Heriberto Cortes

1
Jet Noise Its Reduction

• Heriberto Cortes
• Alex Dugé
• Ana Erb

2
Sound Review

• Sound is a compression waveform that moves
  through air or other materials. Sound waves are
  created by the vibration of some object and are
  detected by when the sound wave causes a sensor
  to vibrate.
• The characteristics of sound are frequency,
  wavelength, amplitude and velocity.
• velocity wavelength x frequency
• The frequency is the number of oscillations of a
  particular point in the course of sound waves in
  a second. (Hz)

Compression Wave (sound)
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Noise

• noise is unwanted sound it is derived from the
  Latin word nausea, meaning seasickness.
• Problems related to noise
• Hearing loss
• Stress
• High blood pressure
• Distraction
• Sleep loss
• Headaches

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Noise Levels
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What is a decibel?

• Unit used to measure the intensity of sound (dB)
• Any sound above 85 dB can cause hearing loss.
• Loss is related to length of exposure as well as
  power.
• Any exposure to 140 dB sound causes immediate
  damage.

• Near total silence - 0 dB
• A whisper - 15 dB
• Normal conversation - 60 dB
• A lawnmower - 90 dB
• A car horn - 110 dB
• A rock concert or a jet engine - 120 dB
• A gunshot or firecracker - 140 dB

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Intensity, frequency, and the human ear
The response of the ear is not exactly
proportional to the decibel scale. In addition to
the physical quantities, intensity and frequency,
the psycho-physiological quantities of loudness
and pitch must be considered. The loudness of a
sound depends both on intensity level and
frequency pitch depends chiefly on frequency but
to some extent on intensity.
Contours of equal loudness, plotted against
intensity and frequency for the average ear.
Contour values are the values of SPL at 1
kHz.
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Decibel
dB is used to describe relationships of POWER.
Power of sound varies as the square of pressure.
The SPL (sound pressure level) equations is as
follows
Acoustic parameters using the relationship
between the instantaneous pressure (p) and
velocity (u), prcu, the following acoustic
parameter for intensity (I) can be found
sound calculator
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Lighthill's Eighth Power Law
Lighthill's eighth power law states that the
acoustic power radiated by a jet is proportional
to the eighth power of the jet speed.
Lighthill's eighth power law for jet noise
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History of Jet Noise

• Aircraft noise is hardly a new subject as
  evidenced in the note received by a predecessor
  of United Airlines in about 1927.
• Jets of the 1960 were up to 6 times louder then
  the jets today.
• Today's jets are 20 decibels quieter then jets
  produced in the 1950s.
• Lighthill and Goldstein began research in
  acoustic theory with jet noise.

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Noise Regulations

• Federal Aviation Act (1958)
• Noise Control Act (1972)
• Quiet Communities Act (1978)
• Federal Aviation Administration (FAA)

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What Regulations Do

• Removal or retrofit of all older aircraft by 1985
  and then further in 2000.
• Reduce overall emissions and noise from aircraft.
• Increase in buffer zones around public airports.
• Zoning near airports to reduce complaints.
• Acoustic insulation for building in areas
  effected by noise pollution.

12
Airframe
Aircraft Noise
Engine
Aircraft noise can be separated into two
categories. It is caused by the engine as well as
the airframe.
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Airframe Noise

• Nonpropulsive noise made by an aircraft in
  flight.
• The shear of the boundary layer and unsteady
  vortex shedding from landing gear, landing gear
  doors, and other separated flows as well as flap
  edge flows contribute a significant part of the
  acoustic energy, especially for large aircraft on
  approach.
• Problematic at approach, when propulsion/throttle-
  related noise sources are relatively low.

14
Airframe Noise Reduction

• Slower decent
• The noise generated by the airframe reduces very
  quickly as you slow down, so even a slightly
  slower approach can make a big difference in
  terms of noise.
• Interest in airframe-related noise has grown, but
  engine noise still accounts for most of the
  aircraft external noise
• NASA has developed a flow-physics-based
  technology that reduces the noise created by the
  air passing over flap edges (right).

Flap airframe noise model in Wind Tunnel (NASA)
This technology is a micro piece of hardware that
can be easily added to existing airplanes. Flap
airframe noise reductions of 4 decibels have been
demonstrated experimentally with little
degradation in lift performance.
NASA Landing Gear Research
15
Engine Noise History
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Were Noise is Created (Engine)
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Noise Generation

• Fan, Compressor, and Turbine
• Related to flow inhomogeneities interacting with
  the surfaces of the fan.
• Turbulence created by blade wake between
  stationary and rotating vanes.
• Combustor
• Direct combustion noise due to unsteady
  combustion.
• Mass flux due to accelerated hot shots through
  turbine and nozzle.

• Exit
• Violent mixing between exhaust gasses and
  atmospheric gasses.
• Shock waves caused by exhaust flow that exceeds
  the speed of sound.

18
Jet Velocity vs. BPR
The jet velocity is reduced considerably as the
bypass ratio increased. This is indicated by the
figure that applies to older engines, but is
still representative of the trend observed for
larger modern engines.
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Noise vs. BPR



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Effects of Bypass Flow Increase
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Noise vs. Thrust
If thrust is reduced, not by scaling the engines,
but by reducing the throttle setting, the noise
is reduced much more because the fan tip speeds
and exhaust velocity are reduced.
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Noise Reduction

• Higher bypass flow generally reduces jet noise.
• Mixture of bypass and core flow to reduce jet
  noise.
• Increase acoustic baffling.
• Angle of takeoff and decent.
• Number of engines.

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Inlet

• High pitched whine heard as aircraft approaches.
• Caused by turbulence produced by fan blades.
• Reduced by trailing edge blowing.
• Blowing air across fans trailing edge to reduce
  strength of wake.

25
Acoustic Insulation

• Very effective at reducing noise levels of jet.
• Acoustic properties of lining are match to noise
  created in the location where it is placed.
• Major drawback is the weight and drag increased
  by the lining.

26
Nozzle Changes

• Increase mixing between jet and bypass exhaust.
• Lowers the amount of low frequency sound created.
• Increases high frequency sound which is quickly
  absorbed y the atmosphere.
• Drawbacks
• Increased weight.
• Decreased efficiency.
• Increased drag.

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Reference Locations
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http//adg.stanford.edu/aa241/noise/noise.html
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Example Problem
A test was performed and the collected data is
modeled using the below equation. To filter out
the high frequency, use fft to illustrate the
Fourier transform of the data. Filter out the
High Frequency and compare the filtered time
sequence to the low frequency component of the
signal.
Noise is being introduced using a random variable
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Fourier Transform
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Filtering the High Frequency
Filter 0
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Filtered Spectra Density Plot
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Inverse fft compared to low frequency signal
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Homework problem

• Use the random data given to determine the
  Fourier transform of the element vector. If any
  of the frequencies is higher than 5, develop a
  program to filter the high frequencies then
  invert the filtered time sequence and compare it
  to the original.
• data will be sent through e-mail

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References

• http//www.aia-aerospace.org/issues/subject/noise_
  workshop/nws02_aircraft_noise.pdf
• http//techreports.larc.nasa.gov/ltrs/PDF/2000/mtg
  /NASA-2000-22cicas-cap.pdf
• http//www.combustion-noise.de/subproject.php?proj
  ect8
• http//roger.ecn.purdue.edu/propulsi/propulsion/j
  ets/basics/noise.html
• http//www.nasaexplores.com/show2_5_8a.php?id01-0
  77gl58
• http//www.nasaexplores.com/show2_articlea.php?id
  01-077
• http//www.grc.nasa.gov/WWW/RT2001/5000/5940heidel
  berg.html
• http//adg.stanford.edu/aa241/noise/noise.html
• www.eng.fsu.edu/shih/eml4421
• www.eng.fsu.edu/shih/eml3050
• http//www.aero-space.nasa.gov/library/event_archi
  ves/chicago/airframe.htm