NACA Research Memorandum (RM-A7I30) provides a good description of the 'typical' NACA submerged duct. You can download this PDF file from the NASA web site.

I am not sure I understand what you are describing as a 'reverse' NACA duct. What I am picturing is a duct that sticks out into the airstream, rather than submerged.

The NACA submerged duct works on a principal that the forward curved section creates a vortex on each side of the duct. The vortex helps pull the slip stream air down into the duct. Because of this principal, the duct must be design for a specific application. And should be as far forward in the air stream as possible. The thicker the boundary layer, the less efficient the inlet will be.

The home build airplane industry is full of examples of poor applications of this inlet. So, be careful when you use them as guidance.  Get the NACA document.

I have a PDF copy. If you give me an email address, I will try to send it to you. It is a 16 meg files. Make sure you email address can receive that size of document.

Is this an efficient way to extract air?

Im afraid I don't have an email system that would handle a 16MB file. Do you have a link to the NASA site that it might be posted on?

It doesn't look very efficient, to me. But, what's most important is not shown. That is the inlet (my judgment based on the picture)design. And if there is any power driving the air.

The exit design doesn't extend out of the boundary layer. There will be no 'suction' created from that design. The only air that exits that vent will be the result of the higher pressures created by the air inlet devices.

This NACA inlet is one of the most mis-used items I have ever seen.

Thanks for your insight!
Is there an NACA inlet equivalent designed to extract instead of induce?

I'm still not clear on whether you're trying to get air into an engine compartment or out of it.

The NACA inlet (I just skimmed the report) only works to take air in, and only if it's far enough forward that there's a very thin boundary layer.  The NACA points forward like an arrowhead, and the leading edge lips draw in a pair of vortices.

To get air OUT of the engine compartment, vents are designed to use the principle behind ejector nozzles.  There's a stream of air rushing past the side of the vehicle, with a velocity that is higher than the vehicle's velocity.  The higher velocity, via Bernoulli's principle, has a lower pressure than the static pressure.  The lower pressure will "suck" air out of any vent on the side of the vehicle.

The photo you have given above (nice import trick, by the way) is of a NACA duct that only works as an intake.  This is probably preferred on the F1 racers because it won't ingest FOD as much as a "mouth" intake (ie. F-16).  I bet the F1 cars exhaust their engine compartments thru ducts at the back. Not usually visible on the pretty press photos.

Get the NACA report for yourself at naca.larc.nasa.gov, go to 1948, and use your browser's Find to look up "submerged duct".  The NACA site claims to have a search engine, but I don't find it useful.  The site has about 8000 papers, and there might more there that interests you than just this particular paper.

STF

I am trying to find the most efficient design to get the hot air out of my engine bay via the front fenders.
So given, the same exhaust dimensions, is a single large opening (i.e. reversed "mouth" or viper style vent):

more efficient then the multi louvered "shark gill" vette/Z3 design:

Is there a more efficient design?

I have a NACA Duct (from a Nascar rear window) mounted in the lower engine shroud to bring cool air into the engine compartment. I also have cold air feeding my intake via the front bumper. Now I am just trying to get the hot air out.

You should take a look into a Mustang, Viper, or other vent to see if they have anything to do with engine bay cooling.  They could very likely be for looks alone... or for cooling the brake discs, or there might be some sort of oil cooler in there, but I'm reaching now.  Copying a design is a good idea, but make sure the design is actually for cooling the engine compartment.

STF

The bigger the hole the better the air through it is very true but the idea with multi louver is that the air flow over them causes a reduction in pressure helping to suck the warm air out. (Or they are simply cosmetic). The best example of this is on Le Mans cars, on the front wings.
The questions really should be 'what car is it?' 'why do you need to vent it??' and 'what speed does it run at...' If you ask a question you really should give as much info as possible as so often in engineering the application is specific so the questions aught to be also.

Here's a couple good, intro references to look at:

- Local Flow II, by the CAFE (Comparative Aircraft Flight Efficiency) Foundation.  It's simple, but sometimes it's nice to start with simple.  It's on their website.

- NASA-76-tnd8206.  A google search will direct you to this report (pdf), or search for it on NASA Langley Tech Reports Server (we all have that one bookmarked, don't we? :)  This is summary of a bunch of old NACA drag reduction efforts; there's some stuff on inlets/outlets.

Generally, (and this only my opinion) louvers will not be as efficient as a single outlet--the vanes are simply flow restrictions.  Sheet metal louvers are used because they are fast and cheap to manufacture, and retain alot of the stiffness of the outlet without adding secondary support.  The fancier styled louvers (like the BMW above) are used because they make the car look more like a shark.  (You're shaking your head, but you know it's true)

The NACA inlet was designed to provide the best pressure recovery possible without having any projecting scoop.  That's it.  When you read the reports, the authors repeatedly make the point that a well-designed scoop will have considerably better pressure recovery.  There are three good reasons to use a NACA inlet, however:  attempting to reduce ingestion of debris (as SparWeb has pointed out), reducing tooling and manufacturing costs relative to a smoothly-contoured scoop inlet, and reducing running drag of systems that require only intermittent use of an inlet.

Comments welcome.

Regards

I am kind of in the same boat, different speed though, as rk90gtx. I am looking for a way to vent excess heat in a Jeep cherokee, XJ, when crawling in 4lo1st off road. The engine runs at 210-220F and it gets real toasty under there. The type of vent I am envisioning/looking for is one that I can place at the rear 1/4 of the hood, one on each side and is closeable for on road use and in inclement weather. The 1998ZJ or Grand Cherokee that was equipped with the 5.9L 360 had ONE on the passenger side that was thermostically operated, the amount of heat radiated out of that vent when running off road and after shutdown is impressive to say the least but OEM price on them is over $450 ea and finding one of those hoods in a yard is well nigh impossible let alone trying to get them to seperate the hood and the vent. I have also considered side vents on the top 2" of the fenders, the stucture under there is open to the engine compartment but I am dubious about the amount of hot air that would flow out from there.
I have tried using the hood release to open the front hood about 1", result was that it heated up faster probably due to less air flow thru the engine compartment with an increase of  almost 80F on the exhaust manifold downtube. I also removed the rubber seal at the rear of the hood, my AC really had to struggle as the hot air was sucked into the fresh air intake at the base of the windshield. At first I had considered a NACA vent till I had a better understanding of their purpose, while they look 'cool' I realize now they are really an intake and not used to exhaust.
Suggestions ???

I278 - Thanks for the CAFE info, some very good info!
I have the NACA duct in the lower shrowds so that it will not "ingest" from the road surface.

MATT0000- I have upgraded my motor to that of a Rally car turbo charged motor. The engine bay is small and the Underhood temps are much hotter than I would like. I am trying to minimize the heat soak that occurs on the intake piping. I could use a "wrap" but I would rather remove the heat from the bay. As I am sure the plastic and rubber components life are shortend by the high heat.

I am coming to the conclusion that a combination of hood vent ( to allow the heat to escape through natural convection) and an additonal side or undertray vent (to extract while in motion) is the way to go. As the operating speed is not constant.
If enough positive pressure is forced up from the front engine under tray, I would imagine that it should displace most of the hot air left in the bay while in motion. So the hood vents would just allow the remaining heat to escape. Comments?

Check out the Augist issue of Flying Magazine.  The Melmoth has interesting intake and exhaust ducts.  In the article, the builder says he's still running too hot, therefore a discussion on this very topic would be of interest to him, as well.  See if you can network your way into asking Peter Garrison about how he designed his ducts.

STF