Making a "Kammback" with the Sunroof and Rear Window
 

Lester has a sunroof and rear window. I was wondering what would happen to the aerodynamics of the truck if I were to open both of them. Does anybody have any idea what that would do or can anybody do one of those airflow diagrams to see how it would work?

Thanks! :thumbup:

http://images.thecarconnection.com/m...00027914_m.jpg

I think it would hurt the aerodynamics, especially with the sunroof open. Not sure what to think with just a rear window open. Assuming it is the kind that retracts into the door, I guess it starts to look a little like a box cavity back there, but box cavities have to be carefully tuned and their benefit is very small and highly interactive with other aero features, apparently.

Don't open the back window while driving unless you also have side windows open. You will be sucking your exhaust fumes into the vehicle.

If you want to try it do so on a gravel road, it will be obvious if you are sucking the wake into the cabin by the dust levels well before you get any CO poisoning.

I miss my 4Runner.

open
 

I'll vote 'don't also.
The lowest pressure outside the vehicle is near the windshield.If you open the sunroof and rear glass,the low pressure above the roof will telegraph through the cabin,bringing air and exhaust fumes forward towards that low pressure as it attempts to equalize the static pressure between the two areas.
It not only ruins the airflow,but it poisons you to boot!:(

STOCK :http://farm6.staticflickr.com/5450/9...1aeeaa77_o.jpg
SUNROOF OPEN / BACK WINDOW OPEN : http://farm8.staticflickr.com/7344/9...9ac7e26f_o.jpg
SIDE WINDOWS OPEN / BACK WINDOW OPEN :http://farm6.staticflickr.com/5471/9...7c6540b2_o.jpg

I had pondered the same thing before on this forum. Thanks for the idea to test it in CFD.

I'll test the coefficient of drag later to see how bad it gets with the windows open.

This will sound uninformed, but why does the graphic seem to show slower air flow at the front of the vehicle when the windows are opened downstream? Is it because the open windows slow the ari around the greenhouse which in turn makes the flow at the front of the car slower?

Actually the flow appears to be moving out of the cabin. Here is a video :

http://www.flickr.com/photos/60376566@N00/9693089745/

I'm multitasking here and getting ahead of myself.

This has me puzzled too.
I will test this later as well. I think it's the fact that my computer is bogged down from having too many things going and it is causing everything to act screwwy.

Cd- Thanks for those CFDs. Can you help me decipher them, I don't know what it all means. Is rear window and rear passenger windows all open a good thing? Or is my idea going to work?

Can you do one with the sunroof, rear window and rear passenger windows open, I would like to see what happens then.

Also, the video you posted doesn't seem to work, at least for me.

I ran some quick and sloppy tests of the Cd ( or cD ) and in both cases the drag went way up. More so with both windows down versus just the sunroof ( around 21% with both windows open )

I would guess that the airflow is creating some major turbulence in the area right before it enters the low pressure zone ( the open windows ) as well .

Look at the ( virtual ) increased frontal area that the airflow is causing. This test only took into account the drag coefficient and not the total drag including the frontal area.

So, no apparently this test will not show any benefits in drag reduction.

If you have the free time, and are willing to buy a battery powered carbon monoxide alarm, I say go for it ! A real world test is always nice to have in the archive here, even just to prove that it doesn't work.
It can be referenced to when the question gets asked again.

http://www.hardwareworld.com/Carbon-...d-pYB7HRK.aspx

If I did such a test, I would make sure that the air had as smooth a passage as possible to the back. If the seats in the rear fold down, this would be a good time to do it. Also i would add tufts at the rear of the vehicle ( inside the vehicle ) to show the direction of the airflow.

As far as deciphering the images, I'm clueless and would have to point you to Aerohead.

I can tell that there is a major disturbance in the force ...er of the air entering the open windows / sunroof but that's all i can decipher. I'm puzzled at the change in pressure at the front of the vehicle once the windows are opened.
I suspect this software has some bugginess. ( But hey its FREE ! so no complaints here ! )

If i have time i will try the open window as well as sunroof idea as well. Perhaps tomorrow.

Aerohead (or anybody), can you help me with what the images mean, I have never tried breaking down a cD image before.

I just happened to have bought a CO alarm for my new house (laundry closet is in hallway directly across from my bedroom and there is a gas dryer) and I haven't installed it on the wall yet, so I may put it in the truck while not doing laundry/in the house.

Now I just need to get this rear window fixed, so I can try this mod out! :rolleyes:

breaking down
 

I'm struggling to 'believe' the software's isobaric interpretation.
The image does not appear to do a proper accounting of velocity pressure vs body location.
The thing to bear in mind is that around the side-view mirrors,the air has achieved its maximum velocity and lowest pressure.
With a squareback,these velocities pretty much remain constant until the trailing edges of the body where it breaks away.
*with the window down you destroy the flow support surface and the air must flow into the void.
*This destroys the flow behind the window.
*The lowest pressure is at the side mirrors.
*The pressure behind the vehicle is higher than at the side-
view mirrors.
*If you open the window and backlight,air will flow from back to front.It's got to,based upon the pressure differential.
*This counter-flow triggers shearing forces which generate vortices which rob power.
*Attempting to use the cabin as a duct forces air to flow within an extremely
high-drag inner wall boundary of tremendous surface roughness,not to mention everything within the 'duct.'(seats,headrests,shoulder belts,etc.)
*The interior flow steals kinetic energy from the exterior flow,giving it no chance for any pressure recovery in the wake region.
* In an aircraft,the only reason you'd use a duct,is to channel air to the embedded engine,who's tailpipe and or fans develop a net thrust propulsion.