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НАУЧНАЯ БИБЛИОТЕКА - РЕФЕРАТЫ - Tupolev 154M noise asesment (Анализ шумовых характеристик самолёта Ту-154М)

Tupolev 154M noise asesment (Анализ шумовых характеристик самолёта Ту-154М)

Contents

The Noise Problem

Effects of Noise

1. Hearing Loss

2. Noise Interference

3. Sleep Disturbance

4. Noise Influence on Health

Noise Sources

5. Jet Noise

6. Turbomachinery Noise

Noise Measurement and Rules

7. Noise Effectiveness Forecast (NEF)

8. Effective Perceived Noise Level (EPNL)

Noise Certification

9. Noise limits

Calculations

10. Tupolev 154M Description

11. Noise calculations

1. Take-off Noise Calculation

2. Landing Approach Noise Claculation

Noise Suppression

12. Jet Noise Suppression

13. Duct Linings

1. Duct Lining Calculation

1 The Noise Problem

Though long of concern to neighbors of major airports, aircraft noise

first became a major problem with the introduction of turbojet-powered

commercial aircraft (Tupolev 104, Boeing 707, Dehavilland Comet) in the

late 1950s. It was recognized at the time that the noise levels produced by

turbojet powered aircraft would be unacceptable to persons living under the

take-off pattern of major airports. Accordingly, much effort was devoted to

developing jet noise suppressors, with some modest success. Take-off noise

restrictions were imposed by some airport managements, and nearly all first-

generation turbojet-powered transports were equipped with jet noise

suppressors at a significant cost in weight, thrust, and fuel consumption.

The introduction of the turbofan engine, with its lower jet velocity,

temporarily alleviated the jet noise problem but increased the high-

frequency turbomachinery noise, which became a severe problem on landing

approach as well as on take-off. This noise was reduced somewhat by

choosing proper rotor and stator blade numbers and spacing and by using

engines of the single-mixed-jet type.

2 Effects Of Noise

Noise is often defined as unwanted sound. To gain a satisfactory

understanding of the effects of noise, it would be useful to look briefly

at the physical properties of sound.

Sound is the result of pressure changes in a medium, caused by

vibration or turbulence. The amplitude of these pressure changes is stated

in terms of sound level, and the rapidity with which these changes occur is

the sound's frequency. Sound level is measured in decibels (dB), and sound

frequency is stated in terms of cycles per second or Hertz (Hz). Sound

level in decibels is a logarithmic rather than a linear measure of the

change in pressure with respect to a reference pressure level. A small

increase in decibels can represent a large increase in sound energy.

Technically, an increase of 3 dB represents a doubling of sound energy, and

an increase of 10 dB represents a tenfold increase. The ear, however,

perceives a 10-dB increase as doubling of loudness.

Another important aspect is the duration of the sound, and the way it

is distributed in time. Continuous sounds have little or no variation in

time, varying sounds have differing maximum levels over a period of time,

intermittent sounds are interspersed with quiet periods, and impulsive

sounds are characterized by relatively high sound levels and very short

durations.

The effects of noise are determined mainly by the duration and level

of the noise, but they are also influenced by the frequency. Long-lasting,

high-level sounds are the most damaging to hearing and generally the most

annoying. High-frequency sounds tend to be more hazardous to hearing and

more annoying than low-frequency sounds. The way sounds are distributed in

time is also important, in that intermittent sounds appear to be somewhat

less damaging to hearing than continuous sounds because of the ear's

ability to regenerate during the intervening quiet periods. However,

intermittent and impulsive sounds tend to be more annoying because of their

unpredictability.

Noise has a significant impact on the quality of life, and in that

sense, it is a health problem. The definition of health includes total

physical and mental well-being, as well as the absence of disease. Noise is

recognized as a major threat to human well-being.

The effects of noise are seldom catastrophic, and are often only

transitory, but adverse effects can be cumulative with prolonged or

repeated exposure. Although it often causes discomfort and sometimes pain,

noise does not cause ears to bleed and noise-induced hearing loss usually

takes years to develop. Noise-induced hearing loss can indeed impair the

quality of life, through a reduction in the ability to hear important

sounds and to communicate with family and friends. Some of the other

effects of noise, such as sleep disruption, the masking of speech and

television, and the inability to enjoy one's property or leisure time also

impair the quality of life. In addition, noise can interfere with the

teaching and learning process, disrupt the performance of certain tasks,

and increase the incidence of antisocial behavior. There is also some

evidence that it can adversely affect general health and well-being in the

same manner as chronic stress.

2.1 Hearing Loss

Hearing loss is one of the most obvious and easily quantified effects

of excessive exposure to noise. Its progression, however, is insidious, in

that it usually develops slowly over a long period of time, and the

impairment can reach the handicapping stage before an individual is aware

of what has happened.

Prolonged exposure to noise of a certain frequency pattern can cause

either temporary hearing loss, which disappears in a few hours or days, or

permanent loss. The former is called temporary threshold shift, and the

latter is known as permanent threshold shift.

Temporary threshold shift is generally not damaging to human’s ear

unless it is prolonged. People who work in noisy environments commonly are

victims of temporary threshold shift.

[pic]

Figure 2.1 Temporary threshold shift for rock band performers.

Repeated noise over a long time leads to permanent threshold shift.

This is especially true in industrial applications where people are

subjected to noises of a certain frequency.

There is some disagreement as to the level of noise that should be

allowed for an 8-hour working day. Some researchers and health agencies

insist that 85 dB(A) should be the limit. Industrial noise level

limitations are shown in the Table 2.1.

Table 2.1 Maximum Permissible Industrial Noise Levels By OSHA

(Occupational Safety and Health Act)

|Sound Level, dB(A) |Maximum Duration |

| |During Any |

| |Working Day |

| |(hr) |

|90 |8 |

|92 |6 |

|95 |4 |

|100 |2 |

|105 |1 |

|110 |Ѕ |

|115 |ј |

Noise-induced hearing loss is probably the most well-defined of the

effects of noise. Predictions of hearing loss from various levels of

continuous and varying noise have been extensively researched and are no

longer controversial. Some discussion still remains on the extent to which

intermittencies ameliorate the adverse effects on hearing and the exact

nature of dose-response relationships from impulse noise. It appears that

some members of the population are somewhat more susceptible to noise-

induced hearing loss than others, and there is a growing body of evidence

that certain drugs and chemicals can enhance the auditory hazard from

noise.

Although the incidence of noise-induced hearing loss from industrial

populations is more extensively documented, there is growing evidence of

hearing loss from leisure time activities, especially from sport shooting,

but also from loud music, noisy toys, and other manifestations of our

"civilized" society. Because of the increase in exposure to recreational

noise, the hazard from these sources needs to be more thoroughly evaluated.

Finally, the recent evidence that hearing protective devices do not perform

in actual use the way laboratory tests would imply, lends support to the

need for reevaluating current methods of assessing hearing protector

attenuation.

2.2 Noise Interference

Noise can mask important sounds and disrupt communication between

individuals in a variety of settings. This process can cause anything from

a slight irritation to a serious safety hazard involving an accident or

even a fatality because of the failure to hear the warning sounds of

imminent danger. Such warning sounds can include the approach of a rapidly

moving motor vehicle, or the sound of malfunctioning machinery. For

example, Aviation Safety states that hundreds of accident reports have many

"say again" exchanges between pilots and controllers, although neither side

reports anything wrong with the radios.

Noise can disrupt face-to-face and telephone conversation, and the

enjoyment of radio and television in the home. It can also disrupt

effective communication between teachers and pupils in schools, and can

cause fatigue and vocal strain in those who need to communicate in spite of

the noise. Interference with communication has proved to be one of the most

important components of noise-related annoyance.

Interference with speech communication and other sounds is one of the

most salient components of noise-induced annoyance. The resulting

disruption can constitute anything from an annoyance to a serious safety

hazard, depending on the circumstance.

Criteria for determining acceptable background levels in rooms have also

been expanded and refined, and progress has been made on the development of

effective acoustic warning signals.

It is now dear that hearing protection devices can interfere with the

perception of speech and warning signals, especially when the listener is

hearing impaired, both talker and listener wear the devices, and when

wearers attempt to locate a signal's source.

Noise can interfere with the educational process, and the result has been

dubbed "jet-pause teaching" around some of the nation's noisier airports,

but railroad and traffic noise can also produce scholastic decrements.

2.3 Sleep Disturbance

Noise is one of the most common forms of sleep disturbance, and sleep

disturbance is a critical component of noise-related annoyance. A study

used by EPA in preparing the Levels Document showed that sleep interference

was the most frequently cited activity disrupted by surface vehicle noise

(BBN, 1971). Aircraft none can also cause sleep disruption, especially in

recent years with the escalation of nighttime operations by the air cargo

industry. When sleep disruption becomes chronic, its adverse effects on

health and well-being are well-known.

Noise can cause the sleeper to awaken repeatedly and to report poor

sleep quality the next day, but noise can also produce reactions of which

the individual is unaware. These reactions include changes from heavier to

lighter stages of sleep, reductions in "rapid eye movement" sleep,

increases in body movements during the night, changes in cardiovascular

responses, and mood changes and performance decrements the next day, with

the possibility of more serious effects on health and well-being if it

continues over long periods.

2.4 Noise Influence on Health

Noise has been implicated in the development or exacerbation of a

variety of health problems, ranging from hypertension to psychosis. Some of

these findings are based on carefully controlled laboratory or field

research, but many others are the products of studies that have been

severely criticized by the research community. In either case, obtaining

valid data can be very difficult because of the myriad of intervening

variables that must be controlled, such as age, selection bias, preexisting

health conditions, diet, smoking habits, alcohol consumption, socioeconomic

status, exposure to other agents, and environmental and social stressors.

Additional difficulties lie in the interpretation of the findings,

especially those involving acute effects.

Loud sounds can cause an arousal response in which a series of

reactions occur in the body. Adrenalin is released into the bloodstream;

heart rate, blood pressure, and respiration tend to increase;

gastrointestinal motility is inhibited; peripheral blood vessels constrict;

and muscles tense. Even though noise may have no relationship to danger,

the body will respond automatically to noise as a warning signal.

3 Noise Sources

All noise emanates from unsteadiness – time dependence in the flow. In

aircraft engines there are three main sources of unsteadiness: motion of

the blading relative to the observer, which if supersonic can give rise to

propagation of a sequence of weak shocks, leading to the “buzz saw” noise

of high-bypass turbofans; motion of one set of blades relative to another,

leading to a pure-tome sound (like that from siren) which was dominant on

approach in early turbojets; and turbulence or other fluid instabilities,

which can lead to radiation of sound either through interaction with the

turbomachine blading or other surfaces or from the fluid fluctuations

themselves, as in jet noise.

3.1 Jet Noise

When fluid issues as a jet into a stagnant or more slowly moving

background fluid, the shear between the moving and stationary fluids

results in a fluid-mechanical instability that causes the interface to

break up into vortical structures as indicated in Fig. 3.1. The vortices

travel downstream at a velocity which is between those of the high and low

speed flows, and the characteristics of the noise generated by the jet

depend on whether this propagation velocity is subsonic or supersonic with

respect to the external flow. We consider first the case where it is

subsonic, as is certainly the case for subsonic jets.

[pic]

Figure 3.1 A subsonic jet mixing with ambient air, showing the mixing layer

followed by the fully developed jet.

For the subsonic jets the turbulence in the jet can be viewed as a

distribution of quadrupoles.

3.2 Turbomachinery Noise

Turbomachinery generates noise by producing time-dependent pressure

fluctuations, which can be thought of in first approximation as dipoles

since they result from fluctuations in force on the blades or from passage

of lifting blades past the observer.

It would appear at first that compressors or fans should not radiate

sound due to blade motion unless the blade tip speed is supersonic, but

even low-speed turbomachines do in fact produce a great deal of noise at

the blade passing frequencies.

4 Noise Measurement and Rules

Human response sets the limits on aircraft engine noise. Although the

logarithmic relationship represented by the scale of decibels is a first

approximation to human perception of noise levels, it is not nearly

quantitative enough for either systems optimization or regulation. Much

effort has gone into the development of quantitative indices of noise.

4.1 Noise Effectiveness Forecast (NEF)

It is not the noise output of an aircraft per se that raises

objections from the neighborhood of a major airport, but the total noise

impact of the airport’s operations, which depends on take-off patterns,

frequencies of operation at different times of the day, population

densities, and a host of less obvious things. There have been proposals to

limit the total noise impact of airports, and in effect legal actions have

done so for the most heavily used ones.

One widely accepted measure of noise impact is the Noise

Effectiveness Forecast (NEF), which is arrived at as follows for any

location near an airport:

1. For each event, compute the Effective Perceived Noise Level (EPNL) by

the methods of ICAO Annex 16, as described below.

2. For events occurring between 10 PM and 7 AM, add 10 to the EPNdB.

3. Then NEF = [pic], where the sum is taken over all events in a 24-hour

period. A little ciphering will show that this last calculation is

equivalent to adding the products of sound intensity times time for

all events, then taking the dB equivalent of this. The subtractor 82

is arbitrary.

4.2 Effective Perceived Noise Level (EPNL)

The perceived noisiness of an aircraft flyover depends on the

frequency content, relative to the ear’s response, and on the duration. The

perceived noisiness is measured in NOYs (unit of perceived noisiness) and

is plotted as a function of sound pressure level and frequency for random

noise in Fig. 4.1.

[pic]

Figure 4.1 Perceived noisiness as a function of frequency and sound

pressure level

Pure tones (frequencies with pressure levels much higher than that of the

neighboring random noise in the sound spectrum) are judged to be more

annoying than an equal sound pressure in random noise, so a “tone

correction” is added to their perceived noise level. A “duration

correction” represents the idea that the total noise impact depends on the

integral of sound intensity over time for a given event.

The 24 one-third octave bands of sound pressure level (SPL) are

converted to perceived noisiness by means of a noy table.

[pic]

Figure 4.2 Perceived noise level as a function of NOYs

Conceptually, the calculation of EPNL involves the following steps.

1. Determine the NOY level for each band and sum them by the relation

[pic],

where k denotes an interval in time, i denotes the several frequency

bande, and n(k) is the NOY level of the noisiest band. This reflects

the “masking” of lesser bands by the noisiest.

2. The total PNL is then PNL(k) = 40 + 33.3 log10N(k).

3. Apply a tone correction c(k) by identifying the pure tones and adding

to PNL an amount ranging from 0 to 6.6 dB, depending on the frequency

of the tone and its amplitude relative to neighboring bands.

4. Apply a duration correction according to EPNL = PNLTM + D, where PNLTM

is the maximum PNL for any of the time intervals. Here

[pic],

where (t = 0.5 sec, T = 10 sec, and d is the time over which PNLT

exceeds PNLTM – 10 dB. This amounts to integrating the sound pressure

level over the time during which it exceeds its peak value minus 10

dB, then converting the result to decibels.

All turbofan-powered transport aircraft must comply at certification with

EPNL limits for measuring points which are spoken about in the next

chapter.

5 Noise Certification

The increasing volume of air traffic resulted in unacceptable noise

exposures near major urban airfields in the late 1960s, leading to a great

public pressure for noise control. This pressure, and advancing technology,

led to ICAO Annex 16, AP-36, Joint Aviation Regulation Part 36 (JAR-36) and

Federal Aviation Rule Part 36 (FAR-36), which set maximum take-off, landing

and “sideline” noise levels for certification of new turbofan-powered

aircraft. It is through the need to satisfy this rule that the noise issue

influences the design and operation of aircraft engines. A little more

general background of the noise problem may be helpful in establishing the

context of engine noise control.

The FAA issued FAR-36 (which establishes the limits on take-off,

approach, and sideline noise for individual aircraft), followed by ICAO

issuing its Annex 16 Part 2, and JAA issuing JAR-36. These rules have since

been revised several times, reflecting both improvements in technology and

continuing pressure to reduce noise. As of this writing, the rules are

enunciated as three progressive stages of noise certification. The noise

limits are stated in terms of measurements at three measuring stations, as

shown in Fig. 5.1: under the approach path 2000 m before touchdown, under

the take-off path 6500 m from the start of the take-off roll, and at the

point of maximum noise along the sides of the runway at a distance of 450

m.

[pic]

Figure 5.1 Schematic of airport runway showing approach, take-off, and

sideline noise measurement stations.

The noise of any given aircraft at the approach and take-off stations

depends both on the engines and on the aircraft’s performance, operational

procedures, and loading, since the power settings and the altitude of the

aircraft may vary.

The sideline station is more representative of the intrinsic take-off

noise characteristics of the engine, since the engine is at full throttle

and the station is nearly at a fixed distance from the aircraft. The actual

distance depends on the altitude the aircraft has attained when it produced

maximum noise along the designated measuring line. Since FAR-36 and

international rules set by the International Civil Aviation Organization

(ICAO annex 16, Part 2) which are generally consistent with it have been in

force, airport noise has been a major design criterion for civil aircraft.

Stricter noise pollution standards for commercial aircraft,

established by the International Civil Aviation Organization, came into

effect worldwide on 1 April. Most industrialized countries, including all

EU states, enforced the new rules and the vast majority of airliners flying

in those states already meet the more stringent requirements. But some

Eastern European countries are facing a problem, especially Russia. Eighty

percent of its civilian aircraft fall short of the standards, meaning it

will not be able to apply the new rules for domestic flights. Even more

worrisome for Moscow is the fact that Russia could find many of its planes

banned from foreign skies. Enforcement of the new rules could force Russia

to cancel 11,000 flights in 2002, representing some 12 percent of the

country's passenger traffic.

The new rules have been applied only to subsonic transports, because

no new supersonic commercial aircraft have been developed since its

promulgation.

5.1 Noise Limits

As mentioned above, all turbofan-powered transport aircraft must

comply at certification with EPNL limits for the three measuring stations

as shown in Fig. 5.1. The limits depend on the gross weight of the aircraft

at take-off and number of engines, as shown in Fig. 5.2. The rule is the

same for all engine numbers on approach and on the sideline because the

distance from the aircraft to the measuring point is fixed on approach by

the angle of the approach path (normally 3 deg) and on the sideline by the

distance of the measuring station from the runway centerline.

[pic]

Figure 5.2 Noise limits imposed by ICAO Annex 16 for certification of

aircraft.

On take-off, however, aircraft with fewer engines climb out faster, so they

are higher above the measuring point. Here the “reasonable and economically

practicable” principle comes into dictate that three-engine and two-engine

aircraft have lower noise levels at the take-off noise station than four-

engine aircraft.

There is some flexibility in the rule, in that the noise levels can

be exceeded by up to 2 EPNdB at any station provided the sum of the

exceedances is not over 3 ENPdB and that the exceedances are completely

offset by reductions at other measuring stations.

6 Noise Level Calculations

17 Tupolev 154M Description

For most airlines in the CIS, the Tupolev Tu-154 is nowadays the

workhorse on domestic and international routes.

[pic]

Figure 6.1 Tupolev 154M main look

It was produced in two main vesions: The earlier production models

have been designated Tupolev -154, Tupolev -154A, Tupolev -154B, Tupolev

-154B-1 and Tupolev -154B-2, while the later version has been called

Tupolev -154M. Overall, close to 1'000 Tupolev -154s were built up to day,

of which a large portion is still operated.

Table 6.1 Tupolev 154M main characteristics

|Role | |Medium range passenger aircraft |

|Status | |Produced until circa 1996, in wide |

| | |spread service |

|NATO Codename | |Careless |

|First Flight | |October 3, 1968 |

|First Service | |1984 |

|Engines | |3 Soloviev D-30KU (104 kN each) |

|Length | |47.9 m |

|Wingspan | |37.5 m |

|Range | |3'900 km |

|Cruising Speed | |900 km/h |

|Payload Capacity | |156-180 passengers (5450 kg) |

|Maximum Take-off | |100'000 kg |

|Weight | | |

The Tu-154 was developed to replace the turbojet powered Tupolev Tu-

104, plus the Antonov - 10 and Ilyushin - 18 turboprops. Design criteria in

replacing these three relatively diverse aircraft included the ability to

operate from gravel or packed earth airfields, the need to fly at high

altitudes 'above most Soviet Union air traffic, and good field performance.

In meeting these aims the initial Tupolev -154 design featured three

Kuznetsov (now KKBM) NK-8 turbofans, triple bogey main undercarriage units

which retract into wing pods and a rear engine T-tail configuration.

The Tupolev -154's first flight occurred on October 4 1968. Regular

commercial service began in February 1972. Three Kuznetsov powered variants

of the Tupolev -154 were built, the initial Tupolev -154, the improved

Tupolev -154A with more powerful engines and a higher max take-off weight

and the Tupolev -154B with a further increased max take-off weight. Tupolev

-154S is a freighter version of the Tupolev -154B.

Current production is of the Tupolev -154M, which first flew in 1982.

The major change introduced on the M was the far more economical, quieter

and reliable Solovyev (now Aviadvigatel) turbofans. The Tupolev - 154M2 is

a proposed twin variant powered by two Perm PS90A turbofans.

6.2 Noise Calculaions

Noise level at control points is calculated using the Noise-Power-

Distance (NPD) relationship. In practice NPD-relationship is used in the

parabolic shape:

[pic],

where coefficients А, В, С are different for different aircraft types and

engine modes. For Tupolev-154M the coefficients А, В, С are shown in the

table 6.2 in respect to Tupolev-154.

Table 6.2 Noise-Power-Distance coefficients of similar aircraft.

| |Tupolev-154 |Tupolev-154M |

|Weight, kg |80000 |76000 |72000 |68000 |68000 |

|Vapp, m/s |74,8 |72,91 |70,964 |68,965 |66,91 |

|Thrust, kg |8445,63 |8024,67 |7601,88 |7179,66 |6758,58 |

|LA, dBA |96,74 |96,05 |95,35 |94,66 |93,97 |

|EPNL, EPNdB |112,17 |111,32 |110,48 |109,64 |108,79 |

|?LA, dBA |0 |0,69 |0,7 |0,69 |0,69 |

|?EPNL, EPNdB |0 |0,85 |0,84 |0,84 |0,85 |

|SQRT (Wing |21,082 |20,548 |20 |19,437 |18,856 |

|Load) | | | | | |

|Thrust To |0,10557 |0,105588 |0,105582 |0,105583 |0,105603 |

|Weight rt. | | | | | |

Tupolev 154M has the same aerodynamics as Tupolev 154, thus the

necessary thrust for both of them during approach is almost the same.

Tupolev 154M has more powerful engines and it can carry more payload. Its

maximum landing weight is 2 tons greater than that one of 154. Noise

parameters are different for these aircraft (table 6.2), and the calculated

noise levels slightly differ as well.

7 Noise Suppression

7.1 Suppression of Jet Noise

Methods for suppressing jet noise have exploited the characteristics

of the jet itself and those of the human observer. For a given total noise

power, the human impact is less if the frequency is very high, as the ear

is less sensitive at high frequencies. A shift to high frequency can be

achieved by replacing one large nozzle with many small ones. This was one

basis for the early turbojet engine suppressors. Reduction of the jet

velocity can have a powerful effect since P is proportional to the jet

velocity raised to a power varying from 8 to 3, depending on the magnitude

of uc. The multiple small nozzles reduced the mean jet velocity somewhat by

promoting entrainment of the surrounding air into the jet. Some attempts

have been made to augment this effect by enclosing the multinozzle in a

shroud, so that the ambient air is drawn into the shroud.

Certainly the most effective of jet noise suppressors has been the

turbofan engine, which in effect distributes the power of the exhaust jet

over a larger airflow, thus reducing the mean jet velocity.

In judging the overall usefulness of any jet noise reduction system,

several factors must be considered in addition to the amount of noise

reduction. Among these factors are loss of thrust, addition of weight, and

increased fuel consumption.

A number of noise-suppression schemes have been studied, mainly for

turbofan engines of one sort or another. These include inverted-temperature-

profile nozzles, in which a hot outer flow surrounds a cooler core flow,

and mixer-ejector nozzles. In the first of these, the effect is to reduce

the overall noise level from that which would be generated if the hot outer

jets are subsonic with respect to the outer hot gas. This idea can be

implemented either with a duct burner on a conventional turbofan or with a

nozzle that interchanges the core and duct flows, carrying the latter to

the inside and the former to the outside. In the mixer-ejector nozzle, the

idea is to reduce the mean jet velocity by ingesting additional airflow

through a combination of the ejector nozzles and the chute-type mixer.

Fairly high mass flow ratios can be attained with such arrangements, at the

expense of considerable weight.

The most promising solution, however, is some form of “variable cycle”

engine that operates with a higher bypass ratio on take-off and in subsonic

flight than at the supersonic cruise condition. This can be achieved to

some degree with multi-spool engines by varying the speed of some of the

spools to change their mass flow, and at the same time manipulating

throttle areas. Another approach is to use a tandem-parallel compressor

arrangement, where two compressors operate in parallel at take-off and

subsonically, and in series at a supersonic conditions.

7.1.1 Duct Linings

It is self evident that the most desirable way to reduce engine noise

would be to eliminate noise generation by changing the engine design. The

current state of the art, however, will not provide levels low enough to

satisfy expected requirements; thus, it is necessary to attenuate the noise

that is generated.

Fan noise radiated from the engine inlet and fan discharge (Fig. 7.1)

of current fan jet airplanes during landing makes the largest contribution

to perceived noise.

[pic]

Figure 7.1 Schematic illustration of noise sources from turbofan

engines

Figure 7.2. shows a typical farfield SPL noise spectrum generated by a

turbofan engine at a landing-approach power setting. Below 800 Hz, the

spectrum is controlled by noise from the primary jet exhaust. The spectrum

between 800 and 10000 Hz contains several discrete frequency components in

particular that need to be attenuated by the linings in the inlet and the

fan duct before they are radiated to the farfield.

[pic]

Figure 7.2 Engine-noise spectrum

The objective in applying acoustic treatment is to reduce the SPL at

the characteristic discrete frequencies associated with the fan blade

passage frequency and its associated harmonics. Noise reductions at these

frequencies would alleviate the undesirable fan whine and would reduce the

perceived noise levels.

A promising approach to the problem has been the development of a

tuned-absorber noise-suppression system that can be incorporated into the

inlet and exhaust ducts of turbofan engines. An acoustical system of this

type requires that the internal aerodynamic surfaces of the ducts be

replaced by sheets of porous materials, which are backed by acoustical

cavities. Simply, these systems function as a series of dead-end

labyrinths, which are designed to trap sound waves of a specific

wavelength. The frequencies for which these absorbers are tuned is a

function of the porosity of flow resistance of the porous facing sheets and

of the depth or volume of the acoustical cavities. The cavity is divided

into compartments by means of an open cellular structure, such as honeycomb

cells, to provide an essentially locally reacting impedance (Fig. 7.3).

This is done to provide an acoustic impedance almost independent of the

angle of incidence of the sound waves impinging on the lining.

The perforated-plate-and-honeycomb combination is similar to an array

of Helmholtz resonators; the pressure in the cavity acts as a spring upon

which the flow through the orifice oscillates in response to pressure

fluctuations outside the orifice.

[pic]

Figure 7.2 Schematic of acoustic damping cavities in an angine duct. The

size of the resonators

is exaggerated relative to the duct diameter.

The attenuation spectrum of this lining is that of a sharply tuned

resonator effective over a narrow frequency range when used in an

environment with low airflow velocity or low SPL. This concept, however,

can also provide a broader bandwidth of attenuation in a very high noise-

level environment where the particle velocity through the perforations is

high, or by the addition of a fine wire screen that provides the acoustic

resistance needed to dissipate acoustic energy in low particle-velocity or

sound-pressure environments. The addition of the wire screen does, however,

complicate manufacture and adds weight to such an extent that other

concepts are usually more attractive.

[pic]

Figure 7.3 Acoustical lining structure.

Although the resistive-resonator lining is a frequency-tuned device

absorbing sound in a selected frequency range, a suitable combination of

material characteristics and lining geometry will yield substantial

attenuation over a frequency range wide enough to encompass the discrete

components and the major harmonics of most fan noise.

7.1.2 Duct Lining Calculation

First we have to determine the blade passage frequency:

[pic],

where z is number of blades, n is RPM.

Blade passage frequencies for different engine modes are given in table 7.1

Next we determine the second fan blade passage harmonic frequency, which is

two times greater than the first one: [pic].

Table 7.1 Fan blade passage frequencies for different engine modes.

|Take-off |Nominal |88%Nom |70%Nom |60%Nom |53%Nom |Idle | |RPM |10425

|10055 |9878 |9513 |9315 |8837 |4000 | |1st harmonic freq., Hz |5386,25

|5195,083

|5103,633

|4915,05

|4812,75

|4565,783

|2066,667

| |2nd harmonic freq., Hz |10772,5

|10390,17

|10207,27

|9830,1

|9625,5

|9131,567

|4133,333

| |

Using experimental data, we determine lining and cell geometry:

For the first harmonic, parameters will be:

. Distance between linings 28.5 cm;

. Lining length 45 cm;

. Lining depth 2.5 cm;

. Cell length 2 cm..

For the second harmonic, parameters will be the following:

. Distance between linings 4.5 cm;

. Lining length 5 cm;

. Lining depth 2.5 cm;

. Cell length 0.4 cm.

Figure 7.4 shows the placement of the lining in engine nacelle.

[pic]

Figure 7.4 Lining placement in the nacelle.

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