Re: Kamm and exhaust smells

[C M Smith <cmsmith@xxxxxxxxxxxxxxx>]

As anyone who has a station wagon (sort of the ultimate expression of the 
Kamm tail) knows, the shape drags a blob of turbulent air behind it. That 
is why the back window gets so dirty so quickly, unless a suitable spoiler 
is mounted to the end of the roof to deflect airflow over the back window 
(not recommended as this increases drag a lot) or one does as SAAB has done 
for many years and design hatchbacks or sedans with proper rear 
aerodynamics to keep a decent boundary layer on the rear surfaces to allow 
good airflow over the rear of the car.

The effect of the chopped tail is felt if the exhaust happens to exit into 
this area of turbulence the exhaust will be drawn into the blob and, 
because the effectively stationary blob of turbulent air is at higher 
pressure than the flowing air, and often the air inside the cabin, exhaust 
can be pushed into (or drawn in if you prefer)  the cabin area, or over the 
cockpit of an open car.

In my desire to be brief in my earlier post on this subject ( 
brief??  8-)  )  I gave a slightly inaccurate description of the drag 
advantage of the Kamm tail, other posts were more accurate. I had also 
forgotten about the practical difficulty of achieving laminar flow over the 
entire surface of a teardrop shape. Notwithstanding this, it is possible to 
design a real shape that maintains laminar flow over most, if not all of 
the surface area, just not in the shape of an automobile that will actually 
fit on a road or race track. Laminar flow is more often relevant in boat 
hulls, sails, and aircraft.

The Kamm tail does develop more form drag than the teardrop shape but less 
skin drag. Skin drag becomes a significant factor at very low speeds and 
also higher speeds, in terms of the proportion to total in the former case 
and effect on total drag in the latter case. In effect, by chopping the 
tail you can reduce total drag as compared to total drag from the 
reasonably practicable alternative shapes.

Also, the Kamm tail is much better from the point of view of lift forces on 
the bodywork. It is the development our understanding of aerodynamic 
downforce that has made variations of the Kamm tail more or less essential. 
In this case, minimum drag is definitely not the objective as generally 
speaking downforce is obtained at the expense of increased drag. The long 
tail and short tail versions of the Porsche 917 explored most of these 
issues in the development of that very successful race car.

As for the boat tail versus the Kamm tail Spider it should be clear that an 
open top car, whether the top is up or down is inherently bad from an 
aerodynamic point of view. Even if the top goes up, the skin drag is just 
awful, the form drag is adversely affected by distortion of the top 
material and it is impossible to design a convertible roof that gives you 
good drag numbers. Top up or down, by the time airflow reaches the back of 
the cockpit it doesn't matter much what the end of the car looks like.

Finally, low coefficient of drag is one thing, total drag is entirely 
another. Basically, a low coefficient of drag can be obtained with a large 
cross sectional area and not very good airflow but the total drag developed 
may be much higher than a shape with a higher coefficient of drag. This Cx 
number is merely the drag of the shape divided by the drag of a flat plate 
of the same cross sectional area, perpendicular to the flow. That's why the 
number is expressed without units, there aren't any it's just a ratio. 
Think of the old Chevy Caprice which achieved a lower coefficient of drag 
by bloating out the form in its final version. No one ever claimed that the 
new bigger frontal area car was actually more efficient than the former 
smaller creased bodywork design. In fact, creased bodywork can give better 
total drag numbers than rounded shapes for reasons analogous to the so 
called Kamm effect. Airflow that detaches cleanly from a  sharp edge can be 
more efficient than airflow that is encouraged to be laminar by smoothly 
contouring the shape but then breaks away in a turbulent fashion.

Aerodynamically efficient shapes have small cross sectional areas together 
with low coefficients of drag yielding low total drag numbers. Whether they 
will remain stuck down to the road at high speeds is another question 
entirely, ask Mercedes about Le Mans and Audi about the TT on the Autobahns!

Cheers


Michael Smith
White 1991 164L
Original owner
--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided