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

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 02-21-2012, 06:29 PM   #261 (permalink)
Master EcoModder
 
aerohead's Avatar
 
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 12,784
Thanks: 20,500
Thanked 6,307 Times in 3,907 Posts
HA

Quote:
Originally Posted by ChazInMT View Post
All I have to say about this is HA! It illustrates what I've been saying all along that if the pressure on the front is the same as that on the back, then the air sliding down the backside is returning the energy to the car that it took to move it out of the way, this to me is the fundamental basis for aero design.
Yes,it's all about pressure recovery.If there's any separation,the wake base pressure will the same as at the point of separation.

  Reply With Quote
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 02-22-2012, 09:15 PM   #262 (permalink)
EcoModding Apprentice
 
ERTW's Avatar
 
Join Date: Jan 2012
Location: Toronto
Posts: 120

Bu - '08 Chevrolet Malibu LS
90 day: 32.29 mpg (US)
Thanks: 50
Thanked 65 Times in 31 Posts
Quote:
Originally Posted by KamperBob View Post


Thanks, dude! Now I'm really looking forward to how this compares with your prismatic template model.

Your earlier point, ERTW, about modeling spheres as known problem solutions is actually a great idea!

So what was the trick for cutting computation time?
Actually Bob, the sphere example is in the software tutorial

It seems there is no trick I need a multicore cpu! Volvo quotes 500 to 1000 iterations for solid results, however, they did complex bodies and I'm doing simple shapes I find that the solver never quite converges - it just oscillates about a value. It 'stabilizes' within 10 iterations I should take the average of a bunch of data points.

My research yesterday totally blew me away! ...
  Reply With Quote
The Following User Says Thank You to ERTW For This Useful Post:
aerohead (02-27-2012)
Old 02-23-2012, 03:17 AM   #263 (permalink)
kah
EcoModding Lurker
 
Join Date: Jan 2012
Location: Denmark
Posts: 33

The Mii - '12 Seat Mii Reference
90 day: 54.83 mpg (US)
Thanks: 2
Thanked 6 Times in 5 Posts
Saw a Citroen Xsara Picasso on the streets yesterday and this thread immediately came to mind.. I wonder how good the aero of this car is in reverse. It actually looks like PSA had the teardrop in mind when they designed it, but somehow got confused as to which way it should face.
  Reply With Quote
Old 02-23-2012, 03:19 AM   #264 (permalink)
kah
EcoModding Lurker
 
Join Date: Jan 2012
Location: Denmark
Posts: 33

The Mii - '12 Seat Mii Reference
90 day: 54.83 mpg (US)
Thanks: 2
Thanked 6 Times in 5 Posts
Photo didn't attach to my previous reply.. here's hoping it works.
Attached Thumbnails
Click image for larger version

Name:	2011-Citroen-Xsara-Picasso-White-Side-View-Pictures.jpg
Views:	69
Size:	15.8 KB
ID:	10318  
  Reply With Quote
Old 02-23-2012, 10:02 AM   #265 (permalink)
Rat Racer
 
Fat Charlie's Avatar
 
Join Date: May 2011
Location: Route 16
Posts: 4,150

Al the Third, year four - '13 Honda Fit Base
Team Honda
90 day: 42.9 mpg (US)
Thanks: 1,784
Thanked 1,919 Times in 1,246 Posts
Try it again. I think you need 10 posts before you can post a pic.
__________________

Quote:
Originally Posted by sheepdog44 View Post
Transmission type Efficiency
Manual neutral engine off.100% @MPG <----- Fun Fact.
Manual 1:1 gear ratio .......98%
CVT belt ............................88%
Automatic .........................86%

  Reply With Quote
Old 02-23-2012, 12:16 PM   #266 (permalink)
Master EcoModder
 
euromodder's Avatar
 
Join Date: Jul 2010
Location: Belgium
Posts: 4,667

GasDwarf - '13 Volkswagen up! EcoFuel CNG
Thanks: 176
Thanked 643 Times in 510 Posts
Quote:
Originally Posted by kah View Post
Saw a Citroen Xsara Picasso on the streets yesterday and this thread immediately came to mind.. I wonder how good the aero of this car is in reverse. It actually looks like PSA had the teardrop in mind when they designed it, but somehow got confused as to which way it should face.
LOL.
There's actually a thread on reversed-aerodynamics as well
  Reply With Quote
Old 02-25-2012, 10:03 PM   #267 (permalink)
EcoModding Apprentice
 
ERTW's Avatar
 
Join Date: Jan 2012
Location: Toronto
Posts: 120

Bu - '08 Chevrolet Malibu LS
90 day: 32.29 mpg (US)
Thanks: 50
Thanked 65 Times in 31 Posts
Phil, do you have a front view and a plan view of the template? I don't think I'm getting the shape right. I set ground clearance at 10% of the body thickness with sharp edges and the Cd is coming in at 0.275 :/ Do you have a solid model?

I've also taken a 4:1 rear body with a 1:1 front end (same length as a 2.5:1 ellipse) which comes out to Cd 0.247. Using a 6:1 rear body (used to model dolphins) only dropped it to 0.243. Both had a 10% fillet and 10% ground clearance. What do you think?

edit: I rounded the bottom edge (R5" or 10%)...Cd 0.191, CL ~ 0.055. Sharp leading edges are a no no! I want to protrude the nose a little into the wake, and sharpen up the trailing edge. I believe it "wants" a 20% radius on the leading edge, and ~12.5 max angle. I think 25% ground clearance is closer to your template (and helps get it above the earth's boundary layer). I'll report it asap.
Attached Thumbnails
Click image for larger version

Name:	trajectory.jpg
Views:	145
Size:	108.0 KB
ID:	10346   Click image for larger version

Name:	isolines.jpg
Views:	136
Size:	54.9 KB
ID:	10347  

Last edited by ERTW; 02-26-2012 at 12:48 AM..
  Reply With Quote
Old 02-26-2012, 12:23 PM   #268 (permalink)
Master EcoModder
 
Join Date: Dec 2011
Location: Boise Idaho
Posts: 842
Thanks: 39
Thanked 89 Times in 69 Posts
ertw, the air separates where the body turns down. What is the angle in relation to the floor there?

Also, are you modelling a fixed floor, or a moving floor like the real world is?

Also, can you model with ZERO ground clearance?

The "optimal" shape has always confused me - it violates the 20 degree slope, and I don't understand how the air is supposed to stay attached.

I bet if you change the slope of the roof to be the same all the way back, CD will improve.

I'd be curious if you just chopped the car at the slope change if the CD will improve.
  Reply With Quote
Old 02-26-2012, 02:27 PM   #269 (permalink)
EcoModding Apprentice
 
ERTW's Avatar
 
Join Date: Jan 2012
Location: Toronto
Posts: 120

Bu - '08 Chevrolet Malibu LS
90 day: 32.29 mpg (US)
Thanks: 50
Thanked 65 Times in 31 Posts
I'm already ahead of you drmiller...the solver finished this morning after 12 hrs and 373 iterations!

I learned a huge lesson this morning. The goal plot was pretty flat at 100 iterations. Even at 200...and then around 220 it became a step function It was flat out until 350 iterations - and I was ready to stop the solver there...and then another step. wtf! It goes to show that what I assumed was "substantially" accurate may not be. I'll just let the solver finish from now on!

A moving floor becomes useful below 5" of ground clearance. At 13", it's more free air, than ground proximity. If the under body is rough, then it may be of benefit to drop the car, and force more air over the body. If the under body is smooth, it's more beneficial to have higher ground clearance. It's a trade off that's best explored in a wind tunnel.

I have the tail at 15, and it still appears to detach somewhat. I hypothesise that ~12.5 is max. I believe the largest loss is the vortices. I think we could stop streamlines from migrating from the bottom, around the sides by widening the tail...more like a duck tail than a point.

rho = 1.205 kg/m @ 273.15 K
v = 30 m/s (108 kph)
A = 2.138 m (23.0 ft)
Fx = 108.68 N

Cd = 0.094

Drag index = 0.20 m (2.156 ft)

did I do good papa?
Attached Thumbnails
Click image for larger version

Name:	streamline front.jpg
Views:	90
Size:	94.9 KB
ID:	10350   Click image for larger version

Name:	streamline bottom.jpg
Views:	124
Size:	101.3 KB
ID:	10351   Click image for larger version

Name:	streamline rear.jpg
Views:	113
Size:	124.3 KB
ID:	10352   Click image for larger version

Name:	dimensions side.jpg
Views:	101
Size:	72.0 KB
ID:	10353   Click image for larger version

Name:	dimensions.jpg
Views:	99
Size:	92.3 KB
ID:	10354  


Last edited by ERTW; 02-26-2012 at 02:46 PM..
  Reply With Quote
The Following 10 Users Say Thank You to ERTW For This Useful Post:
blownb310 (02-27-2012), Cd (02-26-2012), ChazInMT (02-26-2012), jime57 (02-27-2012), KamperBob (02-27-2012), NeilBlanchard (02-26-2012), skyking (02-26-2012), slowmover (02-28-2012), t vago (03-21-2012), Vekke (04-14-2012)
Old 02-26-2012, 02:49 PM   #270 (permalink)
Master EcoModder
 
skyking's Avatar
 
Join Date: Apr 2011
Location: Tacoma WA
Posts: 1,367

Woody - '96 Dodge Ram 2500 SLT
Team Cummins
90 day: 23.82 mpg (US)

Avion and Woody - '96 Dodge/Avion Ram 2500/5th wheel combo
90 day: 15.1 mpg (US)

TD eye eye eye - '03 Volkswagen Beetle GLS
90 day: 49.05 mpg (US)

Mule - '07 Dodge Ram 3500 ST
Thanks: 719
Thanked 500 Times in 326 Posts
Cool! Thanks so much for taking a hard look at it.
Quote:
A moving floor becomes useful below 5" of ground clearance. At 13", it's more free air, than ground proximity. If the under body is rough, then it may be of benefit to drop the car, and force more air over the body. If the under body is smooth, it's more beneficial to have higher ground clearance. It's a trade off that's best explored in a wind tunnel.
1) Keep in mind that the real world still uses tires and wheels, and the farther you lift a body up the more the wheel frontal area and disturbance of the rotating disc come into play. There is no practical solution for fairing in the steering axle in particular, unless you have the budget of a race car company.
2) there is virtually always a crosswind component. A shape that kicks butt straight down the road will lose to one that is tested at a 1~3 degree crab angle, because that angle is reality.
3) #2 affects #3 as well. A disturbance from the wheel, projected across the underbody at an angle may really toss a wrench in the works, so to speak.

__________________




2007 Dodge Ram 3500 SRW 4x4 with 6MT
2003 TDI Beetle
2002 TDI Beetle

currently parked - 1996 Dodge 2500 Cummins Turbodiesel
Custom cab, auto, 3.55 gears
  Reply With Quote
Reply  Post New Thread


Thread Tools


Similar Threads
Thread Thread Starter Forum Replies Last Post
Aerodynamic Streamlining Template Part-B aerohead Aerodynamics 8 05-31-2013 12:23 PM
LED Headlight captainslug DIY / How-to 82 11-15-2011 01:32 AM
Maximum angle for boat tail? abcdpeterson Aerodynamics 72 10-25-2009 04:37 PM
Aerodynamic Streamlining Template: Part-A aerohead Aerodynamics 0 07-18-2009 02:37 PM
All items I scanned in the new product showcase dremd The Lounge 0 11-08-2008 04:14 PM



Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2021, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright EcoModder.com