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Old 09-08-2009, 12:36 AM   #1 (permalink)
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Reducing notchback drag; New Ideas

Hey everyone, I've been lurking on this site since 07, finally decided to get registered.

Anyway Ive been thinking of how to reduce the turbulence (and thus the size of the wake) behind the rear glass that occurs on most sedans.

I conjured an idea that would reduce the total size of the vehicle's wake to trunk height whilst allowing the driver a clear view out the rear window.

Here my idea is shown on a B8 A4.







To allow full operation of the vehicle's trunk, these strakes would be mounted to the roof with small hinges, and the trunk would be equipped with guides the strakes could freely slide in.

Any input is welcomed. I think this could be potentially easy to build, but would it pay off?

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Old 09-08-2009, 01:14 AM   #2 (permalink)
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Welcome to EM! I like it but I will wait for the aero-masters to weigh in. Would the interior fins be solid or clear?

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Old 09-08-2009, 01:20 AM   #3 (permalink)
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I feel like they'd have to be closer together to make use of the boundary layer effect... They just seem to be far enough apart that they'd induce more turbulence by separating what would normally be a "sheet" of airflow into a largely serrated cross section.

I could be wrong, though. I'm not up on the finer points of aero, yet.
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Old 09-08-2009, 01:35 AM   #4 (permalink)
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Old 09-08-2009, 02:02 AM   #5 (permalink)
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why not slap some plexi on top of it?
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Old 09-08-2009, 02:07 AM   #6 (permalink)
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it's an interesting idea worth looking into, however what's the advantave over a clear plastic camback? that would be less involving to build less intrusive (could clip onto the trunk etc)

that said, i think the vanes at the sides are going to be critical, the c pillars can often generate vortexes, and while modern cars have much more aerodynamicly shaped pillars you could undo this by placing the touter vanes at the wrong angle (both in top and in rear view)

also it could be interesting to place air intakes like a naca duct there so you'd acually pull the air insight tightening the wake

the resulting profile reminds me a bit of the A2 wich was very low drag
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Old 09-08-2009, 02:29 AM   #7 (permalink)
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First impression is that it won't work as envisioned. Sorry.

I say that because they don't do anything to address the drop-off angle of the back glass that causes the turbulence in the first place, like a proper spoiler might.

If there is flow through them it appears to me there would be significant surface drag- lots of surface area there.

I just don't see the mechanism at work here to reduce turbulence except maybe on the outermost pair, and even then...

Rear visibility... sort of. Through the rear view mirror straight back- yes. All else- no.
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Old 09-08-2009, 06:52 AM   #8 (permalink)
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If the curvature is such that the airflow becomes unattached on the back glass, the fins aren't going to make it reattach. They will increase drag by adding additional skin friction. From just glancing at it, the car in the photo likely has attached airflow down its backglass and loses attachment at the back of the rear trunk lid where there appears to be a flat spoiler molded into the metal.
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Old 09-08-2009, 07:26 AM   #9 (permalink)
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Cd of this A4 is supposed to be 0.27 so i think most of the rear aero will be sorted.

i don't think the fins will aid much in their curent form but perhaps fins and spoilers might be more of a future aid to aerodynamics then we think.

the bmw BMW Vision EfficientDynamics for example has spome strange C pillar configuration.


but more recent just about every car that came out recieved small dams in front of the tires, and on some cars these dams take on alternative forms wich makes me wonder if small devices wich deflect the air in more favorable patterns isn't going to be something we'll see more often in the future.


for example the odd alfa romeo bat (batman would drive one) achieve very low drag coeficients with the aid of finlets (with a Cd that low they have to be functional)

i think fins at the rear can have two funtions

1: prevent C pillar vortexes of make the wake less wide,
2: perhaps small central fins, or ribs running in the same direction of the airflow could have a beneficial effect (see the front undertray of the merc)

the problem with all these things is that the airflow patterns
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Old 09-08-2009, 10:55 AM   #10 (permalink)
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I am not an expert on aero, but my thinking was the flow becomes turbulent because the flow detaches from the body. If the flow has nothing to flow upon, the air goes turbulent. Would these strakes not give the air another axis to flow against?

And perhaps the A4 is not the best example, but the idea is meant for the rest of us with less than stellar aerodynamics from the factory.

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