Low-Drag Air Outlets

[Sven7]

We all know NACA ducts are the preferred low-drag air inlet, but I can't seem to find much on letting air out smoothly. What's best?

[basjoos]

Exhaust into the low pressure zone trailing the car. The shape of the exhaust opening isn't that critical (unlike a NACA inlet skimming air without adding to turbulence downstream) since you are dumping into a zone of already turbulent air.

[Varn]

I was thinking of that. I figured that my 1600 diesel is putting out above 800 cc of air back into the air stream every revolution. Diesel being un throttled of course. If the exhaust is hotter than the ambient it is more volume. Figuring 2500 rpm or 41x800/sec plus some for the added heat, it is 4-5 liters a second, it has to have some significance, measurable at the fuel pump, I dont know.

[metromizer]

I've seen these used as air outlets, to get out from under bodywork.

McKinney Corp. Race Cars - Naca & Outlet Ducts

The pointed end faces rearward.

[ConnClark]

It all depends on what you are trying to accomplish and the amount of air you are moving though the outlet. Are you expecting to get thrust from the outlet? Are you trying to use the airflow to suck the air out of the outlet? Are you trying to minimize drag of the outlet? Or are you trying to minimize drag of the whole object.

Maybe some of these papers may help



http://ntrs.nasa.gov/archive/nasa/ca...1993093623.pdf


http://ntrs.nasa.gov/archive/nasa/ca...1993091791.pdf

http://ntrs.nasa.gov/archive/nasa/ca...1993093645.pdf

[Sven7]

Well I didn't have anything specific in mind... yet. Certainly not on the Probe.

Later on it will involve sucking out interior ventilation air (and by corollary, helping suck it in somewhere else).

Thanks for the help everyone, and the PDFs. I'll have to take a closer look later, but are they transferrable to ground effect?

[freebeard]

Louvers, and lots of them:


This is of interest to me because my platform of choice has a handy source of 1000—1200 cubic feet per minute of engine cooling air pumped through two 4x5" ducts right into the wake. I don't care about the exhaust because it is dirty and corrosive.

I'm wondering whether it would be better to dump into the middle of the wake, or have vents around the periphery. Maybe even a few inches ahead of the rear bulkhead so it adds energy to the air just before it leaves the body. Like the Renault R8:

[Varn]

On the R8, it is a little deceiving, the louvers are for the cooling air intake. The air is pulled down and forward through the radiator by a mechanical fan then forward into the engine compartment.

[gone-ot]

...look at some of the EXHAUST pipe "extractor" shapes: some work well, others work maybe.

[Otto]

Read the NACA reports toward the end of WW2. You want the exit duct to be as parallel to external flow as practical. Turning a NASA inlet around backwards is not the best way to go. Sorta works, but not optimal.

Turns out, exit ducting is about as important as inlet ducting, just as (I suppose) the pressure-recovery aft end of a body is about as important as the nose, drag-wise.