Re: Kamm tails and drag

["Joseph Kanellopoulos" <kanellj@xxxxxxxxx>]

Its been a million years since I took fluids and maybe I did not understand
the email thoroughly but my impression about shapes like that of the
teardrop are that they are able to keep laminar flow going (i.e. avoiding
boundary flow detachment), hence not producing turbelent flow, which in turn
produces a pressure differential between the front and the back of the shape
resulting in pressure drag.

So from reading what you just said any turbelent flow in the back, resulting
in pressure vacuum right behind the car would result in pressure drag.

"Turbelent flow" is only good, in the sense of having air eddies, like those
found in a golf ball, or in certain motor cycle helmets and my understanding
is that again those eddies are there to help keep the air flow laminar in
higher speeds than it would be possible without them, and then again, those
little dimples cause higher air drag at lower speeds than a smooth golf ball
would have at those lower speeds possibly because the total surface area of
the ball might have increased but I never got the answer to that one...

Now I am a lowely EE so all you MEs out there please jump in and clear this
up for all of us....

J-

----- Original Message -----
From: "C M Smith" <cmsmith@domain.elided>
To: <alfa@domain.elided>
Sent: Saturday, April 12, 2003 9:56 PM
Subject: Kamm tails and drag


> I thought the Kamm tail effect resulted from the fact that the Kamm tail
> caused the tail to drag a blob of turbulent air behind the tail that
> approximated the airflow of the long tailed tear drop shape previously
> thought to be ideal, which reduced skin drag due to air to air friction
> being lower than air to bodywork friction. The increase in efficiency is
> due to the reduction in skin drag resulting from the chopped tail and the
> fact that air to air friction is lower than air to bodywork friction of
the
> long tail version. Certainly Porsche experimented successfully with the
> long tail Le Mans design that also had a Kamm tail but further back along
> the ideal teardrop shape. I do not think that turbulence associated with
> tear drop shape could be a factor since the tear drop has very little
> turbulence associated with it. Skin drag is a huge factor at high speeds.
> Turbulence drag is a problem at low speeds also, hence the amount of money
> devoted to reducing turbulence drag from door mirrors and rain gutters to
> cite two mundane examples.
>
> cheers etc
> Michael Smith
> White 1991 164L
> Original owner
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