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Old 05-29-2011, 07:24 AM   #1 (permalink)
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Stepped shape for flow attachment.

Hello,

I had a brainwave for a passive way of flow attachement with a great angle;
Letting flow stay attached over a surface passively about 17*degrees is the maximum.
Therefor a boattail has to be very long, then I thought of a the way a prius etc. uses a sharp leading edge for clean seperation of the flow.
Vortexes form becouse of the low pressure behind the car.
Vortexes can also be used for sucking in reattaching flow.
Thus a stepped slope would reattach the flow by creating small vortexes.
Then I searched on the web and found some interesting things.

A CFD simulation
323Ci Header/Full Exhaust/CAI/Chipped/3.46:1 Final Drive Build - Page 6 - E46Fanatics

Look at these race mirrors, implaying the same princible.

http://www.racemirrors.com/engineering.html

Look at this hungarian eco car, it's got a slopped rearwindow, exactly what i thougt of.
Antro



EU Transport Research-Fundamentals of Actively Controlled Flows with Trapped Vortices

Attachment browser: Fertis - New Airfoil.jpg by Dickeroo - RC Groups

http://ltces.dem.ist.utl.pt/lxlaser/...ers/11.3_4.pdf

I tried to simulate this with a CFD program on my computer, but as present don't see a change with a normal slope and a tepped one.

I would't love to have more info about this, imaging what's possible you could make a earodynamic side mirror or a rear window and still see enough through it.


Last edited by beer; 05-29-2011 at 07:36 AM..
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Old 05-29-2011, 08:47 AM   #2 (permalink)
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i have seen scans from a textbook showing a shape like that mirror, and the stepped extension does help improve the Cd of the shape.
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Old 05-29-2011, 08:56 AM   #3 (permalink)
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Thanks for the reply
I thought so, do you still have the scans, or do you know the name of the textbook?
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Old 05-29-2011, 08:58 AM   #4 (permalink)
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sorry, i can't remember where the scans are. hopefully someone else can.
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Old 05-29-2011, 10:08 AM   #5 (permalink)
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reminds me of louvers on old muscle cars
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Old 05-29-2011, 10:22 AM   #6 (permalink)
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Indeed it looks like louvers on old cars, perhaps it's was used for aerodynamics already back then.
But if so, it makes one wonder why they won't apply
http://louver.co/a511049-window-louv...-looks.cfmthis presently

Last edited by beer; 05-29-2011 at 10:33 AM..
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Old 05-29-2011, 10:55 AM   #7 (permalink)
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I'm guessing that the steps on the rear window of that "eco-car" are there for the same reason that there were louvers on a lot of the fastback cars. It's nothing to do with aerodynamics, and everything to do with keeping the car from becoming a solar oven.
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Old 05-29-2011, 01:41 PM   #8 (permalink)
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The stepped approach works.
It's filling in the wake, so it helps.
If you keep the rear edge of each step on the aerodynamic template, it'll work fine.
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Old 05-29-2011, 03:02 PM   #9 (permalink)
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beer -

As others have said, the Flow illustrator program we have used in the past isn't perfect, but I think it's good to use for this thread :







I like the louvers as a compromise because you can preserve rear visibility. "Eco-Louvers" would be different from the muscle-car louvers because they would NOT follow the slope of the rear window :



Instead the last louver would end up resembling a "CRX-style" flat window shape :



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Old 05-29-2011, 04:47 PM   #10 (permalink)
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@cfg83
That's great info you give there.
The fluid simulations show indeed that the louvres approach works.
Thanks for confirming my thoughts about this way of flow attachment.

Shame my fluid simulations (other program) did not show any effect.
When I had the oem rightmirror still, I did some tuft testing with a thin sheet of metal attached to the mirror, which i bended in stepped (louvre) form, but my setup was rough so I didn't get clear signs that it worked.

And I knew that the louvre form on the good aerodynamic Antro Solo was there for a reason.

This is great info, becouse in the Netherlands by law you must have a left side mirror and 1 other mirror, that may be a right side mirror or rearview mirror, or both.
So a louvre approach would be great for the rearview and still have some kind of kammback.
And the left side mirror can reasemble a druplet shape with the help of a kind of louvre, while still having the side view.

I'm also wondering if it's even possible to make the angle greater and thus the lenght of a louvre type kammback smaller then its smooth counterpart.
I've acces to a flowbanch so I think I'm going to test it there.
Keep you posted.

Still any more info would be great, thanks alot for al the reactions I got sofar, keep it up.

another pic of louvres on the "rear hood" of a ferrari f40


Last edited by beer; 05-29-2011 at 05:12 PM..
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