
 
Piwoslaw's Avatar
 
Join Date: Oct 2008
Location: Warsaw, Poland
Posts: 3,691

Svietlana II - '13 Peugeot 308SW e-HDI 6sp
90 day: 58.1 mpg (US)
Thanks: 1,268
Thanked 721 Times in 458 Posts
It looks like the minimum is around 6% of the car's height.

Of course, this is probably different for every car. I'm sure that it depends on whether the car has a bellypan, among other things.
__________________
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


[Old] Piwoslaw's Peugeot 307sw modding thread
  Reply With Quote
Old 03-26-2010, 05:15 PM   #4 (permalink)
Master EcoModder
 
aerohead's Avatar
 
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 15,861
Thanks: 23,922
Thanked 7,207 Times in 4,640 Posts
other source

I've got a small book published by SAE entitled:Aerodynamics:Recent Developments,published maybe late 1980s/early 90s.
In the book,another marque,maybe Fiat ( I'll look) had a 2/3rd scale composite car model which could be assembled to create a myriad of car configurations.
What I remember off the top of my head ( and my memory is dubious ),is that they found no hard and fast rules,and air directed up or down or around could play havoc with the Cd,say lowering upper body drag while increasing under body drag.
I'll dig that out for tomorrow.
  Reply With Quote
Old 03-26-2010, 07:09 PM   #5 (permalink)
Left Lane Ecodriver
 
RobertSmalls's Avatar
 
Join Date: Aug 2008
Location: Buffalo, NY, USA
Posts: 2,257

Prius C - '12 Toyota Prius C
Thanks: 79
Thanked 286 Times in 199 Posts
The minimum drag for the 1983 Audi 100 occurs with a stagnation point height of about 8cm/3in. It seems like a stagnation point as low as is achievable, is best.
  Reply With Quote
Old 03-26-2010, 10:24 PM   #6 (permalink)
Master EcoModder
 
Join Date: Oct 2008
Location: Mid-Atlantic
Posts: 491

OurInsight - '06 Honda Insight
Thanks: 170
Thanked 69 Times in 44 Posts
Quote:
Originally Posted by RobertSmalls View Post
The minimum drag for the 1983 Audi 100 occurs with a stagnation point height of about 8cm/3in. It seems like a stagnation point as low as is achievable, is best.
What do you make the Insight stagnation point to be? I took a tape to mine, but with the blunt lower bumper, I couldn't envision where the point might be.
  Reply With Quote
Old 03-26-2010, 11:03 PM   #7 (permalink)
Left Lane Ecodriver
 
RobertSmalls's Avatar
 
Join Date: Aug 2008
Location: Buffalo, NY, USA
Posts: 2,257

Prius C - '12 Toyota Prius C
Thanks: 79
Thanked 286 Times in 199 Posts
All I have to go on are Cd's tuft testing and the following image, which is the only one I've seen of an Insight in a wind tunnel.



Obviously the Insight's stagnation point is below that streamline. On Cd's Civic, which has an upper grille block, it's between the upper and lower grilles. It's probably around the same height on the Insight.
Attached Thumbnails
Click image for larger version

Name:	classic_windtunnel.jpg
Views:	328
Size:	49.5 KB
ID:	5859  
  Reply With Quote
Old 03-27-2010, 07:40 AM   #8 (permalink)
Master EcoModder
 
Join Date: Oct 2008
Location: Mid-Atlantic
Posts: 491

OurInsight - '06 Honda Insight
Thanks: 170
Thanked 69 Times in 44 Posts
The is another image of a lower smoke trail on IC at this link:

InsightCentral.net - Encyclopedia - Honda Insight Aerodynamics

But, it seems to me that neither smoke picture really shows us the stagnation point. Cd's tuft testing seems to show that blocking the lower grill, as completely as possible, would lower stagnation point.

It is interesting to note that basjoos created a pointed and much lower front end on the aerocivic to lower the stagnation point. Kinda makes one wonder if Honda had to make a compromise on the Insight/Civic nose shapes. Obviously, there were aero compromises in general, though the .25 Cd in near record for production cars.
  Reply With Quote
Old 03-27-2010, 02:40 PM   #9 (permalink)
Master EcoModder
 
aerohead's Avatar
 
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 15,861
Thanks: 23,922
Thanked 7,207 Times in 4,640 Posts
Source

Quote:
Originally Posted by aerohead View Post
I've got a small book published by SAE entitled:Aerodynamics:Recent Developments,published maybe late 1980s/early 90s.
In the book,another marque,maybe Fiat ( I'll look) had a 2/3rd scale composite car model which could be assembled to create a myriad of car configurations.
What I remember off the top of my head ( and my memory is dubious ),is that they found no hard and fast rules,and air directed up or down or around could play havoc with the Cd,say lowering upper body drag while increasing under body drag.
I'll dig that out for tomorrow.
The paper I have is SAE 860212,'Car Underside,Upperbody and Engine Cooling Systems Interactions-and Their Contributions to Aerodynamic Drag,by A.Garrone and M.Masoero,Fiat Auto Wind Tunnel Dept.,1986.
The paper runs to 8 pages.
It was a full-scale model,not a scale model.
* Underbody drag of bellypan,suspension,wheels and wheelhouse was found to be nearly same as upperbody drag.
* Cooling system drag was measured at Cd cs 0.017.
* Base drag of upperbody was 50% of upperbody drag ( the car looks like a Scion xA).
* Sealing the cooling sys. lowered underbody drag but increased upperbody drag.It should be 'optimized' as Korff/Morelli have done.
* these folks also came up with the 2.5-degree diffuser for bellypan.
* For a low-drag car,a front airdam increases drag.
* In a low-drag car,the underbody can constitute 50% of the drag.
From their diagrams you get the idea that moving the stagnation point around could conceivably degrade performance if you weren't careful.
From Hucho's book,which cites the 'ideal' nose,if you ascertain the stagnation point there,and then place the grille opening there,then maybe that's a good bet for general purposes.
If you have the 'length',then GM's 'bottom-breather' appears to be a great way to go.
Historically,moving the cooling system to the rear,as with Probe-IV,V,GM Ultralite,GM Precept,appears to offer even better savings with zero opening at the front at all.
  Reply With Quote
The Following 2 Users Say Thank You to aerohead For This Useful Post:
jime57 (03-27-2010), Piwoslaw (03-27-2010)
Old 03-27-2010, 02:48 PM   #10 (permalink)
aero guerrilla
 
Piwoslaw's Avatar
 
Join Date: Oct 2008
Location: Warsaw, Poland
Posts: 3,691

Svietlana II - '13 Peugeot 308SW e-HDI 6sp
90 day: 58.1 mpg (US)
Thanks: 1,268
Thanked 721 Times in 458 Posts
Quote:
Originally Posted by aerohead View Post
Historically,moving the cooling system to the rear,as with Probe-IV,V,GM Ultralite,GM Precept,appears to offer even better savings with zero opening at the front at all.
Should we call it a "rear-end-breather"?

__________________
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


[Old] Piwoslaw's Peugeot 307sw modding thread
  Reply With Quote
Reply  Post New Thread


Thread Tools


Similar Threads
Thread Thread Starter Forum Replies Last Post
Rerouting exhaust to a low pressure point ConnClark Aerodynamics 19 10-06-2012 02:24 PM
Newbie's staring point pyrogaz Introductions 7 01-31-2010 04:18 PM
At what point is it advantageous to shut the engine off while driving? Mazda_2.3 Hypermiling / EcoDriver's Ed 33 02-20-2009 01:55 AM
Hit a 3 point buck with a '94 Escort SW. TestDrive DIY / How-to 22 12-09-2008 01:38 AM
At What Point Does A Lowered Car Hurt You? kdmy0 Aerodynamics 5 07-24-2008 12:23 PM



Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2024, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright EcoModder.com