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Old 03-14-2015, 08:24 AM   #1 (permalink)
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Has there been any experimentation with redirecting window/sunroof air to the rear?

Has there been any experimentation with redirecting window/sunroof air to the rear?

By experimentation, I mean something like attaching a duct to a slightly open window or sunroof area and piping it to the rear of the car to help fill the low pressure air vacuum behind it.

Has it ever been done or is there any reason why it can't/shouldn't be done?

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Old 03-14-2015, 09:04 AM   #2 (permalink)
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I've wondered the same thing too. I did a really sloppy CFD test with the sunroof open, and then later with the back windows and rear window down.

IIRC, all tests showed an increase in drag.

And then there is the issue of crowding the cabin with ducting. The idea seems feasible though. Thanks for posting.

There was a post or two on that topic here before BTW.

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Old 03-14-2015, 09:12 AM   #3 (permalink)
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I was looking at the image i just posted and was noticing how 'buggy' the software i was using was.
Just look at the high pressure areas in the image ( the red ) - everything is backwards of what it should be !
The front of the vehicle is shown to have dead air, while the true dead zone is shown to have the most pressure.
The airflow in this image is correct though ( front to back )
So take the above image at face value.

We need more testing ! I'd be interested in this topic.
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Old 03-14-2015, 10:21 AM   #4 (permalink)
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Old 03-14-2015, 10:40 AM   #5 (permalink)
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I experimented with this idea and it failed miserably, to the point it was bringing exhaust fumes into the car.
The biggest reason for the experiment is that the civic hatchback has very inadequate ventilation to begin with.
My plan, use the same vent idea that cars already use, the flapper that opens when you shut your car door or trunk.
So I cut a suitable sized hole in the tailgate of my civic hatchback behind the license plate.
I then installed a flapper from an old volvo approx 5" x 7".
The plan was to vent any excess pressure from inside the car and hopefully fill the low pressure air vacuum behind it.
Well for some reason it drew the exhaust into the car.
Then I had the bright idea to install a 120mm fan to force the air out, end result was the vacuum was to hard to over power.
So now I have gorilla tape blocking the vent inside and out.

I think the idea to fill the low pressure air vacuum behind the car would be better implemented via duct work from under the front of the car and exited in the void area, sort of like and exhaust system.
If I were to experiment with that plan I think I would start the duct work from the grill as its a great catchment area.
If it was done right the duct work could double as part of a belly pan.
Seems to me someone already implemented something similar to that on a VX, but I would exit it up higher in the void.
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Old 03-14-2015, 01:22 PM   #6 (permalink)
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redirecting

Quote:
Originally Posted by mikesheiman View Post
Has there been any experimentation with redirecting window/sunroof air to the rear?

By experimentation, I mean something like attaching a duct to a slightly open window or sunroof area and piping it to the rear of the car to help fill the low pressure air vacuum behind it.

Has it ever been done or is there any reason why it can't/shouldn't be done?
The lowest pressure on the car is in the windshield area/A-pillars.If you were to open a pathway from a window or sunroof to the rear of the car,the air from the wake would travel 'FORWARDs' into the duct and come out the windows and sunroof.
A Baumann scoop or NACA submerged inlet could allow some dynamic pressure ram-air to force air in at the front,but the additional wetted area and internal wall friction of the long,restrictive ducting would cut benefits;and by robbing air from the outside of the car you'd jeopardize the boundary layer's ability to receive energy from the redirected flow,triggering premature separation on the outside of the car.
Active boundary layer control is proven to work,however it requires the addition of power from some external source.
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Old 03-14-2015, 01:23 PM   #7 (permalink)
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I think I recall Basjoos having a couple of ports at the end of his tail (in the cabin) for just this purpose.
I wish he would post some close ups of that set up.
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Old 03-14-2015, 06:14 PM   #8 (permalink)
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I agree with aerohead. Ducts need to be short with a large cross-section. And the volume of air moving through the interior is [orders of magnitude] less than the air moving over the outside. The edge case would be a convertible with the top down.

Here's one that does/should work:
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Old 03-15-2015, 11:00 PM   #9 (permalink)
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How come a lower pressure area occurs above the car..."throwing air"?

Quote:
Originally Posted by WD40 View Post
Well for some reason it drew the exhaust into the car...
If I were to experiment with that plan I think I would start the duct work from the grill as its a great catchment area.
Interesting, if I have it right, the key point here seems to be that pressure is -lower- in the sunroof area, for example, than behind the car, which reverses the desired air flow (doh!).

However, I'm wondering -why- this is true...
A guess (please feel free to correct)
*) The roof and windshield area do not curve smoothly/continuously into each other, causing the windshield to "throw" the air over the sunroof and create a low-pressure/turbulence area

Yes, I'm considering the idea of reshaping/custom-creating the front/roof joining area (technical term for this?) to prevent said "throw" if this is the case.

The speed lost in the actual duct due to it likely not being perfectly smooth does seem to be a definite issue.
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Old 03-15-2015, 11:02 PM   #10 (permalink)
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Meant to ask as well...where can I find the other/previous threads on this topic?
I was/am unsure what keywords to search for...

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