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In this answer gravel kits are mentioned. I then looked at the wikipedia page and I noticed that they are quite different from what I was expecting:

Gravel kit tube in front of engine intake

I was expecting some kind of "shield", plate or extended inlet on the bottom side of the engine. Wikipedia says that these instead are

vortex dissipators, using compressor bleed air

How do they work? How does a beam and a sidewise jet of air prevent FOD ingestion?

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    $\begingroup$ Not sure it's possible to "ingest" damage?! $\endgroup$ – Lightness Races with Monica Aug 26 '15 at 17:01
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    $\begingroup$ @LightnessRacesinOrbit admittedly is not the most clear of acronyms. According to Boeing it stands for Debris, not Damage (for example) boeing.com/commercial/aeromagazine/aero_01/textonly/s01txt.html $\endgroup$ – Federico Aug 26 '15 at 17:05
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    $\begingroup$ @LightnessRacesinOrbit I've never heard anyone expand FOD to include "damage". It's always "Foreign Object Debris". $\endgroup$ – egid Aug 27 '15 at 0:19
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    $\begingroup$ @egid I've only heard FOD expanded as "foreign object damage." (USMC air wing, 1996-2001). This despite the fact that in context it usually refers to the objects rather than the damage they cause, i.e., "FOD bag", "FOD can", "FOD walkdown". "Foreign object debris" doesn't even make sense... it would just be "foreign objects." $\endgroup$ – Kevin Krumwiede Aug 27 '15 at 2:56
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    $\begingroup$ @egid: Seems to have been a terminology shift, possibly from before you entered the industry. Sadly, Kevin is right and the new meaning is all sorts of buggered-up in English! $\endgroup$ – Lightness Races with Monica Aug 27 '15 at 11:29
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Notice the path of condensation leading up from below this engine into it:

enter image description here

That image was taken from this video, and it clearly demonstrates that there exists a path of air which flows from ground level up into the engine. Any debris on the ground is liable to be sucked up in that vortex. The nozzle is designed to break up the flowing air on that specific, well-known path.

Note that even when there is no condensation to make the vortex visible and obvious, it still might exist.

Here is a nice intake vortex video, and here you can see the vortex swirling on the ground. This video really shows off a C-17's huge vortex during landing.

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Vortex Dissipators

Prevent vortices forming at the engine intakes which could cause gravel to be ingested by engine. These consist of a small forward projecting tube which blows pressure regulated (55psi) engine bleed air down and aft from 3 nozzles at the tip to break up the vortices.

Source
So basically it works by blowing air out of the tube to prevent vortices from forming. It is the vortex that sucks the stones up. No vortex = no FOD. The 'shield' kind of protection is installed on nose gear to prevent damage to the under belly.

**note main gear is behind engines so no FOD from main gear affect engines.

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    $\begingroup$ The key thing to realize here is that the main risk of FOD comes from a vortex (like a small tornado) forming ahead of the engine and bending to the ground and sucking the stones up. If such vortex is prevented, the stones won't come flying on their own. $\endgroup$ – Jan Hudec Aug 26 '15 at 11:49
  • $\begingroup$ Thanks @JanHudec. Already edit that to my answer. Feel free to improve it. $\endgroup$ – vasin1987 Aug 26 '15 at 12:08
  • $\begingroup$ could you consider expanding your answer with additional information from that source you posted? in particular the image where the airflow from the 3 nozzles (and I thought it was only 1!) is shown is really interesting and educative. [said otherwise, I would be looking for a slightly more detailed "how"] $\endgroup$ – Federico Aug 26 '15 at 21:45

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