7 replies to this topic

[Nuvolari]

I'm sure one of you aerodynamicists out there will be able to sort out a discussion I'm having with a friend...

My understanding is that a NACA duct works well as a low drag scoop, employing the "slow" moving boundary layer and sharp edges of the duct to form two contra-rotating vortices which assist in sucking in the air. The scoop works best in areas of attached flow, and less well where the B/L is thick or detached.

My friend says that if reversed these can be used to evacuate air efficiently, as the convergence of the duct will help to re-accelerate the air to a velocity approaching that of the surface airflow.

I can see this may work if located in an area of low pressure, but surely there must be better ways to evacuate air?!

Cheers,

Nuvolari

[rdrcr]

The only current application I know of NACA ducts being used to help extract air, is on Top Fuel Dragsters.



Besides the spectacular photo, one can see the reverse NACA ducts on top of the body just ahead of some smaller ones for intake to the drivers compartment.

There appears to be one on the side as well.

Why they use them in this fasion instead of another port design is beyond me. But they are pretty common I guess.

Perhaps the amount of pressure that is built up inside the body work requires it.

I shall await someone with an aerodynamic background to chime in now...

[Paolo]

Lotus 49 used Naca ducts to extract air from its frontal radiator

[desmo]

IIRC several F1 as well as some sports prototypes cars with front-mounted rads used exrtractor ports resembling NACA ducts to vent the hot air from the rads, although they weren't symmetric with the car's centerline- the "tails" angled away from the centerline apparently to direct the flow away from the cockpit area.

[rdrcr]

Good call Paolo, I should have noted that example too, especially since it's one of my favorite racing cars.

The early version of the 49 had a proper small one facing forward, then the nose was changed with those reverse NACA ducts to help with cooling in the later versions.

I was looking at the 49 and while they resemble NACA ducts, are they really? desmo, Your comment about them not being symmetric leads me to wonder does it matter if they are perfectly symmetrical? The problem was solved no? Would the problem have been equally as remedied with another style of opening?

[Nuvolari]

The inference here is that they probably can be used to evacuate air, although more elegant and efficient ways to do so exist...

As I think Desmo pointed out in another thread, the effectivness of the extractor is linked to the component difference of the velocity of the two merging flows. That is, if the vent is perpendicular to the surface air flow, gas extraction is maximised. The flow exiting from a NACA is at a very acute angle to the surface flow, so I can't see it being much good...

Nuvolari

[Jhope]

Is that a NACA type duct Benetton used to extract hot air from the sidepods?

[Timm]

Bear in mind that when trying to get air out of a sidepod, the gradient is more favourable than on entry. That is to say, on an inlet NACA, you're trying to persuade the air to stick to the surfaces and follow the duct floor into the pod. On exit, the air is being pressed against the floor naturally and your biggest concern is prevention of seperation once it reaches the outside.