Stagnation point

[Piwoslaw]

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.

[NeilBlanchard]

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?

[Piwoslaw]

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.

[aerohead]

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.

[RobertSmalls]

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.

[jime57]

Originally Posted by RobertSmalls

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.

[RobertSmalls]

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.

[jime57]

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.

[aerohead]

Originally Posted by aerohead

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.

[Piwoslaw]

Originally Posted by aerohead

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"?