Does re-attached flow negate prior drag ?

[Piotrsko]

And to be even more obscure, sometimes turbulent airflow produces less drag than laminar airflow on some NASA profiles. It's a boundary layer and may not be very thick or measureable

[aerohead]

Originally Posted by Cd

I remember reading in a magazine decades ago, when a "GM Engineer " explained that the cD was nearly identical for both the 3rd generation Camaro and Firebird, despite the Camaro having bucket headlights.
( CarCraft magazine )
He explained that the air pooled and created a swirling pocket of air that oncoming air skipped around.
So the drag that was being created by the turbulence in the headlight buckets was "cancelled out" by the airflow skipping over the turbulent air.
Even back then as a teen, that didn't make sense to me.
Any oncoming air would BECOME turbulent air as it met the "swirling pocket of air "and it would create drag.
Also, all air flowing over a turbulent body eventually becomes stagnant again farther downstream anyway.
I'm guessing the amount of drag created is a result of how quickly the airflow goes back to it's stagnant state.
It doesn't cancel out the drag that was created.

But I sure would like to be wrong.

Anybody have any comments ?

As the GM engineer rightly commented, it's not 'turbulence' in the bucket, it's just 'pooled' air. Dead air.
Oncoming flow cannot displace it, and it cannot compress it. It experiences it as if it were a solid structure, and just skips over the bubble formed out in front of it.
https://ecomodder.com/forum/member-a...ing-follow.jpg

[aerohead]

Originally Posted by Cd

I remember reading in a magazine decades ago, when a "GM Engineer " explained that the cD was nearly identical for both the 3rd generation Camaro and Firebird, despite the Camaro having bucket headlights.
( CarCraft magazine )
He explained that the air pooled and created a swirling pocket of air that oncoming air skipped around.
So the drag that was being created by the turbulence in the headlight buckets was "cancelled out" by the airflow skipping over the turbulent air.
Even back then as a teen, that didn't make sense to me.
Any oncoming air would BECOME turbulent air as it met the "swirling pocket of air "and it would create drag.
Also, all air flowing over a turbulent body eventually becomes stagnant again farther downstream anyway.
I'm guessing the amount of drag created is a result of how quickly the airflow goes back to it's stagnant state.
It doesn't cancel out the drag that was created.

But I sure would like to be wrong.

Anybody have any comments ?

The drag we're interested is 'pressure drag', which is the difference in static pressure between the forward stagnation point, and the base pressure of the wake.
A perfectly-streamlined car will still have drag, owing to kinetic energy lost to viscous shearing within the turbulent boundary layer.
Any car short of fully streamlined will have additional drag due to flow separation, along with induced drag due to vortices, which are a separation-induced phenomena.
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Some 'drag' caused by separation, can be mitigated by a spoiler, which allows flow re-acceleration, elimination of the adverse pressure gradient responsible for the separation, re-attachment onto the spoiler, and 'planned' separation off the spoiler, where the flow is decelerated, slower, of higher pressure, creating higher base pressure, lower pressure drag, lower drag.
Goro Tamai refers to this process as 'mending' the flow.
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Any surviving turbulence caused by separation reflects energy completely lost to heat. This chaotic kinetic energy of the turbulence is dissipated far downstream of the car by the viscous shearing, just as stirring a beaker full of liquid in a chemistry lab will ultimately raise its temperature.