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Old 10-20-2010, 06:34 PM   #1 (permalink)
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Sawtooth edges on 747 engines

I saw this on another forum and thought it might make an interesting discussion.
I have no idea what the sawtooth edges are for on these engines, but my guess would be to manage the airflow better by creating small vortices.

Since the airstream then meets the turbulent jet-wash, I'm puzzled as to how that this could be useful.

So what is the real purpose ?

Photos: Boeing 747-8R7F/SCD Aircraft Pictures | Airliners.net

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Old 10-20-2010, 06:57 PM   #2 (permalink)
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Cd -

Some say it's primarily for reducing noise :

Quote:
Originally Posted by Sayyad View Post
The image I've attached is that of the engine of a 787 Dreamliner. The guy who worked on the sawtooth design wrote that it reduces noise. This means less energy wasted energizing air to produce that noise. I'd guess that there would be small vortices forming at each point on the trailing edge, but that all the vortices combined would take less energy to maintain than a possible single large one trailing from a smooth trailing edge.
Others say it's just a Batman thing, :



What *is* interesting is that the design has evolved. In the pictures above, it's on the external cowling only. In this picture it is also part of a second cowling closer to the exhaust :



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Old 10-20-2010, 07:10 PM   #3 (permalink)
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...look up "scalloped fins" on whales.
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Old 10-21-2010, 10:37 AM   #4 (permalink)
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Quote:
Originally Posted by Cd View Post
So what is the real purpose ?
Noise reduction.

Boeing: Boeing, Rolls-Royce Verify Jet Noise Reductions

The technology behind serrated trailing edge design is described in US patent 6612106 filed by Boeing in 2001.

Segmented mixing device having chevrons for exhaust noise reduction in jet engines - Patent 6612106


I wouldn't be surprised if they also reduced wake drag.
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Old 10-21-2010, 10:48 AM   #5 (permalink)
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The idea is that the cold, slow surrounding air mixes smoothly with the hot, high speed jet exhaust.

In fact the jet exhaust is also the bypass air surrounding the core air, and if all these can mix with minimum turbulence that's as quiet as you can get.

Anyone old enough to remenber the lobed jetpipes of 707s - same solution to a much worse problem
illustrations and explanation here:
Noise Control/Supression
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Old 10-21-2010, 11:28 AM   #6 (permalink)
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Their usage is a balance between performance (thrust) and noise (i.e. a trade off). Mixers have been around since the 60's (the sooner you get the flow mixed the quieter the engine is). What's new is the use of them on the fan cowl. The chevrons increase drag but decrease noise. Noise is a major issue these days. Ecomodders have no use for "sawtooth edges".
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Old 10-21-2010, 12:07 PM   #7 (permalink)
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stealth radar dissipators?
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Old 10-21-2010, 02:36 PM   #8 (permalink)
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