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northseatiger
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Gas Turbine Engine Noise

Fri Apr 09, 2004 7:48 am

Hi,
Can anyone tell me what causes the charecteristic noise of a gas turbine engine (large fixed wing) on accent and descent, you know the "eeeeerrrrrrggghhhhh" noise ?.
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rockyracoon
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RE: Gas Turbine Engine Noise

Fri Apr 09, 2004 7:55 am

try tech ops


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brons2
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RE: Gas Turbine Engine Noise

Fri Apr 09, 2004 8:02 am

fan blades going supersonic, is what I have read before.
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DC-10Tech
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RE: Gas Turbine Engine Noise

Fri Apr 09, 2004 4:52 pm

Actually its the fan blades approaching the speed of sound.
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Starlionblue
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RE: Gas Turbine Engine Noise

Fri Apr 09, 2004 8:06 pm

I imagine you don't want supersonic fan blades.
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EconoBoy
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RE: Gas Turbine Engine Noise

Fri Apr 09, 2004 8:50 pm

Modern turbofan engines produce a ‘buzz-saw’ sound which is projected forward from the fan, and is caused by the blade tips going supersonic. You could eliminate this noise by having a slower running fan, but then to get the same thrust, the fan would have to be a lot bigger, and the whole engine consequently heavier and thirstier. Some piston driven prop tips (did) go supersonic, leading to a big increase in noise - the North American T6 Harvard is/was a good example.
Another major source of noise in older, low by-pass engines used to be caused by the hot, high speed jet exhaust encountering the cold, still ambient air and causing noisy vortexes. This has been greatly reduced in modern turbofan engines which have a high by-pass ratio; the fan exhaust forms an intermediate, insulating layer between the turbine exhaust and the surrounding air.

 
AUAE
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RE: Gas Turbine Engine Noise

Fri Apr 09, 2004 8:50 pm

I don't think you want supersonic fan blades, along the same reasons you don't want supersonic props. I am pretty sure that the airflow through a jet engine does not reach supersonic speeds. The shock waves would be a huge efficiency loss. The only exception would be the exhaust exit where you try to get as much velocity as possible.

So as far as sound, I believe it has many roots. 1. Disturbance of the air by the compressor, turbine and exhaust (ie just like the noise a regular box fan makes), 2. combustion, and 3. the vibration of all the rotating machinery. As far as the ere-ere-ere, or the arm-arm-arm, that is just the natural frequency or the resonance of the engine. Different engines sound different because they are designed differently, giving rise to a different natural frequency.
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MITaero
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RE: Gas Turbine Engine Noise

Sat Apr 10, 2004 12:10 am

With respect to the flow, the blade tips do go supersonic, if I'm not mistaken.

>The only exception would be the exhaust exit where you try to get as much velocity as possible.

With the convergent nozzle used by commercial aircraft, Mach 1 is the maximum Mach number of the exit. Velocity can be higher if the exit temperature is higher, though.
 
aeroguy
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RE: Gas Turbine Engine Noise

Sat Apr 10, 2004 2:19 am

You can definitely have supersonic flow in a jet engine. As was mentioned the fan is the most obvious place. With the steady trend of increasing bypass ratio, tip speeds have gone supersonic. It seems logical that you wouldn't want a supersonic fan but it's a reality of modern fan design and actually is an interesting problem. Consider the recent trend of using wide-chord fan blades. Also consider fan blades in terms of a high speed aircraft wing. Thickness to chord ratio (t/c) is a critical parameter in determining drag divergence Mach Number, critical Mach Number, shock wave formation, etc. Obviously for high speed aircraft/blades you want t/c to be small. There's a feasible limit on how thin jet engine fan blades can be structurally, so to improve t/c, increased chord is the way to go. Now out at the tips especially, an interesting approach to the supersonic flow problem is to treat the interblade airfoil passages as converging-diverging nozzles. Ideally you can minimize shock losses without limiting the amount of flow that can pass through the fan.

But supersonic flow is not limited to the fan. Keep in mind that compressor design has progressed such that we are no longer constrained to keeping blade tip Mach Numbers below 1. It is possible to have supersonic flow in the compressor without introducing excessive losses.

Also, during certain conditions, parts of the turbine section can become choked (yielding Mach 1 flow). For a twin spool engine for example, the LP turbine can be choked over much of its operating range.

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