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Old 03-16-2015, 12:41 AM   #11 (permalink)
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Quote:
Originally Posted by mikesheiman View Post
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.
The reason it throws the air over the sunroof is purposeful, so that the occupants don't get blown around with the fast moving air coming into the sunroof. The low pressure above the sunroof is the design intent. Have you seen and wondered about the little flip up spoilers that pop up at the front of the opening when you open the sun roof? Additionally, throwing the air over the top helps lessen the drag, opposed to, if the air actually enters the cockpit area.

Every benefit has an opposing compromise.


Last edited by doviatt; 03-16-2015 at 12:48 AM..
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Old 03-16-2015, 12:46 AM   #12 (permalink)
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Quote:
Originally Posted by Cd View Post
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.

You will notice the back window seems to be open or missing in this simulation (no room to roll it down). I would ignore this image as it has many problems.
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Old 03-16-2015, 02:22 AM   #13 (permalink)
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aerohead has the best handle on this, but I would suggest that the shape doesn't "throw" air. The air is just sitting there, the car moves through it, and it tries to get back to where it was. The shape causes pressure and velocity to trade off. The velocity can have a component at right angles to the direction of travel. That probably sounds like the same thing.

Consider this:

http://ecomodder.com/forum/showthread.php/another-vortex-generator-application-2871-2.html#post88536

I found this by going to Google Images and entering <<site:ecomodder.com cfd windshield header>>. The local search only finds words [mostly]. There have been threads on open window vs air-conditioning and internal ducts. How about rear mounted radiator?

Rear Mounted rad n other trickery!

Using A-pillars as tubes to duct hot engine area/radiator air to the roof?

Aero design question

maybe you can find something in Random Wind Tunnel and Smoke Pictures Thread . It's only 43 pages.
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Old 03-16-2015, 03:57 AM   #14 (permalink)
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I've wondered for a while about exhaust gases.

Is there enough exhaust gas to make a difference in a car's aerodynamics?
Would pointing the tailpipe in a particular direction have an effect?
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Old 03-16-2015, 05:47 AM   #15 (permalink)
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Quote:
Originally Posted by doviatt View Post
You will notice the back window seems to be open or missing in this simulation (no room to roll it down). I would ignore this image as it has many problems.

That is because I removed it from the model.
I was testing whether or not it would be beneficial to have air flow through the cabin and out the back.

I did this test quite some time ago. I'll see if i can find the additional images, as well as any that show the correct pressure.
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Old 03-16-2015, 12:58 PM   #16 (permalink)
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Quote:
Originally Posted by Cd View Post
That is because I removed it from the model.
I was testing whether or not it would be beneficial to have air flow through the cabin and out the back.

I did this test quite some time ago. I'll see if i can find the additional images, as well as any that show the correct pressure.
I see. Sorry I missed that part.
I couldn't figure out why I didn't see the wake behind the vehicle.
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Old 03-16-2015, 03:35 PM   #17 (permalink)
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Quote:
Originally Posted by markweatherill
Is there enough exhaust gas to make a difference in a car's aerodynamics?
Would pointing the tailpipe in a particular direction have an effect?
Exhaust gases are corrosive. IMHO best to just get them away from the car. A VW beetle has about 1500cf/m of exhaust and 1500cf/m of pumped engine cooling air. My thought was to boat-tail the car and then inject the engine cooling air into the truncated boat-tail. It would look like this (on the left):


The actual vent is designed as a Coanda nozzle.
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Old 03-17-2015, 05:06 PM   #18 (permalink)
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Quote:
Originally Posted by markweatherill View Post
I've wondered for a while about exhaust gases.

Is there enough exhaust gas to make a difference in a car's aerodynamics?
Would pointing the tailpipe in a particular direction have an effect?
Hoerner reported that the Messerschmitt Me 109,at 380-mph,at say,90% power,enjoyed 250-pounds of thrust from the engine exhaust.
It's the only citation I've seen in 40-years which actually quantified such a thing.
NASCAR,while having the potential to benefit from this sort of thing,specifies a side outlet so that following drivers aren't exposed to the undiluted exhaust while drafting.

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Last edited by aerohead; 03-17-2015 at 05:06 PM.. Reason: correction
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air pressure, cda, duct, redistribute, windows up

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