I am curious as to the effect that having the rear hatch open along with the front windows rolled down all the way have on the drag of a Civic Hatchback.
I have a nifty little device that can hold open the rear hatch of almost any car. I've used it on my CRX's hatch and now my VX Hatch. It was originally designed for 4th and 5th Generation Corvettes, but the guy made one up special for me so he could branch out his application list. Corvette Gadget Man is the site I got the item from.
Anyways... With the windows down (when extremely hot and humid out as well as my daughter not being in the car) I would put this in the hatch glass latch and VIOLA!
When in this configuration, I don't get any cross-winds or turbulence inside of the car. I do get a small whiff of fumes when taking off from a stop, but nothing horrible. Does this configuration reduce some of the negative pressure behind my car, make it worse, or no change at all?
My theory is simple. If one can divert some of the high pressure air in front of the car "through" the car, overall drag should lower. So, for example, opening the vent with an opening in the back should help (immeasurable agree but..)
But here is the odd thing with my VX. Vent open at speed with everything else closed produces a fairly low amount of flow into the car (less than my other cars). Opening the hatch back seemed to help, but just a little. But when I cracked the door window I got lots of flow.... but not from the vent... from the hatch back back into the car....???? It seemed like the vacuum created by an inch or more of door window opening was pulling interior air from everywhere it could.
I "think" this can be explained using Bernoulii's principle in that the velocity of the air passing the side of the car is the highest... much higher than the slow or zero velocity cushion of air at the cowl, and still higher than the slower moving wake behind the hatch. The problem is that this is not the effect I was looking for.
I believe that your explanation is correct. Hmm, that is kind of a drag though, since you don't want to suck air in thru the hatch... I liked your original idea because it seemed like it could help a lot of different issues... Keep trying!
I noticed that with only the drivers window open and the hatch cracked that air would enter from the cracked hatch as well. When both passenger and driver widows open all the way no air would enter through the cracked hatch as well as no air entering from the open windows, unless a large enough gust of crosswind came along which only happened as a semi passed close by me traveling at almost 70mph (the semi was going that fast, I was going 60)
Since Summer is coming up and my three months of freedom from my tech job at school I will have a chance to test out more of the flow by placing the tape/yarn/string pieces INSIDE the vehicle to truly measure where the flow is coming from.
I thought this through and came to the conclusion that air will always enter the hatchback when the windows are open, and this will hurt mileage (small maybe). I believe this is because, while there is a low pressure zone behind the car, the pressure outside the windows is even lower (highest velocity if still streamlined). This whole, suck air in from anywhere, including the back, and suck it out the window will just do more to disrupt the overall streamline effect and hurt things.
Oh, lastly, on my last try my daughter said, "What is that smell?". I thought, "Oh, ****. I forgot about the exhaust!". A long time ago I dropped to the ground one day due to CO exposure on a construction site.... be careful... it does not sneak up on you.... one second you are fine and the next second you are heading towards unconciousness.... it is that fast.