Does re-attached flow negate prior drag ?

[Cd]

Dumb question of the week : Does re-attaching flow negate drag prior to the cause of the drag ?
Example : Darin's Insight and Civic boattail designs with open tail lights.
There is turbulence just after the tail lights, but it reataaches farther on.

I would think that of coarse it doesn't - it just smooths the flow later on, so that the air does not continue to produce even more drag.
Duh.

[freebeard]

Does drag only have affect in the longitudinal direction? There would be a trig function in resolving the portions at a shallow angle?

[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 ?

[Piotrsko]

As I understand it: drag is the moving air gaining then losing energy and becoming at rest again. Stagnant? Perhaps but an odd choice of word.

Moving air takes energy from the device moving it and causes ancillary after effects because air has mass. Even parked cars exhibit drag but it's tiny and perhaps immeasurable but does exist during say tornadoes.

At this point I suspect zero drag in an atmosphere is impossible. Perhaps even in intersteller space. Feel free to prove me wrong. Eventually musks tesla will be parked someplace in the universe and fall into a gravity well.

[Cd]

Thanks for the reply, and thank you for explaining it in such easy to understand language.
I have a learning disability, but also a fascination with aerodynamics. This means I have a lot to learn.
I would imagine I am not the only person that visits the forum that could benefit from answers to idiotic questions like mine.
I would imagine there are some children with these same questions.

So from what I undesrand, I think you are saying that, like I suspected, once drag is created, there is no way to cancel it out by bringing the airflow back to a steady state ?
What about the Camaro, vs. Firebird example I mentioned earlier ?
Is it possible that some air gets trapped in the headlight buckets and really causes air to skip over it ?
I would think that any oncoming air would simply get sucked into the vortex there and result in a cycle of continued drag, rather than skipping over the area.
However, I also think back to how air curtains can act.

Thanks for your feedback.

[Piotrsko]

Take a big rock in the middle of a fast river. Somewhere down stream the flow is smoothed again but the rock is still creating drag which attempts to move the rock downstream as long as the river is flowing. Putting another rock behind the first rock and you get more drag, but maybe not twice as much total drag as the first rock even if the second rock is the same size or even bigger, mostly because the flow around the second rock had to slow down after the first rock. So drag is speed related.

A vortice in a headlight bucket can make the headlight bucket look bigger because it extends past the bucket and disturbs air flow outside of the bucket. Sometimes those vortices reduce the airflow speed hitting the windshield (or something else). Big second rock has reduced drag, so does the windshield. Sometimes you can fool flows into smoothness with shapes that dont look smooth. THAT is magic to me.

Another concept is that drag adds but doesn't subtract. Once you have it, it doesn't really go away, even after some distance. Second rock has visible effects in the stream for a good distance until the flow has had a chance to speed back up. You could build channels in the river to help re-direct and re smooth that flow, but I do not know how to do that, but I can do it just a little bit with wings and flaps because I know how to trick the air into flowing smoother. Most of the time I make things worse.

The difference between a car and the rock is what is moving to disturb the flow. The air is mostly still and the car moves, whereas the rock is stationary and the river moves.

I grok the learning difficulty. There are things in my head that I cannot share because the language portion goes on strike or makes me look foolish.

[Cd]

Piotrsko - That was PERFECT !
I wish I could find a book ( or video ) that explained aerodynamics like that.

[Piotrsko]

Well thank you. However, being in the same learning boat helps me understand your needs. Hopefully you realize that was a really long distance standoff observation probably prone to inaccuracy.

[aerohead]

Originally Posted by freebeard

Does drag only have affect in the longitudinal direction? There would be a trig function in resolving the portions at a shallow angle?

There's no general comment that can be made. We'd have to limit things to a specific case, conditions.

[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 ?

One reason for bringing the smoke flow images back was so, everyone could 'see' the Lanchester/Prandtl surfaces of discontinuity out ahead of all the cars.
This is exactly the evidence that laminar flow will travel over stagnation bubbles, and that 'buckets' aren't necessarily 'parachutes.'
Allowing for anthropomorphism, the air is already trying to figure out how to get around the car, long before it actually 'arrives.'