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).

http://ecomodder.com/forum/attachmen...6&d=1269631462

[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.

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?

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.

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.

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.

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.

http://ecomodder.com/forum/attachmen...1&d=1269660868

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.

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.

Source
 

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.

Should we call it a "rear-end-breather"? ;)

breather
 

Why not!?
And they could have a 'coefficient of inhalation' CI!
Just be careful inside elevators after eating alot of beans.

LOL--I used to explain the engine EGR system as "fart sniffing" for cars.

sniffing
 

They'd probably never let us teach at high school level.

...ah, but it's OK at the next level up, at the college-level (ha,ha).

Taking those measurements from that diagram to my car, its (Hsp) is 2 inches and its (Hnadw) is 44 inches, so its ratio is 4.5%, right in the middle of the negative range.

Thanks joos,

From previous pictures, I though you might be close to the minimal value.:thumbup:

Farts aside;) -

is there some sort or internet reference where we might see what this looks like?

crawl under
 

If you run across a Corvette or Trans Am Firebird it'd be worth your while to crawl under for a peak.( be prepared to explain yourself to the owner ).
There might be some stuff on the web but there's nothing like looking the beast in the face.
The distance ahead of the radiator is a big issue,as is air-tightness.The thing won't work without the airdam and they ride very low to the ground.
Four-season driveability could be an issue.I've never 'lived' with one of these,don't know.
They cut the Firebird's Cd by 0.011 so it's something worth looking into,no pun intended.

Neighbor has a late model Corvette. Next time he has the car cover off, I'll ask for a look.

Take your camera with you and post some pics, please.

Hmm, I don't think that's happened before: For a moment, I was contemplating going to a Chevy dealership to look at the inventory.

Where can I read about cars with radiators at the rear? I would love to seal the engine bay against salt, have finned cooling pipes running the length of the car, and have enough thermal capacitance on the cold side of the thermostat to complete my commute without any airflow across the radiator.

My boss at work has a Camaro with this system. There's a wide thin radiator air inlet just ahead of the airdam and the radiator air is exhausted behind the airdam. It uses the high pressure ahead of the airdam combined with the low pressure behind the airdam to drive air through the radiator. He mentioned a friend with a Camaro who over ran a parking curb when parking at a store, breaking off the airdam, and then it started overheating when he drove it home afterwards. So the airdam is critical for the proper functioning of the cooling system.

It has had slight discussion here:

http://ecomodder.com/forum/showthrea...etup-3651.html

I had some other sites, but seem to have lost them. Did get a look at neighbors C4 Vette and I must say that it is a very impressive setup. Front completely devoid of any openings. Big underbumper intake just in front of a thin air dam. extensive internal ductwork. I was impressed. The "Insight" question might be, "Is it worth giving up the really smooth front underbody in order to execute a bottom breathing arrangement?" The current arrangement may be just as effective, but interesting question.

Robert,I would try a GOOGLE search for the Ford Probe-IV concept car.
That car was front-engine,but they piped the coolant to a quarter-panel located forced-air heat-exchanger which when under electric fan power,would actually propel the car to 4-mph.
The AC condenser was located on the opposite side.
Also,the coolant traveled to and from in an extruded aluminum heat-sink tube.

Made an arrangement with my neighbor to take pictures of his Corvette this weekend. Now if I hope I can figure out a way to get them posted.