Aerodynamics

This is the place for posts that don't fit into any other category.

Moderator: RichardW

yangreen
Posts: 381
Joined: 25 Jan 2005, 14:11
Location:
My Cars:
Contact:

Post by yangreen »

I was thinking about that actually. Clarkson always claims that the All-agro was more aerodynamic going backwards. I'd love someone to research this! What about the 2cv? The DS? Most sporty coupes? The only thing that would probably be worse is a Volvo Estate. Or a Ford Anglia...
tomsheppard
Posts: 1801
Joined: 19 Dec 2002, 14:46
Location: United Kingdom
My Cars:

Post by tomsheppard »

Cobblers FandF. Estates are longer than hatchbacks, therefore have a lower drag coefficient due to reduced Form Drag. With your professed qualifications, shouldn't you have known that?
User avatar
Kowalski
Posts: 2557
Joined: 15 Oct 2003, 17:41
Location: North East, United Kingdom
My Cars: Ex 05 C5 2.0 HDI Exclusive 145k
Ex 97 Xantia 1.9TD SX 144k
Ex 94 Xantia Dimension 1.9TD 199k

Post by Kowalski »

Estates tend to be more vertical at the rear than saloons and hatchbacks, and one thing smooth air flows don't like doing is going around corners, i.e. going smoothly along the roof then going vertically down the back of the car. Directing the air down the rear screen of a hatchback or saloon will tend to produce lift, that was part of the reason that people had accidents with the first Audi TTs.
tomsheppard
Posts: 1801
Joined: 19 Dec 2002, 14:46
Location: United Kingdom
My Cars:

Post by tomsheppard »

But estates are longer. I agree that air causes turbulence when it closes behind an object but the Kamm tail is cut off sharply and it works.
adamskibx
Posts: 250
Joined: 29 Nov 2004, 01:46
Location: United Kingdom
My Cars:

Post by adamskibx »

I think im right in saying that drag co-efficient refers to the efficiency in which an object travels through the air in relation to its front on cross sectional area-therefore a big car with a drag co-efficient of 0.34 will take more energy to travel at a given speed than a small car with the same drag co-efficient of 0.34. Thing is its easier to make a big car have a low drag co-efficient as the passenger area and position makes up a smaller area of the total car so the shape can be for aerodynamic reasons rather than to fit the passengers in comfort. I remember reading that the AX has a drag coéfficient of .27 but I may be wrong, which is impressive for a small car. Im not sure what part lenght plays but I know estate cars always have a slightly lower top speed when you read the back of car manuals.
p20
Posts: 143
Joined: 05 Nov 2004, 21:29
Location:
My Cars:

Post by p20 »

Anyone remember the Volvo 850 btcc touring cars?? http://www.supertouring.co.uk/the_cars/ ... ate_2.html
According to this site, there wasn't much between the estate and the saloon, but the estate had better downforce at speed.[:D]
tomsheppard
Posts: 1801
Joined: 19 Dec 2002, 14:46
Location: United Kingdom
My Cars:

Post by tomsheppard »

AND LESS DRAG!!
FORM DRAG IS A FUNCTION OF LENGTH .
(As Mr R should have known with his flying experience if he is to be believed.)
Now how many times do you all need telling?
User avatar
fastandfurryous
Posts: 1388
Joined: 07 Jul 2004, 17:57
Location: On the road, travelling at high speed. Meep Meep.
My Cars:
x 4

Post by fastandfurryous »

My knowledge and understanding of fluid dynamics, Drag, Cd, Cl, CFD, flow divergance/convergance, etc.etc. is just fine!
Personally, I think you may be referring to reynolds number, and boundary layer thickness, which is indeed a function of length. Above Re=2000, when the boundary layer becomes turbulent, drag is indeed reduced, although this is more in reference to skin friction rather than turbulence and eddys generated behind a non-convergant surface.
After all, in aircraft basics, the larger the chord of the wing (from leading to trailing edge), the greater the drag. (and usually the greater lift too).
You don't see it these days, but when car manufacturers used to quote fuel consumption figures in Urban/56mph/75mph it was always noted that in steady-speed situations, the saloon/hatch version of a particular model would always achieve 3-4mpg more than the estate. I have the figures for a 1.7TD 405 somewhere.
yangreen
Posts: 381
Joined: 25 Jan 2005, 14:11
Location:
My Cars:
Contact:

Post by yangreen »

Not sure the AX is that aerodynamic Adam! They did produce a one off super-smoother AX for fuel efficiency tests. I think they were trying to get 100mpg out of an AX diesel. Can't remember what they actually achieved.
adamskibx
Posts: 250
Joined: 29 Nov 2004, 01:46
Location: United Kingdom
My Cars:

Post by adamskibx »

Umm yeah not sure where I got that figure from yangreen- does sound a bit silly looking at it now- Im still not sure what the overall conclusion is here- all these theories of drag are great but at the end of the day estates are slower than saloons with the same engine in general, and like fastandfurious says, use a bit more fuel- top speed wise this can only be down to better aerodynamics of the saloons-surely?
DoubleChevron
Posts: 622
Joined: 22 Sep 2003, 18:06
Location: Australia
My Cars:
Contact:

Post by DoubleChevron »

Hi Guys,
if you look at the spec's for the base model, they are alway quite a bit more slippery. eg: The CX super with narrow 175/80 tires is considerably more slippery than the GTi counterpart with wide tires and spoilers ... Mainly I'm guessing due to the tire width.
I remember reading about this previously the Renault 25 was very good from memory, the GS was exceptional for it's time (and apparantly the only car around the gave good results in everyones windtunnels --not just the manufacturers) ... Amazing facts, the Etype Jag had a cd - .44 ... More than a VW Kombi [:0]
seeya,
Shane L.
Homer
Posts: 1503
Joined: 26 Feb 2003, 10:52
Location: Yorkshire
My Cars: Current:
Volvo V60 D4 180

Previous:
BX16RS (two of),
BX19TZI,
Xantia 2.0i saloon,
Xantia 2.0 Exclusive CT turbo Break,
Peugeot 807 2.0 HDi 110,
Renault Grand Scenic, 2.0 diesel (150bhp)
C5 X7 2.0 HDi 160 which put me off French cars possibly forever
x 16

Post by Homer »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by adamskibx</i>

Umm yeah not sure where I got that figure from yangreen- does sound a bit silly looking at it now- Im still not sure what the overall conclusion is here- all these theories of drag are great but at the end of the day estates are slower than saloons with the same engine in general, and like fastandfurious says, use a bit more fuel- top speed wise this can only be down to better aerodynamics of the saloons-surely?
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
You need to take gearing into account as well.
User avatar
fastandfurryous
Posts: 1388
Joined: 07 Jul 2004, 17:57
Location: On the road, travelling at high speed. Meep Meep.
My Cars:
x 4

Post by fastandfurryous »

<blockquote id="quote"><font size="1" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by DoubleChevron</i>

Amazing facts, the Etype Jag had a cd - .44 ... More than a VW Kombi [:0]
<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">
true indeed, But:
The actual Drag force on a car is the product of Cd, Air density, Frontal area, and the squate of the speed. The Cd of an e-type is higher than that of a VW combi (and of a large number of other cars too) but it's frontal area is tiny. Thus, if you do the calculation from D=0.5xCdxPxAxUxU (that should be "half Cd Rho a U squared") the total drag for any given speed on an E-type is substantially less than that of a bigger vehicle with a better drag co-efficent.
The best description of Cd I've heard is that it's the relationship between the frontal area of an object, and the area of turbulence it leaves behind. This is why any object with a smooth taper at the back (like say an aircraft), which leaves very little turbulent air behind it, has a low Cd.
FrenchLeave
Posts: 359
Joined: 18 Jan 2005, 21:47
Location: 5 miles N. of Boston, Lincs
My Cars:

Post by FrenchLeave »

Whilst our discussions have concentrated on the visible body shape, there are two other major hidden factors:
Firstly the underbody shape. There have been some real horrors with footwells, spare wheels, exhaust systems and all sorts of rude mechanical bits hanging down and upsetting the smooth airflow. I remember that when the DS was first introduced, the Motor Show stand featured a DS body suspended to show its smooth undersurface, this must have contributed to the low Cd figure. Nowadays, some manufacturers fit a "chin" below the front bumper to deflect the air around the sides of the underbody.
The other hidden factor is the engine cooling air supply which has to find its way through all sorts of obstructions before exiting under the car.
As far as aircraft are concerned, the generation of lift automatically produces a disturbed airflow behind the aircraft, referred to as the wake. The wake behind a Jumbo has been known to flip a light aircraft onto its back ten miles behind it. And on final approach, with flaps and wheels down, it's even worse!
James.UK
Posts: 1169
Joined: 14 Dec 2003, 23:12
Location: United Kingdom
My Cars:
x 2

Post by James.UK »

The "E" type Jags windscreen was very upright, I imagine that made things far worse. [V]
Re Tyres, doe the fact that the they are rotating, and thus creating their own tubulance reduce the drag they create? [?]
Useless piece of information.. Hulls.. Most powerboat hulls have a severe step in them (step up toward the stern) so as to create a mixture of air and water (bubbles-foam) under the hull to reduce suction.. [8)] [:)]
.
Post Reply