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

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 09-03-2020, 03:40 AM   #1 (permalink)
Master EcoModder
 
JulianEdgar's Avatar
 
Join Date: Nov 2017
Location: Australia
Posts: 1,804
Thanks: 98
Thanked 1,451 Times in 1,025 Posts
Models that don't work on real cars

Aerohead has a belief, that has unfortunately been apparently adopted by many people here, which is quite wrong.

That belief is that The Template (and roughly similar shapes) have zero lift. Of course, these shapes have in fact very high lift.

Aerohead often quotes references to support this incorrect belief, but the clearest and most accessible he has quoted is Figure 2.4, page 51 of Hucho (second edition).

So let's look at that diagram. (Note that I not going to (mis)quote or paraphrase, as Aerohead does so often. Instead, I will reproduce the diagrams and text.)

The diagram Aerohead references:



Now, what does Aerohead say about it? He said:

Quote:
Originally Posted by aerohead View Post
If you'll revisit Figure 2.4, page 51 of Hucho, you can see how,over the last 14.5% of the body, local pressure rises all the way back to local barometric pressure. Depending on rear overhang, and low pressure under the body, due to a diffuser, rear lift can be zero, like the VSPORT's.
Note how there are no caveats or qualifications in what Aerohead says - he's accepting the diagram as being applicable to real cars. But look at the caption. The diagram is for inviscid fluid - that is, an imaginary fluid without viscosity!

And does that matter? Let's look at what is written on the next page:



Note from the passage:

On the rear part of the vehicle's upper surface a steep pressure rise occurs, and it is in this region where considerable differences exist between the real flow of a viscous fluid and the inviscid flow shown here.

and

If all X-components of the pressure distribution on the vehicle surface are integrated, the result for the drag will be D=0.

and

In the real, viscous flow there exists a drag force, but it cannot be explained by considering an ideal, inviscid fluid.

None of this is much of a surprise to me - if you measure real stuff on real cars on real roads, you quickly find where people have misunderstood (or mis-applied) theory.

The idea that Aerohead constantly pushes that low drag = low lift is simply rubbish. It's one of many misconceptions he has, most of which are very easily shown by getting away from the keyboard and doing some real-world measurements on the road.

It's not hard - and it might prevent silly mistakes like referencing a diagram for inviscid fluid as if it applies to real cars.

  Reply With Quote
The Following User Says Thank You to JulianEdgar For This Useful Post:
aerohead (09-04-2020)
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 09-03-2020, 10:07 AM   #2 (permalink)
EV convert
 
oil pan 4's Avatar
 
Join Date: Jul 2011
Location: NewMexico (USA)
Posts: 9,172

Sub - '84 Chevy Diesel Suburban C10
SUV
90 day: 19.5 mpg (US)

camaro - '85 Chevy Camaro Z28

Riot - '03 Kia Rio POS
Team Hyundai
90 day: 30.21 mpg (US)

Bug - '01 VW Beetle GLSturbo
90 day: 26.43 mpg (US)

Sub2500 - '86 GMC Suburban C2500
90 day: 11.95 mpg (US)

Snow flake - '11 Nissan Leaf SL
SUV
90 day: 141.63 mpg (US)
Thanks: 222
Thanked 3,002 Times in 2,337 Posts
When I drove a "new beetle" which is about as close as you can get to the template on a street driven car, that thing generated lift and had horrible cD.
It had to use a spoiler so the back wheels would stay planted to the road surface.
But quoting 2 generations out of date stuff is kind of his thing.
__________________
1984 chevy suburban, custom made 6.5L diesel turbocharged with a Garrett T76 and Holset HE351VE, 22:1 compression 13psi of intercooled boost.
1989 firebird mostly stock. Aside from the 6-speed manual trans, corvette gen 5 front brakes, 1LE drive shaft, 4th Gen disc brake fbody rear end.
2011 leaf SL, white, portable 240v CHAdeMO, trailer hitch, new batt as of 2014.
  Reply With Quote
The Following User Says Thank You to oil pan 4 For This Useful Post:
aerohead (09-04-2020)
Old 09-03-2020, 10:43 AM   #3 (permalink)
Master EcoModder
 
mpg_numbers_guy's Avatar
 
Join Date: Feb 2018
Location: VA
Posts: 1,305

His (Benny) - '06 Honda Insight MT
Team Honda
Gen-1 Insights
Last 3: 78.73 mpg (US)

Hers (Colson) - '13 Volvo S60
Team Turbocharged!
90 day: 34.88 mpg (US)
Thanks: 305
Thanked 449 Times in 341 Posts
Beetles slop down way too fast to conform to the template anyway, not exactly an apples to apples comparison.
__________________
2006 New Formula Red Honda Insight MT, 284k: (Build thread)

2013 Ember Black Volvo S60 automatic, 101k (Fiance's car):
  Reply With Quote
Old 09-03-2020, 11:45 AM   #4 (permalink)
Ecomodding amateur
 
M_a_t_t's Avatar
 
Join Date: Nov 2015
Location: Indiana
Posts: 598

The Van - '95 Chevy Astro Cl V8 Swapped
Team Chevy
90 day: 10.75 mpg (US)

The new bike - '17 Kawasaki Versys X 300 abs
Motorcycle
90 day: 59.04 mpg (US)

The Mercury - '95 Mercury Tracer Trio
Team Ford
90 day: 37.79 mpg (US)
Thanks: 110
Thanked 212 Times in 156 Posts
The simplest way to look at it:

Rounded surface creates lower pressure. Look at an air foil, the top surface creates lift by lowering the pressure. Overall shape of the car is rounded, thus creates lower pressure on that surface (varying levels). Low pressure on the top half of the car is lift.

Flip a car upside down and you get down force right?
__________________
Věci získávají svou skutečnou krásu pouze ve vztahu k našemu životu
-Jiří Mucha (1915-1991), Publicist

1973 Fiat 124 Special
1975 Honda Civic CVCC 4spd
1981 Kawasaki KZ750E
1981 Kawasaki KZ650 CSR
1983 Kawasaki KZ1100-A3
1986 Nissan 300zx Turbo 5 spd
1995 Chevy Astro RWD (current project)
1995 Mercury Tracer
2004 Chevy Astro AWD
2017 Kawasaki VersysX 300

https://www.youtube.com/channel/UC6s...LulDUQ8HMj5VKA
  Reply With Quote
Old 09-03-2020, 05:24 PM   #5 (permalink)
Master EcoModder
 
freebeard's Avatar
 
Join Date: Aug 2012
Location: northwest of normal
Posts: 18,901
Thanks: 5,261
Thanked 6,247 Times in 5,004 Posts
It's been done.



Quote:
When I drove a "new beetle" which is about as close as you can get to the template on a street driven car, that thing generated lift and had horrible cD.
That's because it isn't. At least the first generation. The styling trope can be traced back to the Audi Avus or Quattro, the stylistically arched daylight opening. It made production as the Audi TT. Spoilers save TT owner's lives.

To follow The Template, max camber needs to be at 30% of the length.

Thanks OP, I hadn't seen this one before:



What do you make of the internal dotted lines? They seem to imply that pressure differentials transmit through a solid body. ???

I'm patiently waiting for the inexorable: duckduckgo.com/?q=CFD Open+VDB and Blender. The best link appears to be:
Quote:
Fluid Simulation with Blender - FetchCFD
Search domain http://www.fetchcfd.com/view-project...n-with-Blender
Blender's physics fluid simulator uses the Lattice Boltzmann methods to simulate the fluids and allows easy adjustment of resolution. Blender source file also attached. THIS WORK CAN BE REUSED UNDER THE CC BY LICENSE.

https://www.fetchcfd.com/view-project/192

This will bring it down from supercomputer level to the hobbyist. You don't even need to have a garage full of 2nd hand game consoles Beowolfed into a render farm.
__________________
.

Cold hearted orb that rules the night, Removes the colours from our sight, Red is grey and yellow white, But we decide which is right, And which is an illusion.
The Day Begins -- Moody Blues
_________________
  Reply With Quote
Old 09-03-2020, 05:44 PM   #6 (permalink)
Master EcoModder
 
JulianEdgar's Avatar
 
Join Date: Nov 2017
Location: Australia
Posts: 1,804
Thanks: 98
Thanked 1,451 Times in 1,025 Posts
Quote:
Originally Posted by freebeard View Post

Thanks OP, I hadn't seen this one before:



What do you make of the internal dotted lines? They seem to imply that pressure differentials transmit through a solid body. ???
The lines in the upper part of the diagram show flow directions. The dotted lines show flow around the car.

The key point is that the pressures shown on this diagram do not apply to real cars.
  Reply With Quote
The Following User Says Thank You to JulianEdgar For This Useful Post:
freebeard (09-03-2020)
Old 09-03-2020, 05:57 PM   #7 (permalink)
Master EcoModder
 
freebeard's Avatar
 
Join Date: Aug 2012
Location: northwest of normal
Posts: 18,901
Thanks: 5,261
Thanked 6,247 Times in 5,004 Posts
Got it. According to Simulation Theory it's all varying degrees of verisimilitude.
__________________
.

Cold hearted orb that rules the night, Removes the colours from our sight, Red is grey and yellow white, But we decide which is right, And which is an illusion.
The Day Begins -- Moody Blues
_________________
  Reply With Quote
Old 09-03-2020, 08:06 PM   #8 (permalink)
Master EcoModder
 
kach22i's Avatar
 
Join Date: Jul 2011
Location: Ann Arbor, Michigan
Posts: 3,804
Thanks: 46
Thanked 2,528 Times in 1,769 Posts
Quote:
Originally Posted by M_a_t_t View Post
The simplest way to look at it:

Rounded surface creates lower pressure. Look at an air foil, the top surface creates lift by lowering the pressure. Overall shape of the car is rounded, thus creates lower pressure on that surface (varying levels). Low pressure on the top half of the car is lift.

Flip a car upside down and you get down force right?
I found the first few sections of the link below informative.


https://www.sciencedirect.com/topics...rodynamic-lift
Quote:
Vehicle body aerodynamics
Heinz Heisler MSc., BSc., F.I.M.I., M.S.O.E., M.I.R.T.E., M.C.I.T., M.I.L.T., in Advanced Vehicle Technology (Second Edition), 2002
Quote:
The aerodynamic lift coefficient CL is a measure of the difference in pressure created above and below a vehicle's body as it moves through the surrounding viscous air..........
Quote:
14.3.2 Vehicle lift (Fig. 14.20)
When a car travels along the road the airstream moving over the upper surface of the body from front to rear has to move further than the underside airstream which almost moves in a straight line (see Fig. 14.20). Thus the direct slower moving underside and the indirect faster moving top side airstream produces a higher pressure underneath the car than over it, consequently the resultant vertical pressures generated between the upper and under surfaces produce a net upthrust or lift.
Attached Thumbnails
Click image for larger version

Name:	3-s2.0-B9780750651318500154-f14-20-9780750651318.gif
Views:	29
Size:	30.9 KB
ID:	29089  
__________________
George
Architect, Artist and Designer of Objects

1977 Porsche 911s Targa
1998 Chevy S-10 Pick-Up truck
1989 Scat II HP Hovercraft

Chin Spoiler:
http://forums.pelicanparts.com/off-t...effective.html

Rear Spoiler Pick Up Truck
http://forums.pelicanparts.com/off-t...xperiment.html

Roof Wing
http://ecomodder.com/forum/showthrea...1-a-19525.html
  Reply With Quote
Old 09-03-2020, 08:37 PM   #9 (permalink)
Tyrant-at-large
 
Vman455's Avatar
 
Join Date: Feb 2012
Location: Champaign, IL
Posts: 1,822

Pope Pious the Prius - '13 Toyota Prius Two
Team Toyota
SUV
90 day: 53.63 mpg (US)

Tycho the Truck - '91 Toyota Pickup DLX 4WD
Thanks: 193
Thanked 1,625 Times in 858 Posts
Quote:
Originally Posted by JulianEdgar View Post
The dotted lines show flow around the car.
I don't think this is the case; Hucho is quite clear that this is a two-dimensional section. I believe the dotted lines are meant to represent possible duct paths between intakes and outlets for cooling and cabin ventilation air. The paragraph after the one you posted explains:

Quote:
From the pressure distribution shown in Fig. 2.4, suitable positions for the air intake and outlet for the cooling and ventilation system can be selected. The intake should be placed in regions of high pressure, e.g. in the nose region or in the scuttle, whereas the outlet may be placed in a region of suction.
__________________

  Reply With Quote
The Following 2 Users Say Thank You to Vman455 For This Useful Post:
freebeard (09-03-2020), samwichse (09-04-2020)
Old 09-03-2020, 08:41 PM   #10 (permalink)
Master EcoModder
 
JulianEdgar's Avatar
 
Join Date: Nov 2017
Location: Australia
Posts: 1,804
Thanks: 98
Thanked 1,451 Times in 1,025 Posts
Quote:
Originally Posted by Vman455 View Post
I don't think this is the case; Hucho is quite clear that this is a two-dimensional section. I believe the dotted lines are meant to represent possible duct paths between intakes and outlets for cooling and cabin ventilation air. The paragraph after the one you posted explains:
Yes I think you are right.

  Reply With Quote
Reply  Post New Thread


Tags
uote or paa

Thread Tools




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