lehpron
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Source Of Noise From A Jet Engine?

Sat Nov 19, 2005 4:58 am

What makes that whistling whine noise? actual fuel burn? friction along axial-bearing? pressure difference at exhaust?
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LimaFoxTango
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RE: Source Of Noise From A Jet Engine?

Sat Nov 19, 2005 5:36 am

At first guess I'd probably say its either the compressors or turbines spining at a whopping 20,000 RPM or more.
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DeltaGuy
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RE: Source Of Noise From A Jet Engine?

Sat Nov 19, 2005 6:31 am

Some of it is from air being sucked into the engine itself. Stand near one on startup especially, or during idle...you can hear/feel the air being sucked.

On the S-3 Viking (Navy Anti-Sub Jet), the suction noise is so loud as to cause the jet to whine really loud all the time, and occasionally let out a huge "fart' when the throttle is moved at lower settings...hence why we call it the "Hoover". It actually shares the same engine as the CRJ and the A-10 Warthog.

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airportugal310
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RE: Source Of Noise From A Jet Engine?

Sat Nov 19, 2005 7:21 am

Nothing like watching a 777 sucking up a nice tornado-style water spout when it revs those huge engines...its a cool phenomena
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HiFi
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RE: Source Of Noise From A Jet Engine?

Sat Nov 19, 2005 11:18 am

I am no specialist, but I do know that pressure/temperature difference of the air at the exhaust is a MAJOR contributor.. Just compare turbojet vs turbofan. A 5:1 BPR turbofan has 1/6 of the air coming out at high temperature, reducing noise considerably if compared to a turbojet, where all of the air sucked into the engine comes out hot relatively to OAT.

Chevron nozzles are supposed to reduce noise by better mixing cold and hot air at the exhaust:

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Photo © Mario Nonaka



As to suction noise, there are also some ways to reduce it.. negative scarfing, for instance, consists in using a negative-angle inlet (the lower part of the inlet is more to the front of the aircraft than the upper part), although it reduces performance a bit.. you might want to check your market requirements for take-off distance  Confused I don't know if any successful airliner has used that.
Helmholtz ressonators are little holes on the inner surface of the inlet. They are commonly used. Swept stators also, on more modern engines.
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kaddyuk
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RE: Source Of Noise From A Jet Engine?

Sat Nov 19, 2005 11:22 am

Vibration... There are probably reaching on a hundred different sources of sound coming from a turbine, not limited to the whirring of the gearbox to the clink of the fan blades knocking together...

All i know is... wear ear defenders when standing next to one...
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HAWK21M
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RE: Source Of Noise From A Jet Engine?

Sat Nov 19, 2005 12:04 pm

Air Flow thru Fan,Compressor & Turbine sections.
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Fly2HMO
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RE: Source Of Noise From A Jet Engine?

Sat Nov 19, 2005 3:16 pm

On some engines, especially on the JT8Ds when viewed from the rear, all you really hear from that angle is the combustion when on idle, and when they throttle up, the shear roar of all the air being pushed back out.

At least that's my quickly improvised theory...  innocent 
 
kl671
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RE: Source Of Noise From A Jet Engine?

Fri Nov 25, 2005 10:32 am

The whining noise comes from the intake. When a fan blade passes a point on its way round a revolution, it produces a change in air pressure at that point as the bit of air in front of the blade is compressed. As the blade moves on, the pressure recovers and the cycle repeats when the next blade comes along. Multiply the number of blades by the rotational speed of the rotor in revs/sec to get the frequency of the whine. The same effect occurs with second and subsequent stages of the compressor to give a sound unique to each engine model. Each engine manufacturer uses different number of blades and rotational speeds.

The low frequency noise comes from the turbine exhaust and contains contributions from both combustion noise and from the differential between the high velocity jet gases and the relatively low speed outside air. Chevron nozzles increase the perimeter of the exhaust duct and this reduces the velocity of the turbine gases where they meet the outside air. The lower the difference between exhaust gas and outside air speeds, the lower the noise.

Hope this answers you question.
 
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HAWK21M
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RE: Source Of Noise From A Jet Engine?

Fri Nov 25, 2005 12:43 pm

Quoting KL671 (Reply 8):

Very Well Explained  bigthumbsup 

Quoting KL671 (Reply 8):
Chevron nozzles

Similiar to those on the B787/B748 Engines.
regds
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manzoori
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RE: Source Of Noise From A Jet Engine?

Fri Nov 25, 2005 6:54 pm

Quoting Kaddyuk (Reply 5):
clink of the fan blades knocking together

But surely only when the engine is windmilling (i.e. shutdown). I'm no expert in Fan Blades (More of a Turbines man myself  Wink ) but I'm pretty sure as the CF loads come on with rotational speed the fan blades wouldn't be knocking together.

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F14D4ever
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RE: Source Of Noise From A Jet Engine?

Sun Nov 27, 2005 4:38 am

Quoting FLY2HMO (Reply 7):
the shear roar of all the air

Ding ding ding! You got it!  optimist 
Most of the rear-lobe noise is shearing due to the velocity differential between the high-velocity core nozzle airflow and the low-velocity fan nozzle airflow.
Not metal clanking together, not pressure differential. Shearing.
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airfoilsguy
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RE: Source Of Noise From A Jet Engine?

Sun Nov 27, 2005 6:37 am

Quoting Manzoori (Reply 10):
But surely only when the engine is windmilling (i.e. shutdown). I'm no expert in Fan Blades (More of a Turbines man myself ) but I'm pretty sure as the CF loads come on with rotational speed the fan blades wouldn't be knocking together.

The fan blades expand at speed closing the gap and stopping the "knocking"
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liedetectors
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RE: Source Of Noise From A Jet Engine?

Sun Nov 27, 2005 11:40 am

Quoting HAWK21M (Reply 6):
Air Flow thru Fan,Compressor & Turbine sections.
regds
MEL

You are correct. The majority of the noise is the fan blades breaking the sound barrier.

On a side note, the low rumble sound that people complain about on start up is the combustors when they start up. BR710 used to be notorious for this and the engine used to rumble for a long time. RR has since fixed the problem for us.
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HAWK21M
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RE: Source Of Noise From A Jet Engine?

Sun Nov 27, 2005 4:43 pm

Quoting Liedetectors (Reply 13):
RR has since fixed the problem for us.

What was the Fix.
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lehpron
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RE: Source Of Noise From A Jet Engine?

Mon Nov 28, 2005 3:54 pm

Quoting KL671 (Reply 8):
Hope this answers you question

Yes it does, and everyone else, thank you.

Secondary question, what about pulsejets like those on the V1 buzz bomb of WW2 fame? (or infame, no pun intended) Is that the actual fuel combustion pulsing or the shearflow in that case?
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kl671
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RE: Source Of Noise From A Jet Engine?

Wed Nov 30, 2005 11:00 am

Quoting Lehpron (Reply 15):
Secondary question, what about pulsejets like those on the V1 buzz bomb of WW2 fame? (or infame, no pun intended) Is that the actual fuel combustion pulsing or the shearflow in that case?

Throw lots of fuel into a tube and light it. An explosion occurs and causes a flap in the intake to close. The gases and noise that were heading towards the intake now do a U turn and start harassing those that haven't yet discovered the intake is closed. We now have sound waves adding and canceling, turbulent flow noise from the mixing of different velocity gases within the pipe, combustion noise, shearing, standing waves and I suspect the odd shock wave from localised supersonic flow.

I have no idea of the answer to your question but I am sure someone could earn themselves a Phd by working it out.
 
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HAWK21M
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RE: Source Of Noise From A Jet Engine?

Wed Nov 30, 2005 5:30 pm

Quoting Kl671 (Reply 16):


LINK

regds
MEL
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jarheadk5
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RE: Source Of Noise From A Jet Engine?

Thu Dec 01, 2005 10:57 pm

Quoting Liedetectors (Reply 13):
The majority of the noise is the fan blades breaking the sound barrier.

Fan blades don't break the sound barrier. Nor do any of the other blades in the engine, for that matter.
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joness0154
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RE: Source Of Noise From A Jet Engine?

Fri Dec 02, 2005 12:10 am

Quoting JarheadK5 (Reply 18):
Fan blades don't break the sound barrier. Nor do any of the other blades in the engine, for that matter.

If the propellor tips on a stearman biplane can break the sound barrier by spinning at somewhere near 2500RPM, I have no doubt that a fan blade spinning upwards of 15,000RPM can break the sound barrier.
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jarheadk5
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RE: Source Of Noise From A Jet Engine?

Fri Dec 02, 2005 2:15 am

Quoting Joness0154 (Reply 19):
If the propellor tips on a stearman biplane can break the sound barrier by spinning at somewhere near 2500RPM, I have no doubt that a fan blade spinning upwards of 15,000RPM can break the sound barrier.

Sure, they CAN. I'm sure in an overspeed event, some of the HPC/HPT blades will experience supersonic tip speeds. But it's not exactly conducive to longevity.

BTW, last time I looked at TCDS's on ATP Navigator, the fan speed at 100%N1 on an airliner-sized turbofan (PW4000-series, I think) was ~8000RPM.

The Stearman is an interesting case - I've been to many airshows with Stearmans performing (the Red Baron Stearman Squadron was always one of my favorite civilian shows to watch), and I've always noticed that I can feel the "snapping" of the prop in my chest. Obviously, there's a lot of vibration going on there... it can't be good for the aircraft or engine from a longevity/durability standpoint. Just how bad it is, I don't know.
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liedetectors
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RE: Source Of Noise From A Jet Engine?

Tue Dec 13, 2005 9:10 am

Quoting JarheadK5 (Reply 20):
BTW, last time I looked at TCDS's on ATP Navigator, the fan speed at 100%N1 on an airliner-sized turbofan (PW4000-series, I think) was ~8000RPM.

More nonsence. You need a new source. I work regularly with the BR710 and the Tay 611-8C and they get much more RPM then that. BR710 gets close to 15,000 RPM at take off.
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Jetlagged
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RE: Source Of Noise From A Jet Engine?

Tue Dec 13, 2005 12:29 pm

Quoting Liedetectors (Reply 21):
More nonsence. You need a new source. I work regularly with the BR710 and the Tay 611-8C and they get much more RPM then that. BR710 gets close to 15,000 RPM at take off.

15,000 core rpm, not fan? Just a guess. As for the PW4000, 8,000 rpm definitely sounds like 100% core rpm (N2). Fan rpm would be around 3,600 at 100%.

Quoting JarheadK5 (Reply 20):
Sure, they CAN. I'm sure in an overspeed event, some of the HPC/HPT blades will experience supersonic tip speeds. But it's not exactly conducive to longevity

Fan blade tips may well go supersonic at takeoff power, that would explain the buzz-saw sound they make. As long as the blade section is designed for supersonic flow it won't be damaged in any way. Some military engine cores are designed with supersonic compressor blades.
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phollingsworth
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RE: Source Of Noise From A Jet Engine?

Wed Dec 14, 2005 1:16 am

Quoting JarheadK5 (Reply 18):
Fan blades don't break the sound barrier. Nor do any of the other blades in the engine, for that matter.

There is all sorts of localized supersonic flow in modern gas-turbine engines. A significant amount of effort in what is sometimes called "3-D aero" CFD analysis goes into analyzing and handling the effects of shock waves coming of a blade and impinging on a trailing blade or a down stage stator vane.

Quoting Jetlagged (Reply 22):
15,000 core rpm, not fan? Just a guess. As for the PW4000, 8,000 rpm definitely sounds like 100% core rpm (N2). Fan rpm would be around 3,600 at 100%.

Also, even without any incident velocity a 71" fan would be sonic on a sea-level, standard day at 3600 rpm. Of course at take-off thrust there will be a substantial incident velocity of the inlet flow.
 
liedetectors
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RE: Source Of Noise From A Jet Engine?

Wed Dec 14, 2005 7:57 am

Quoting Jetlagged (Reply 22):
15,000 core rpm, not fan?

Core. Sorry should have clarified.
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RichardPrice
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RE: Source Of Noise From A Jet Engine?

Wed Dec 21, 2005 1:40 am

In this video, you can distinctly hear the A330s engine spool up to takeoff thrust in two seperate parts, can anyone tell me what part is making what noise? Cheers!

http://flightlevel350.com/viewer.php?id=4392
 
Lemurs
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RE: Source Of Noise From A Jet Engine?

Wed Dec 21, 2005 6:20 am

Here's an excellent discussion of Boeing's findings during the QTD2 testing earlier this year. (Quiet Technology Demonstrator 2) There are some eye-popping results in there, particularly the fan-noise reduction numbers they mention early. 15db reduction is beyond huge when you consider decibles are on a logarhtmic scale, and it's not exactly quiet in a cabin.

http://seattlepi.nwsource.com/business/250230_air30.html
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lehpron
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RE: Source Of Noise From A Jet Engine?

Wed Dec 21, 2005 7:19 am

Quoting Lemurs (Reply 26):
Here's an excellent discussion of Boeing's findings during the QTD2 testing earlier this year. (Quiet Technology Demonstrator 2) There are some eye-popping results in there, particularly the fan-noise reduction numbers they mention early. 15db reduction is beyond huge when you consider decibles are on a logarhtmic scale, and it's not exactly quiet in a cabin.

I hear one way of doing so is increasing the blade chord, making an extreme form look almost like a ship's propeller, eh?
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lightsaber
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RE: Source Of Noise From A Jet Engine?

Wed Dec 21, 2005 3:23 pm

Quoting Joness0154 (Reply 19):
If the propellor tips on a stearman biplane can break the sound barrier by spinning at somewhere near 2500RPM, I have no doubt that a fan blade spinning upwards of 15,000RPM can break the sound barrier.

Fan blades need to be slightly supersonic at full takeoff otherwise there isn't enough surge margin on the fan. Thus, the tips are in the mach 1.1 to 1.2 range at takeoff. Also, recall the fan is "oversped" at takeoff: otherwise, the low turbine is operating at such a low mach number that the engine's efficiency would be quite poor.

Most of the noise from a turbofan at higher thrust ratings is the turbine exhaust. The high compressor creates quite a whine that can be heard on landing planes. While it is theoretically possible to hear the combustor, I'm not aware of an engine where that sound isn't drowned out by other components.

A good graph from NASA(scroll down half way), the article itself is so-so as an introduction to aircraft noise, but the graph answers this topic's question on where the noise comes from:
http://www.nasa.gov/centers/glenn/about/fs03grc.html

Now generally I dealt with charts a little more broken down (these have fan and compressor noise lumped together). I've also seen charts that usually put more of the noise on the turbine exhaust, but the ratio varies engine to engine and even a bit test to test.

Please notice how airframe noise on approach is a large contributor? That's from flaps, gear, and other components extended in the "unclean" state aircraft must be in for landing. At some point the airframe has to be cleaned up if engine noise is reduce. Note: Both A and B are doing this. (e.g., A380, 787, 747-8 vs. 744)

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