NACA inlets with VG's 1

[ALMartins]

I heard something about the use of sharpened edges installed along the lateral edges of NACA submerged inlets, to boost their pressure recovery power. Apparently, the working principle is the generation of vortices on the sharp edges (with an associated drag penalty).
Does anybody know of any reference and/or site where I can find information on this?

 

[buzz41]

I've seen these NACA scoops ( usually used for cooling aft equipment ) implemented with both overhanging edges and raised VG edges on mil acft even years ago. I have yet to find a good tech ref with boundary layer details, etc. for these scoops. Anything on the web?

Any efficient scoop has to allow diffusion for pressure recovery without having the BL spoil things. My view is that the best these things can do is mix the BL with higher energy air prior to diffusion. That is better than having the diffuser totally stalled, but pressure recovery will be more modest than otherwise.

My take is that lately they have been misapplied to applications where a lot of energy is involved - not that efficient. They do have the advantage of not eating a lot of rain and bugs. So, they should be good for cooling the pilot, maybe, but not the engine.

 

[ALMartins]

buzz;
I've been doing some research on the matter of NACA inlets since I wrote my question, and I found some very interesting stuff. No surprise most of the information came from the NACA Reports site kept by Langley Research Center. You can find dozens of reports on the subject, but I found the TN 2323 specially interesting. It is a theoretical approach to the inlet problem. One thing I did not know is the S-shaped inlet edges are actually meant to generate vortices already, helping to control boundary layer separation on the pressure recovery ramp. That measure also reduces deceleration, reducing efficiency, like you said. Finally, the same report SUGGESTS the use of raised lateral walls, forming "raised VG's" (page 29). However, no quantitative or experimentai info is given. Check it out:

http://naca.larc.nasa.gov/reports/1951/naca-tn-2323/

 

[mrvortex]

Al, The NACA submerged inlet was my inspiration for my first VG patent in 1984. The idea was: If thetrailing edge of the floor of a NACA duct is made re-entrant with the original flow control surface, (instead of leading to the interior of the body to take air onboard) you have an exquisitly low-drag vortex generator because you create then release a pair of counter-rotating vortices onto the skin - without any protrusion. The vortices are formed from the slowest, low-energy air available, but because they are forced vortices with solid-body rotation, they are strong & useful. For more info, see my original U.S. patent 4,455,045 which I sold to Cessna in 1992.

I originally used NACA ducts on my Indy Eagles in the early 1970's and discovered that they could be souped-up a bit by erecting lips onto the sidewalls that protruded into the boundary layer. That adds a bit of device drag of course, but does create a larger vortex along the inside of each sidewall to assist the inlet in providing a good mass flow despite the adverse pressure gradient of a downstream heat exchanger for example.

At Wright Patterson, there is an F-86 with its fuselage NACA duct "assisted" by the addition of several tiny conventional vane VG's on the fuselage skin immediately adjacent to the NACA duct edges! Obviously an attempt to improve it. - - mrvortex